MOLLUSCOUS AN1MAL3. 



579 



within the tissue of the mantle, the animals 

 are called naked mollusca. The common 

 field-snail, (Limax agrestis, pi. 24, f. 5.) and 

 the black-snail, (L. ater, pi. 27, f. 25.) are 

 examples of animals of this kind. More fre- 

 quently however, this shelly matter becomes 

 so greatly developed that the animal, in a 

 state of repose, can contract itself within it ; 

 it is then termed a shell, or a testaceous ani- 

 mal, which is exemplified by the common 

 garden snail-shell. (Helix aspersa, pi. 24, f. 

 1 5.) The epidermis which covers these ani- 

 mals, is very thin and sometimes dessicated, 

 or of a woolly texture- 



The variety of forms, surface, colour, bril- 

 liancy, and substance isalinost infinite. They 

 are nearly all calcareous, although some are 

 simply of a horny consistence ; but in both 

 cases they consist of matter deposited in lay- 

 ers, or exuded from the skin under the epider- 

 mis, in the same manner as the hardened 

 mucous covering, nails, hairs, horns, scales, 

 and even teeth. The shelly tissue differs ac- 

 cording as its transudation is deposited, either 

 in parallel laminae, or in close set vertical fila- 

 ments. 



Every mode of mastication and deglutition 

 is illustrated in the mollusca; for we find their 

 stomachs simple, complicated, and frequently 

 provided with a peculiar armature ; the ali- 

 mentary canal is variously prolonged. Most 

 species have salivary glands, and always a 

 liver, but neither pancreas nor mesentery. 

 Several have peculiar secretions. 



Several of the mollusca are bisexual. Some 

 produce their young alive, while others are 

 oviparous. The eggs in some are covered by 

 a shelly envelop, and others only by a simple 

 viscosity. 



The turbinated mollusca appear to be ani- 

 mals in a slight state of development; they 

 possess but little industry, and are only pre- 

 served by their great fecundity, and extreme 

 tenacity of life. They are all oviparous; their 

 reproduction consequently dependingon sexual 

 impregnation. 



Some genera of mollusca inhabit the sea 

 and fresh waters, while others are entirely 

 terrestrial: and a few species are amphibious, 

 as may be exemplified in the succinea, amphi- 

 bia, pi. 3\,f. 75. 



They feed on all substances, both animal 

 and vegetable; some will eat these even in a 

 putrid state, while others will only consume 

 them fresh. 



The uses of this numerous class are ex- 

 tremely varied. Many of them are taken as 

 food by man, and others supply nutritious prey 

 for birds and fishes. Their shelly coverings 

 are converted into many useful articles of com- 

 merce ; and from the veins of certain species 

 of shells called purpura, is extracted the beau- 



tiful dye of the ancients called Tyrian purple. 

 The P. lapillus, which is common on the Bri- 

 tish shores, adhering to stones, yields this dye ; 

 it is represented in pi. 24, f. 79. For a des- 

 cription and figures of the different parts of 

 testaceous mollusca, see the note on Concho- 

 logy, in this volume, p. 350 358. 



CLASS I. 



CEPHALOUS MOLLUSCA, OK SOFT ANIMALS WITH A HEAD, 



Lamarck divides the turbinated testaceous mollusca 

 into five orders, depending upon the complication of 

 their internal organization. These are first: 



ORDER I. HETEROPODA. 



Destitute of arms arranged around the head; unpro- 

 vided with a foot under the belly or throat for locomo- 

 tion, but furnished with one or more irregularly set 

 fins. 



The animals of this order are distinguished from all 

 the other mollusca by the form of their foot, which, in 

 place of a horizontal disk, is compressed into a circular 

 lamina, and is used as a fin, and on the edge of which, 

 in several species, is a dilatation forming a hollow cone, 

 representing the disk of the other orders. Their bran- 

 chiae are situated on the posterior part of the back, and 

 composed of plumiform lobes, directed forward; imme- 

 diately behind these are the heart, a small liver, with 

 part of the viscera and internal organs of generation. 

 Their bodies are of a gelatinous, transparent substance, 

 lined with a muscular layer, elongated, and terminated 

 by a compressed tail, in most species. There is a mus- 

 cular mass belonging to the mouth, and the tongue is 

 provided with little hooks; the oesophagus is very long; 

 the stomach thin; on the right side of the visceral mass 

 are two prominent tubes affording a passage for the 

 faeces, ova, &c. They generally swim on their back, 

 with the foot upwards. They possess the property of 

 distending their body by filling it with water, in a way 

 which is not yet properly understood. Carnaria Gym- 

 bium, pi. 31, f. 1. 



ORDER II. CEPHALOPODA. 



Lower part of the body enveloped in a bag-shaped 

 mantle, with the head issuing from the sac, and having 

 inarticulated arms, to which are attached cups, sur- 

 rounding the mouth of the animal, which is furnished 

 with two horny mandibles. The mantle unites under 

 the body; in several species the sides are extended into 

 fleshy fins: head projecting from the sac, is provided 

 with two large eyes, and crowned with longer or shorter 

 fleshy arms or feet, capable of motion in all directions, 

 their surface provided with cup-shaped suckers, by 

 means of which they can adhere firmly to other bodies; 

 these feet are used for prehension, natation, and walk- 

 ing. They swim with the head behind, and crawl in 

 all directions, with the head under the body. At the 

 opening of the sac in front of the neck, is placed a fleshy 

 funnel as a passage to the excretions. See the note on 

 Conchology, p. 350 358. 



Division I. CepJtalopoda Sepiaria. 

 Loligo vulyaris. Medium cuttle-fish, pi. 27, f. 6. 

 Division II. Cephalopoda Monotlialama. 



Shell unilocular, entirely external, and enveloping 

 the animal. Arqonauta Argo. Paper Nautilus, pi. 

 31, f. 2. and pi. 24, f. 1. 



Division III. Cephalopoda Polylhalama. 

 FAMILY i. AMMONACEA. 



Septa sinuous, lobed and cut at the margin, meeting 

 together upon the inner wall of the shell, and articu- 

 lated by jagged sutures. Shell multilocular, partly or 



