to the intestine (Fig. 61, 62,* e), which like the oesophagus, winds upwards along the surface of the liver to terminate in d Fig. 61. Fig. 62. The stomach of Loligo vulgaris. the funnel, which is the common vent of all the excrements. The liver is very large in all the genera of this class, and must furnish a copious supply; but, besides this, and the * In reference to these figures, it may be observed that they are copied from nature; a remark which seems necessary, since they differ entirely from Sir E. Home's figure of the stomach of Loligo vulgaris, or the Sepia Loligo of Linnæus. Sir Everard's figure appears to have been taken from a species of Octopus. secretions of the other accessary organs to good digestion, Sir E. Home believes that the inky fluid is intended also to have some effect upon the lower portion of the intestinal canal, to enable this to extract from its contents " secondary kind of nourishment," an opinion not very probable in itself, and with but a few fanciful analogies in its support. *Comp. Anat. i. 369 and 393. a *** Troschel and, more particularly, M. Löven, have sought in the form and various armature of the tongue of the Cephalophorous mollusca (viz., Cephalopoda, Pteropoda, and Gasteropoda), a character on which to distinguish their natural alliances and affinities. The researches of Löven into this subject seem to have been extensive, and are very interesting; but I am only acquainted with them through the medium of an imperfect translation, with the perusal of which I was favoured by Mr. Alder of Newcastle. Hence, I cannot presume to offer the results, at which Löven has arrived, to the student. 324 LETTER XIX. THE PHYTIVOROUS MOLLUSCA. THE herbivorous, or, as they are frequently named, the phytophagous mollusca, belong exclusively to the class of Gasteropods, and embrace, with few exceptions, the Pulmones, about half of the Nudibranches, the Inferobranches, a great proportion of the Tectibranches, the Scutibranches, and Cyclobranches, and such of the Pectinibranches as have no slit or siphon in the collar of the mantle, or, which is the same thing, whose shell has an entire aperture.* The mouth of the animal is not elongated into a proboscis, but is furnished with two jaws; and the tongue is usually broad and membranous, although sometimes long and filiform. What proportion these herbivorous tribes united may bear to the zoophagous, I am not prepared to say, for we are not in possession of a complete catalogue of the species; but to make a conjectural approximation to the solution of this question, let us assume that the enumeration of the species given by Lamarck is equally defective in all the families, (and I know of no fairer mode of coming at the truth,) when we shall find that the phytivorous are fewer by a third than the zoophagous, so that Meckel errs in asserting that the great bulk of the Gasteropods live on vegetable matters: * That is to say, the families Trochoides and Capuloides of Cuvier, Règ. The marine tribes live on sea-weed, "part single, or with mate, Graze the sea-weed their pastures, and through groves and it is probable that the species in general are not limited to any particular weed, nor are even very nice in their selection. At the same time it may be observed, that some are rarely found except on one and the same plant, as, for example, the pretty Patella pellucida, which pastures almost uniformly on the broad frond of Laminaria digitata, and has shown both sense and taste in choosing so wide and tender a field. Still more provident in its way, is another common species of Patella, or, perhaps, a variety only of that just mentioned, which is usually found occupying a cavity or cell in the root of the same sea-weed. Insinuating itself amid, or under the entangled root, it eats into its very core until a cavity is made sufficiently large, where it may find a full repast ever at hand, and at the same time a snug residence, secure from almost any foe, and sheltered from every ordinary storm. I have often admired the apparent sagacity of the creature in its choice, and amid the bustle of life, have sometimes envied its quiet seclusion, which is not so strict as to exclude all society, for I have frequently seen two Patellæ dwelling together in the same "peaceful hermitage." The land tribes seem to refuse no tender herb: we know that they will eat with avidity the spring corn, clover, peas, and turnips; they are very fond of all kinds of fruit; and the mushrooms afford the slugs a grateful delicacy, even the poisonous, acrid, and milky species being greedily devoured by them.* To me, they seem to prefer all these in a fresh state; my personal observation leads certainly to this conclusion, though I am aware it has been said that they, like Anim. iii. 72 and 86.-Adanson had given the distinguishing characters between the zoophagous and phytophagous pectinibranchial mollusca very correctly. Hist. Nat. du Senegal, p. 80, 81, 193. The Phytophaga are "eminently distinguished from the carnivorous race by two characters; their mouth does not form a proboscis: and the aperture of their shell is entire,-in other words, without the notch or canal for the passage of the siphon."-SWAINSON'S Malacology, p. 56 and 158. *See Ann. Nat. Hist. ix. 73. It is stated somewhere in the Gardener's Magazine, that Helix aspersa has been seen to feed on the fiery-flavoured foliage of the Clematis flammula! Of a very acrid agaric described by Lister he says, "I observed these mushrooms even then, when they abounded with milk (not to be endured upon our tongues), to be exceeding full of flymaggots; and the youngest and tenderest of them were very much eaten by the small, gray, naked snail."-Corresp. of Ray, p. 100. modern epicures, are fonder of their food when it has advanced some way to putrefaction. Dr. Fleming says:"Those which are phytivorous, appear to prefer living vegetables, and refuse to eat those which are dried. We are not aware that putrid vegetable matter is consumed by them, although many of the snails and slugs are found under putrid leaves and decayed wood. In those places there is shelter from the sun, together with dampness; so that it is difficult to determine whether they sojourn in an agreeable dwelling or a well-stored larder."* On occasions they eat voraciously; but, when necessary, they can sustain a fast longer, perhaps, than any other animated beings; snails having been kept for upwards of a year, nay for years, and the Limnäeæ and Planorbes for many months, without any food, except that small and tenuous portion which they might extract from the air or water. The mouth in this tribe, as in other mollusca, is always anterior and terminal, with often an inferior aspect; in Doris, and Cyclostoma, and a few others, it is prolonged into a sort of snout, which can be shortened or elongated to a small extent; in Aplysia and Pleurobranchus, there are labial tentacula at the sides; and it is overshadowed in the Tritonia by a deeply crenate veil, which receives a very remarkable developement in the Tethys, of which I have already given you a figure. It is in no instance furnished with the complicated retractile proboscis of the pectinibranchial Zoophaga; but, on the contrary, we very generally find, within the lips, jaws of a cartilaginous or horny texture, fitted for dividing their food into appropriate portions. In the marine tribes there is a pair of these instruments acting horizontally; but they differ so much in size, form, and even consistence, in the different genera, that no general description could be made applicable. Usually they are merely oblong pieces of cartilage; sometimes thin reticulated plates: whereas, in Tritonia, they are composed of solid horn; and, in reference to their form, Cuvier compares them to the scissors with which sheep are shorn, the blades being large, oblong, curved, deeply emarginate behind, and partially serrulated on the upper edge, (Fig. 63, 6). The slugs and snails (Pulmonifera), whether terrestrial or aquatic, have a single jaw‡ placed on the upper side of the oral aperture; and it acts in cutting the herbage by being Edinb. Encyclop. xiv. 602. + Doris, Pleurobranchus, and the pectinibranchial Phytophaga (Littorina, Trochus, Nerita, &c.) in general are exceptions. Hook, in his Micrographia, has given a magnified view and description |