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by 21 (2 — y)
a + b
a + o
2ab EXAMPLES.-(1.) Divide by a
11. Divide 8zy by
20. Divide by
(20 x - y).
12. Divide 180x by Ans. Here, we have
21. Divide 1 + by 1 bc
3m To understand the reason of the rule, let it be premised that 13. Divide 18 (q + x)
**-18 the product of any fraction by the same fraction inverted is
3c + d
ax + aba a'z+a
y+b always a unit.
14. Divide 2a + by x +
by b ab
d Thus Х
24. Divide 1 by 1 + But a quantity is not altered by multiplying it by a unit.
25. Divide * + 4a? + by Therefore, if the product of the dividend by the divisor inverted
3x + 1
- 4 be multiplied by the divisor itself, the last product will be equal 17. Divide by
* + 2a + to the dividend. Now, by the definition, " division is finding a
y - 2 y + 2 quotient, which, multiplied into the divisor, will produce the 18. Divide by
26. Divide 9ra - 28 + dividend.” And as the dividerd multiplied by the divisor in. verted is such a quantity, the quotient is truly found by the rule. 19. Divide by
by 3x - 4- 3h (2.) Divide by
ข Here we have
KEY TO EXERCISES IN LESSONS IN ALGEBRA. 21 3h 6h
bdy 148. To divide a fraction by an integer.
4ah 4- 48h 2.
2abh - 2abd
7. Divide the numerator by the given integer, when it can be done
my - 2y
5am without a remainder; but when this cannot be done, multiply the denominator by the integer.
3a – 3n
hnr + 2hn
14. h + 3d. am
4a + 4h-am-hum
- 18ad Thus the quotient of divided by m, is b 3c + cd + 3y + dy 7dhy + 7hy
h 149. To divide an integer by a fraction.
10. Reduce the integer to the form of a fraction, and proceed as before. Or, multiply the integer by the denominator, and divide
EXERCISE 22. the product by the numerator.
1. EXAMPLE.—Divide a by
7. 1. Here, a = ; and ( divided by i
a's a xd ad Or, a Ans, as before. 21
1 + 13*
i 6. Divide
4 + 5y
18r8 + 1974 - 93x + 36
17. 2. Divide by 7. Divide
36.46 by 6.
52.* + 964 - 43x2 + 12
8. Divide ay by ".
LESSONS IN FRENCH.-XLIX.
9. Divide ab + ex by 3y
SECTION XCVIII. - PRACTICAL RÉSUME OF THE RULE 3 5. Divide 10. Divide Sac – x by
ON THE PAST PARTICIPLE (continued). a + 1*
The participle past is INVARIABLE150. By a former definition " the reciprocal of a quantity is the quotient arising from dividing a unit by that quantity.”
1. In active verbs, when the direct regimen follows the pai b
ticiple :Thus the reciprocal of is 1 + 6 = 1 x
Mes nièces ont étudié leurs leçons, My nieces have studied their lessesi
Elles ont négligé leurs études, They have neglected their studies. Hence, the reciprocal of a fraction is the fraction inverted. For
b instance: the reciprocal of
2. In neuter verbs conjugated with avoir :-
Mes cousines ont disparu,
My cousins have disappeared. 1
or 3y; the reciprocal of, is 4. Hence the reciprocal Les cinq heures qu'elles ont dormi, The five hours which they have sley
1 of a fraction whose numerator is 1, is the denominator of the heures :
In the latter sentence, the word pendant is understood aft
1 fraction. Thus, the reciprocal of - is a ; of is a + b, etc. Les cinq heures pendant lesquelles The five hours during which ?
a + b
3. In unipersonal verbs, whether conjugated with être or w Sabe
avoir:1. Divide by 3ab. 6. Divide by
Les chaleurs qu'il a fait cette The heat there has been this year. 101xx + 15ab 2. Divide by 5ax. . 7. Divide
Il est arrivé bien des malheurs, Many misfortunes have happened 8x + 11 3. Divide by 32, 8. Divide
4. In reflective or pronominal verbs, of which the second p
noun is an indirect regimen, when no direct regimen precedes Divide
Elle s'est proposé de partir, She proposed to herself to lears.
