(See CRUSTACEA.) The THIRD, or CYCLO-GANGLIATED or mol luscous DIVISION of the animal kingdom, com prehends five classes, viz. :— 14. Tunicata, soft, aquatic, acephalous animals, breathing by internal branchiee, never in form of four pectinated laminae, and covered by a close external elastic tunic furnished with at least two apertures. The exterior tunic is lined by a muscular coat; sympathetic ganglia are observed in the abdominal cavity, and the respiratory organs are ciliated as in higher molluscous classes for the production of the respiratory currents. The mouth, unprovided with tentacula or other organs of sense, opens at the bottom of the abdominal cavity,as seen in the cynthia dione. (Fig. 39. a.) The short cesopha gus leads to a capacious stomach (b), sometimes surrounded by the lobes of a small liver, which pours its secretion into that cavity as in higher molluscs. From the stomach a short wide convoluted intestine proceeds to near the veu tral orifice (d) of the sac, where it terminates in the anus (c). The thoracic orifice (e), or the entrance to the respiratory cavity, is generally provided with numerous delicate tentacula ( f) and a nervous longitudinal filament (h) is ge nerally observed to encompass that opening, and to terminate in a small glanglion (g). These ani mals are entirely marine, most are fixed, some are free ; they are all female, like the conchifera ; the circulation is aided by a muscular heart. Many are organically connected in groups, others are isolated, (See TUNICATA.) 15. Conchifera, acephalous, aquatic ani mals, covered with a solid calcareous shell, consisting of at least two pieces, and breathing by internal branchiae in form of four pectinated laminae. These bivalved animals have the mouth, as in the former class, situated at the bottom of the respiratory or thoracic cavity; the stomach is surrounded and perforated by the lobes of the liver; the circulation is aided by a bifid or a divided auricle and by a mus cular ventricle, which is generally perforated by the rectum, as seen in the annexed figure of the organs of the spondylus, (fig. 40.) The two fimbriated lips (a) which surround the mouth longed laterally into four tapering flat pec tinated tentacular ex pansions (b). The stomach (0 and the intestine are sur rounded by the large mass of the liver (i), and the rectum, near the adductor muscle (m), penetrates the ventricle of the heart (d), at some distance from the anus (e). The branchial veins (g, h) return the aerated blood to the two lateral divisions of the auricle, these pour it into the ventri cle, by which it is pro pelled forwards and backwards through the system, so that the heart is here, as in other invertebrated classes, a systemic organ.
(See CONCHIFERA.) 16. Gasteropoda, body invertebrate and in articulate, provided with a head which for the most part supports tentacula and simple eyes, and furnished with a muscular foot, extended under the abdomen, and adapted for creeping. These animals are sometimes naked, more generally covered with a univalve, unilocular, solid, external shell. Some gasteropods breathe by a pulmonary cavity, most by branchim va riously disposed on the surface or under an open mantle. Most are marine, many inhabit fresh waters, and some reside on land. The higher forms are mostly carnivorous, and the lower orders phytophagous, and this difference affects principally their alimentary apparatus, as seen by comparing that of the carnivorous buccinum undatum, (fig. 41,) with the same apparatus in the phytophagous patella vulgata, (fig. 42.) Like most of the predaceous gas teropods the buccinum is provided with a long muscular proboscis, (fig. 41, a, b,) capable of being extended to a distance from the mouth, and enclosing a bifid tongue covered with sharp recurved teeth. The oesophagus near the sto mach dilates into a small crop (c), and to this succeeds a round membranous stomach (d, e). The Whole remaining intestine is shorter than the (Esophagus, and dilates into a wide colon (f,) before terminating in the anus (g), on the right side of the body under the open mantle. The liver, of great size, and accompanying the testicle (i) in the turns of the spire, pours its secretion into the stomach as in the acepha lous classes. The vas deferens following the right side of the body terminates at the end of the male organ (h) in a small tubular styliform duct. In the patella, (fig. 42,) however, which feeds on marine plants, the mouth (a) is provided with a long slender convoluted tongue covered with numerous rows of teeth like a long file. The wide and sacculated oesophagus (d) leads to a capacious and lengthened stomach (f, g), surrounded by the large liver, and the long convoluted intestinal canal (h) makes several turns imbedded in the mass of the liver before it arrives at the short dilated rectum (i) and anus (k). The salivary glands are generally of great size in this class, and present some times in the same species both the simple follicular and the conglome rate forms. The pancreas likewise is often present in form of a single follicle opening into the sto mach along with the biliary ducts. 'The inferior orders are mostly male and female, but in the higher forms the sexes are distinct. (See