7ab 10. Divide 21 abc by
5. When the participle precedes an infinitive, and is prece by a direct regimen, and this direct regimen is not the act
bat the object acted upon. In this case the infinitive is gene- favour (en faveur) of the architect. 11. The ten miles which rally rendered in English by the passive voice:
he has run have fatigued him. 12. Have your sisters injured Les chansons que j'ai entendu The songs whish I heard (boing) each other? 13. They have flattered themselves. 14. Did my chanter, 8ưng.
friends present themselves ? 15. There came three of your 6. When the direct regimen preceding a participle is not the idea of reading Tasso (Le Tasse). 18. Have you seen them (m.)
sisters. 16. What did they imagine ? 17. She conceived the object of this participle, but of a verb following:
steal my apples? 19. I saw them steal your peaches. 20. Have La règle que je vous ai conseillé The rule which I advised you to you heard them (f.) sing? 21. I have heard them sing. 22. d'étudier, study.
The songs which I heard sung are not new. 23. I found in your 7. The participle of faire, fait, followed by an infinitive, is room the books which I had forbidden you to take. 24. The always invariable:
peaches which I have forbidden you to eat are not ripe (mûres). Jo les ai fait raccommoder, I have had them mended.
25. Have you seen those soldiers ? 26. I saw them pass last 8. After the pronoun en, when no direct regimen precedes :-week: 27. I saw them carried to the hospital (a l'hôpital) this
morning. 28. Have you brought oranges from France ? 29. Vous a-t-on donné des fleurs ? Have they given you flowers ? I brought some. 30. The oranges which I brought from it (en) On m'en a donné,
Thoy have given mo (some) of thom. are good. 31. Have you brought silk goods ? 32. I have RÉSUMÉ OF EXAMPLES.
brought some. 33. I have brought none. Elles nous ont donné de bons con. They have given us good advice.
seils. Elles nous en ont donné. They have given us some.
KEY TO EXERCISES IN LESSONS IN FRENCH. Les trois lieues qu'il a couru. The three leagues which he ran.
EXERCISE 147 (Vol. II., page 387). Les années que ces édifices ont The years that those edifices have duré. lasted,
1. Why do you not cease reading ? 2. I should be wrong to cease La belle journée qu'il a fait hier! What a beautiful day it was yester- gardener to water those powers ? 4. On the contrary, I had ordered
reading before knowing my lesson. 3. Have you forbidden your
day! C'est la plus belle fête qu'il y ait it is the finest feast that there has
him to water them. 5. Why has he neglected doing it? 6. Because
he has forgotten to bring the watering-pot. 7. What does Mr. F. wisk Il s'est présenté deux de vos amis. There came two of your friends.
to do? 8. He longs to continue the study of medicine. 9. Are you Ces demoiselles se sont nui. Those young ladies have injured one it. 11. Have you not refused to render that service to your enemy?
not wrong to visit that gentleman? 10. I should be wrong to neglect
another. Les Asiatiques se sont fait une The Asiatics have made the education 12. I should have been wrong to refuse rendering it to him. 13, What espèce d'art de l'éducation de of the elephant a kind of art.
conveyance have you advised us to take? 14. I have advised you to l'éléphant.
take the steamboat. 15. Have you threatened to strike that child ? Elle s'est imaginé l'idée de pouvoir She conceived the idea that she might 16. I have threatened to correct him. 17. Have you refused to sell
goods to my brother ? 18. I have refused to sell them to him on réussir. succeed.
credit. 19. Have you told my son to repair to my house ? 20. I Les fruits que j'ai va voler. The fruits which I saw being stolen.
begged him to go straight there. 21. Do you propose coming ChristLes soldats blessés que j'ai vu por- The wounded soldiers whom I saw ter. (being) carried.
mas Eve ? 22. We propose to come the next day. 23. Does your
companion propose to keep silent ? 24. He proposes to impart it to La chanson que j'ai entendu chan. The song which I heard sung.
every one. ter, Les pommes que je vous ai défendu The apples which I forbade you to de manger. eat.
COMPARATIVE ANATOMY.-XVI. Je les ai fait partir.
I obliged them to leave.
They have brought me (some) of them. The classes of the Mollusca hitherto described have been pasVOCABULARY.
sive in their habits and methods of feeding. We have seen Auberge, 1., inn. Enterr-er, 1, to bury. Pièce, f., picce. that, for the most part, they are fixed or moved about at the Bien, m., good.
Habill-er, 1, to dress, Racont-er, 1, to relate. mercy of the waves. Even when locomotive, locomotion is Dernier, -e, last. Jou-er, 1, to play. Reven-ir, 2, ir., to re- with them rather for the purpose of change of place than for Disparait-re, 4, ir., to Lion d'Or, m., Golden turn.
definite progression. They are not pursuers of other animals, disappear.
Soieries, silk goods.
por do they graze on the copious vegetation of the ocean, but Dorm-ir, 2, to sleep. Mort, -e, dead.
they are dependent on what the current, which their cilia create, EXERCISE 191.
brings them for nutriment. With them the obtaining of fresh 1. Quelle auberge vous a-t-on recommandée ? 2. On m'a water for breathing and food for nutriment is one and the recommandé l'auberge du Lion-d'Or. 3. Quelles nouvelles same act. In conformity with their habits they are almost avez-vous apportées ? 4. J'ai apporté des nouvelles agréables. devoid of organs of sense, and wholly without instruments for 3. Vos voisines sont-elles habillées ? 6. Elles ne sont pas en seizing prey. They have, it is true, a double nerve-knot above core habillées. 7. Ont-elles bien dormi la nuit dernière ? 8. the mouth, but this, though in the position of a brain, can Elles n'ont pas bien dormi. 9. Quand sont-elles arrivées ? hardly be said to have the function of perception which we 10. Elles sont arrivées à quatre heures et demie. 11. Ont-elles usually associate with the brain. Two other pairs of nervedormi plus de cinq heures ? 12. Les six heures qu'elles ont knots generally communicate with this, and they are often dormi leur ont fait beaucoup de bien. 13. Vos seurs se sont much larger than it. One of them has the two halves which elles amusées ? 14. En jouant, elles se sont fait mal au bras. compose the pair more or less removed from one another, each 15. Se sont-elles raconté notre conversation ? 16. Elles se la lying at the point of junction of the mantle and gills on its sont racontée. 17. Vos amies ont-elles dispara ?
18. Elles own side, and sending nerves to these organs. The other n'ont pas disparu ; elles sont revenues chez elles. 19. Les double knot lies in the foot, and is more or less developed soldats que vous avez vus partir, sont-ils revenus ? 20. Ils according as the foot is large, small, or rudimentary. These sont morts; je les ai vu enterrer. 21. Ne les avez-vous pas fait | three double nerve-knots are called respectively the cephalic étudier ? 22. Je les ai fait lire. 23. Avez-vous apporté des (head), parieto-splanchnic (mantle and visceral), and pedal Boieries ? 24. Je n'en ai pas apporté. 25. Les soieries que j'en (foot) ganglia. In the case of the pecten, fringes of feelers ai apportées sont superbes.
and of eye-spots run round the mantle, and these imperfect and
perhaps questionable organs of sense are supplied from the EXERCISE 192.
parieto-splanohnio ganglia ; and this pair of ganglia no doubt 1. Have you not recommended my nieces ? 2. I have re- represent the only nerve-knot which exists in the Tunicata and commended them. 3. Have you brought me good oranges ? Polyzon. In their case, also, it supplies nerves to the tentacles 4. I have brought you some. 5. Have you given any to my and other organs of sense which they possess. This pair of two daughters ? 6. I have given them some. 7. I would have ganglia may therefore be considered to be the seat of perception given them some, if I had had many. 8. Have you not neglected rather than the cephalic
. Another of the functions of the brain, your studies ? 9. I have not neglected them; I never neglcct | however, may be assumed to belong to the cephalic pair of them. 10. The years which that church has lasted speak in ganglia, and that is the function of volition. All the other
ganglia of the body are in direct communication with the teropods; but in function, of course, the foot of the gasteropods cephalic pair, but not with the other two pairs. We may there is much more like a foot than the same organ in the lower class. fore consider that the cephalic pair is the originator of the Usually the foot is a muscular, elongated sheet, broader and voluntary actions, or is concerned in all those actions which longer than the body of the animal, and acts at the same the creature performs as a whole and individual. This pair is, time as the wall of the body and the means of propelling it so to speak, the central telegraph-station, and therefore may be along. The whole rim of the foot all the way round is usually considered to be
thickened, and in direct commu
can be closely apnication with the
plied to a smooth manager.
surface, while the We have dwelt
central parts can so long on the
be thrown in nervous system
wrinkles. Thus of the conchifer,
the whole acts as partly because we
a kind of sucker had not space for
or holdfast, while it in our last les
all the middle son, and partly
parts, being alto indicate the
ternately applied contrast which it
to the ground presents to that
and dragged over of the gastero
it, effect a movepods or headed
ment in which molluscs. In
the whole animal these, though the
participates. If nervous system
the reader allows no doubt consists
a slug to crawl of the same ele
up a pane of ments, they are 8
glass, and looks more closely as 9
at it through the sociated; and the
transparent meganglia, situated
dium, he will see over the throat,
successive waves behind the cavity
moving all along of the mouth, are
the foot, showin direct commu
ing that, while & nication with the
series of points organs of sense ;
are fixed, the and this is the
parts in between same arrange
are moring, and ment as is found
the moving parts both in verte
then become brates and artiIII.
fixed, allowing culates.
the previously The Gastero
fixed parts to be poda derive their
pushed or pulled name from the
along by the conusual form of the
traction of the locomotive or
muscles embedgan, which is so
ded in the skin. constantly found,
Such a mode though so vari
of progression, ously developed,
which may be in the different
called piecemeal, members of this
is, of course, class. We found
very slow, but the foot in the
it is sure; and Conchifera to be
how should an an organ which, PULMOGASTEROPODA.-I. ARION (THE BLACK SLUG). II. AGATHINA MAURITANICA. III. CYCLOSTOMA
animal without in some, secreted ELEGANS. IV. DIAGRAM OF THE CIRCULATION IN A SNAIL.
limbs move over the byssus or Refs. to Nos. in Figs.-I. 1, orifice of lung-chamber; 2, anus. II. 1, throat; 2, stomach ; 3, a solid surface anchor-cable, in intestines ; 4, anus ; 5, liver ; 6, 6, pulmonary diaphragm ; 7, main vein ; 8, chamber surrounding otherwise ? As. others bored the heart ; 9, auricle (receiver) ; 10, ventricle (distributor); 11, kidney ; 12, generative organs. sociated with this holes, and yet
(N.B. In this diagram the shell has been removed and the skin cut along the back and opened; power of definite in others accom
the floor of the lung also is thrown aside.) III. 1, operculum. IV. 1, lung rein; 2, auricle; locomotion, slow plished jerky
3, ventricle ; 4, main artery ; 5, liver artery ; 6, foot artery; 7, stomach artery; 8, buccal
as it is, the whole movements of
organism is modi the body. In the
fied. swan-müssel of our rivers this instrument is applied to more Let us suppose that a Lamellibranch had the under part of regular and definite locomotion, and with the foot they may be its foot flattened into a broad muscular sheet, capable, not of seen ploughing their way through the soft mud which falls to pushing through soft mud, but of gliding over smooth rock: the bottom of the stream. In their case, however, the foot is how could it make use of its new power of locomotion ? It a rounded organ, and at its end is something like the human would, in the first place, be hampered with two immense shields, tongue, both in shape and structure. In the gasteropods, or which, being ample enough to close upon its whole body, would belly-walkers, the foot is a flat broad surface placed along the certainly have their edges dragged over and ground upon the under side of the body, by means of which the animal can crawl rock over which it passed, and thus wrenched about in relation over solid bodies. In some of the conchifers the shape of the foot to one another and to the soft parts of the animal united to them.
much more like that of the human foot than in any of the gas. Then its large sheets of unprotected membrane, called gills,
would be liable to be torn and bruised. Add to these incon- | into the gasteropod by placing, side by side, some intermediate veniences the fact that it would be without eyes or feelers in the forms between the more typical turbo and the river-mussel fore-part of the body to direct its course, or to take observations (Unio). In patella (the limpet) it will be seen that the gills are of what occurred, and we may judge that the benefits of travel still on both sides of the animal, as are also the muscles, though would be quite outweighed by its dangers and troubles. In the these have no longer the office of closing the shell, which, in this gasteropods, therefore, both shells are consolidated into one, and case, is consolidated into an equilateral cone. In the bonnet drawn out in an upward direction, so that, while the more deli- limpet one side of the breathing organs has been aborted, while cate organs are securely lodged, the edges of the shell's mouth in turbo both the breathing apparatus and muscles of one side are withdrawn from the ground.
are gone, and the whole animal is twisted, in its upper part, The gills are removed out of harm's way in a singular manner. I into a one-sided spire. In this case a rounded horny plate is
1., 11., III., IV. DIAGRAMS OF UNIO, PATELLA, CALYPTREA, AND TURBO, TO SHOW THE STAGES OF TRANSITION FROM THE CONCHIFER TO THE TYPICAL
GASTEROPOD. V. A SINGLE SET OF TRANSVERSE TEETH FROM (1) A SEPHORATED GASTEROPOD AND (2 AND 3) UNSEPHORATED GASTEROPODS. VI.
SHELL OF CASSIS. VII. PALUDINA. VIII. SECTION OF CERITHIUM. Ref. to Nos, in Figs.-I., II., III., IV. 1. l', lips or tentacles ; 2, mantle ; 3, shell muscles ; 4, gills ; 5, foot; 6, position of the liver ; 7,
byssus ; 8, operculum. Ví. 1, spire ; 2, suture ; 3, aperture ; 4, outer lip; 5, inner lip; 6, anterior canal for passage of the siphon ; 7, posterior canal; 8, varices. VII. 1, umbilicus. VIII, 2, columella.
Those on one side (usually the right) are brought right up and developed on the upper part of the foot, or rather tail of the placed on the animal's back, and there enclosed by a fold of the creature, and this, when the animal pulls back its head and leathery skin, and placed partially in the last or largest part of thin foot into the shell, closely closes the aperture. This the shell cavity, while those of the other side are entirely operculum, as it is called, is supposed to be the representative aborted or dispensed with. This arrangement gives a one of the horny byssus of the bivalve, being, as will be seen, sidedness to the animal, and, perhaps, is the determining cause similarly situated. of the shell being made more compact in the method peculiar to The alimentary canal commences with a mouth armed with gasteropods, namely, by being twisted into a one-sided spiral or hard parts. These are different in different creatures; but in helix, as it is technically called. The head, with its feelers, all there is a fibrous plate, bearing teeth, placed on a cushion on eyes, and ears, can be thrust out from the shell and stretched the floor of the mouth. These teeth are usually directed backwell forward, so as to gain some acquaintance with those external wards ; sometimes the plate in which they are set is very long objects which come within the line of march. In the illustration from point to back, the teeth being disposed in small cross it is shown how the conchifer may possibly have been modified rows set in parallel lines from one end of the plate to the other.
This is more especially the case in the carnivorous sea-snails, in All the land, and most of the fresh-water endile have lungs, which it is associated with a long extensible proboscis. In the and belong to the sub-class Pulmonifera, while the sea-snails land and fresh-water gasteropods belonging to the order have gills, and belong to the other sub-classes. Thus we see Pulmonifera, the number in a cross direction is very great, but repeated in the Mollusca the two different kinds of breathing the lingual ribbon is much shorter. This tooth-bearing ribbon organs which are suited to aquatic and aërial life, which, in the is set on a muscular pad, which can move it backward and vertebrates, are represented by the gills of fish and the lungs of forward, so that the little flinty teeth act as a fine file. It is the higher orders. From this we may infer that a gill is the curious that these teeth are composed neither of horn nor shell necessary form of a water-breathing apparatus. (CaCO3), but of silica (SiO,) or flint. They are, of course, liable There is yet another sub-class of gasteropods called to be worn away; but the ribbon is formed from behind as fast Nucleobranchiata, or Heteropoda, which have various forms of as it wears away in front; and in some species, a considerable breathing organs; but these are so different in the whole of length of it lies coiled up in a sac or pouch, which stretches their structure from the rest that it becomes a question whether away from the mouth, ready to supply the place of the continual they should be classed with the gasteropods at all. wear and tear. A few examples of the pattern of the teeth are The central organ, which aids the circulation of the blood, is given in the engraving, in which only one transverse row of situated in the typical gasteropods in the partition or diaphragm, three different species is given. The mouth is very muscular, as it is called, which lies between the breathing chamber and and has on its front and upper wall a broad horny jaw, which the chamber containing the viscera. It is always at the hind is flat, with a cutting edge directed downward. It is of various part of this, and receives the blood from the gills, or central shapes, and is often toothed on its lower edge. In some sea- vessel of the lungs, into a chamber or auricle. From this it snails the mouth-cavity is furnished with a long trunk, which passes through a valve to the more muscular ventricle, and is can be unfolded from within, and used to grasp objects while driven by this into a vessel which almost immediately divides they are played upon by the file-like tongue. Inside these into two, one of which goes forward to the mouth and foot, and trunks there is sometimes a toothed circle or collar of pointed the other backward to the liver and all those organs which are fangs, which very much strengthen the hold that the creature situated in the recesses of the shell or hind cavity of the abdo has on its prey. It is singular that this tooth-bearing tongue men. The blood, thus distributed by vessels, is said to escape is found universally, not only among the gasteropods, but also from them into the general cavity of the body, and from thence among all the higher orders of the Mollusca, so that some enters by wide openings to the veins which convey it to the gills classifiers have associated these together as the Odontophora, or or lungs. In the case of the lung-breathers it enters the tooth-bearers.
diaphragm from behind, and runs in two main vessels along the We proceed to describe the alimentary canal as it occurs margins of this organ, and then sends off smaller vessels or in the arion, or common black slug, noticing such marked dif- sinuses towards the central vessel. In the Prosobranchiata the ferences as occur in some other orders. A very small throat sexes are distinct; but in the Pulmonifera and Opisthobranleads from the roundish buccal cavity, and this gradually dilates chiata the sexes are united in one individual, and the organs in until it ends in a wider stomach. On the sides of the throat the former are of very complicated and peculiar structure. In are situated two large glands for secreting saliva ; but, though the neighbourhood of the heart there is an organ which is conbound to the exterior of the throat by vessels, they discharge sidered to be a kidney, which eliminates the azotised products their secretion into the back part of the mouth by two ducts, caused by the wear and tear of the vital action. This organ which pass between the nervous collar and the attenuated seems also, in some species, to have the office of introducing portion of the throat. The stomach is of various shapes in the water into the blood-system from without, as it has an opening slugs; but in the example before us the hind part forms a kind on the one side into the breathing-chamber, and on the other of globular bag, and the two ducts from the liver enter just into the pericardium or external heart-chamber, before this rounded portion. The intestine leads from the globe The front part of the mantle-fold, which covers in the directly forward, so that this globular part occupies an acute breathing-chamber, is thickened into a collar, and this is the angle in the course of the alimentary canal, and then, after being instrument for secreting the shell. The shape and foldings of bent backwards and forwards two or three times, runs to a small this edge of the mantle give rise, in the process of growth, to all orifice situated at the neck of the animal on the right side, close those beautiful shells whose variety of colours and shape must to the aperture of the breathing chamber. The stomach and be known to the reader. intestine are closely embraced by lobes of the very large liver, One of the characteristics of the gasteropods is the immense which is so bound to them as to be with difficulty unravelled amount of sticky mucus they are constantly exuding, and which In the case of the spirally-coiled and shell-bearing gasteropods, makes, in the land-slugs, a serious draught on their natritive the largest masses of the liver are situated in the small end of system. This is secreted by glands all over the skin, but also, the spiral shell. In the aplysia (the sea-hare), and some other in some species, by special larger glands on the back of the of the gasteropods allied to it, the interior of the stomach is neck. studded with shelly plates and spines, thus converting it into a The nervous ganglia, though they probably consist of the gizzard.
same elements as in the Conchifera, are gathered together so The breathing organs of the Gasteropoda are very various, as to form a ring round the throat, situated at the tarrow part and they have been made use of to divide the class into sub- just behind the buccal mass. The muscular system is almost classes and orders, Thus there are four main sub-classes wholly confined to the skin, except that a broad muscle arising founded mainly on these organs. In the Opisthobranchiata the from the lower part of the body runs to the head, and slips of gills are branched like a tree, or gathered together in bundles this muscular sheet also go up the tentacles, so that, when in and placed on the hind parts of the body behind the heart, and contraction, the tubular tentacle and eye-stalks are pulled into are either naked or only partially protected by a fold of the the body at the same time as the head is withdrawn. In the mantle or shell. In the Pulmonifera there is a chamber common snail the eye-tentacles are the longest, and are set situated over the neck, and covered completely in by a thick highest on the head, while the lower pair is simply tactile. In fold of the mantle
. It only receives air through an aperture, many sea-snails there is only one pair of tentacles, the ends of which can be closed by a muscle running round it. The walls, which are feelers, while the eyes are set on the sides or bases of and especially the floor, of this chamber have large blood-vessels these. The eyes, themselves, are not highly organised, being in them, and so constitute a kind of lung in which the blood is little more than a nerve expanded in front of a dish of black aërated. In the Prosobranchiata the arrangement as to the pigment, and placed behind a transparent cornea. Ear-sacs, chamber is much the same, but in the chamber lie one or two with round ear-stones in them, are found in many Gasteropoda. gills, usually of a comb-like or feathery form, and in these, and The Gasteropoda are the most typical class of the Molluscanot in the walls of the cavity, the blood becomes oxygenated. that is, they are the central group, showing fewer points of In the carnivorous sea-snails the aperture is converted into a relation to the other sub-kingdoms than the other classes, and canal or siphon, which is often very long, and which has an possessing a very large number of species nearly allied to one anterior canal in the aperture of the shell for its accommodation, another, so that there are fewer gaps in the series. They, in thus constituting the difference between the round-mouthed fact, occupy a similar position with regard to the Mollusca 28 (Holostomata) and channelled (Siphonostomata) shells, as shown the insects do to the Articulata, or the osseous fishes do to the illustration.