GASTEROPODA.) 17. Pteropoda, body organized for swim ming, mantle closed above, branchice external, no muscular foot for creeping, shell, when present, always thin, pellucid, unilocular, and inopercu!ate. These soft, minute, floating ani mals are all marine, and are enabled to swim by means of two lateral musculo-cutaneous fin like expansions, on the surface' of which the respiratory branchim or vascular plexuses are placed. These lateral fins are never supported by rays. The head is generally provided with retractile or sheathed tentacula, seldom with eyes. The body is sometimes entirely naked, often protected by a delicate thin transparent shell,which encloses the abdomen and is covered with a fold of the skin. They appear to be most closely allied to the inferior testaceous cepha lopods in the nature and form of their shells and in their locomotive powers, and also in the general simplicity of their internal strut,. ture, especially of their generative organs. The structure of one of the naked pteropods, clip borealis, is represented in fig. 43, where the abdominal cavity is exposed by the mantle being opened from behind. The mouth (a) leads to a long (b), which is sur rounded by a circular series of nervous gan glia (t). The stomach (c c) is imbedded in the lobes of the liver (g), which open by numerous short ducts into its cavity. The oesophagus is accompanied by the two long simple salivary follicles (k), and at the left or pyloric extremity (d) of the stomach is placed the heart (i), en closed in its pericardium, which receives the arterialized blood from the branchial veins, and sends it through the system. The bottom of the abdomen or cavity of the mantle (h) is occupied as in the cephalopods with the gene rative organs, which consist of an ovary (1) and long oviduct (m, o), into which a short wide ccecum (n), commonly considered as a testicle, pours its secretion. The oviduct termi nates on the left side, near the anus (e), in a small glandular sac (q), beneath which is the rhenal sac (p). The pteropods are commonly found floating in immense numbers at the sur face of the water in still warm evenings in tro pical seas; some, as the clip borealis, figured above, abound in the Arctic seas. (See PTE ROPODA.) 18. Ccphalopoda, free cyclo-gangliated or mulluscous animals, with the feet disposed around the head, respiring by internal branchiw, and with the abdominal cavity enveloped by a muscular mantle open anteriorly. The cepha lopods are all marine animals capable of swim ming by means of membranous or muscular expansions, which are never supported by rays. The surface of the body is often naked, some times covered with a shell, which is generally po lythalamous, rarely monothalamous, and always inoperculate. There is often a concealed, loose, dorsal, calcareous or horny shell contained in a shut subcutaneous sac. The mouth is fur nished with two horny or calcified mandibles, and the rudiments of an internal organized cartilaginous cranium and vertebral column are generally perceptible, together with some de tached parts of the skeleton of vertebrata. The cesophagus is surrounded by a nervous collar, from which two supra-abdominal nervous co lunins generally extend along the middle of the back, and sympathetic ganglia are observed in the abdominal cavity as in the inferior mollus cous classes. These are predaceous animals, and the alimentary canal, though generally furnished with three enlargements, forming a crop, a gizzard, and a spiral or proper chylific stomach, is always very short. There are two pairs of salivary glands ; the liver is of great size, and pours its secretion, with that of the pancreatic follicles, into the stomach, as in the inferior classes. There is always a strong mus cular systemic ventricle, and generally a di vided auricle placed at the beginning of the branchial arteries. The common form of the chylopoietic organs is seen in those of the /o/izopsis 2uttata, (fia. 44,) where the liver (a a a a) pours its se cretion by ducts (b), which are surrounded and pene trated by the pancrea tic follicles (c c), and which unite into a single canal before they open by a valvular aperture into the third or chylific stomach (fg). The crop (d) ends in the strong muscular giz zard (e), and from the third stomach (f g) the short intestine (h) ascends in front of the liver to terminate by a valvular anus at the base of the funnel. The naked species have a glandu lar sac for secreting a black inky matter, which appears to be wanting in those protected by an external shell, excepting in the argonauta, where the shell is seen in the ovum, and where there is a slight membranous connexion be tween the animal and its thin delicate calca reous covering. The sexes are generally sepa rate, but the lowest foraminiferous cephalo pods appear to approach to the pteropods in the male and female character of the genital organs. (See CEPIIALOPODA.) The last or highest DIVISION of the animal kingdom, comprehending the vertebrated or red-blooded animals, or SPIN I-CEREBRATA, con sists of five distinct classes, characterised chiefly by their generative, their sanguiferous, and their tegumentary organs, viz.— 19. Pisces, cold and red-blooded oviparous vertebrated animals, with one auricle and one ventricle to the heart, breathing by permanent branchi, and with fins for progressive motion. They have a vertebral column and cranium, enclosing a spinal cord, and brain consisting of a medulla oblongata, optic lobes, cerebral hemispheres, olfactory tubercles, and a cere bellum. The hands and feet are always formed like fins for progressive motion in a watery element. The fins are supported by rays pro longed from the skeleton, the body is generally covered with scales, the trunk is organized for the lateral motion of the tail, there is DO sacrum, and the pelvic arch is unconnected with the vertebral column. The bones are elastic or cartilaginous, and the centres of ossification for the most part remain perma nently detached. The bodies of the vertebrae terminate in two cup-like cavities, they move on elastic tense intervertebral sacs, and the transverse processeS are directed vertically downwards in the coccygeal region of the skeleton to facilitate the lateral motion of the trunk. The muscles, of a white colour, are disposed in oblique strata on the sides of the trunk for the movement of the elastic vertebral column. The mouth, destitute of salivary glands, is generally furnished with numerous unequal, irregular, fangless, osseous teeth, and the wide cesophagus, short like the neck, leads to a capacious stomach, from which the in testine, shorter than in the higher classes, and nearly equal throughout, proceeds, without ccecal enlargement, to terminate in a cloacal sac on the inferior surface of the trunk. The liver is large, and pours its secretion generally by a single duct into the duodenum, near the pyloric extremity of the stomach and close to the opening of the pancreatic duct, as shown in the annexed figures of these parts in the frog-fish (fig. 45, A) and the cod (fig. 45, B). (a) of the frog-fish (fig 45, A) leads to a large globular stomach (c) with a strong muscular cardiac sphincter (b). The pyloric extremity is also surrounded with strong muscular bands (d), and beyond its pyloric valve two pancreatic simple glandular follicles (e e) open into the duodenum (g) close to the opening of the ductus communis chole dochus (f ). In the cod (fig. 45, B) the wide cesophagus (a) leads to a long and capacious muscular stomach shut below, and immediately beyond the pyloric valve, formed by a circular fold of the mucous coat, open the ducts of numerous straight and simple pancreatic folli cies (e c) along with the ductus communis choledochus (I). The cartilaginous plagi
ostome fishes, the most complicated of this class, have a conglomerate form of the pan creas opening in the same situation. In the sturgeon and in the sword-fish an interme diate form is seen between the simple pan creatic follicles of the invertebrated classes and the more complicated conglomerate organ in the higher vertebrata. This is shown in the annexed figure of the chylopoietic viscera as I found them in the xiphias gladius (fig. 46), where the liver (a) is raised up to show the three hepatic ducts uniting with the cystic from the curved gall-bladder (c) to form a very short ductus communis choledochus. The pancreas (d) forms a large reniform mass com posed of numerous straight follicles produced by the successive divisions of the great termi nal duct (e) of this organ. This large inter mediate organ is surrounded with a distinct muscular tunic to force its contents into the duodenum immediately beyond the pyloric • valve (b). The tortuous small intestine ends by a valvular orifice (f) in a very short but distinct colon, which presents no ccecum in its course to the anus (g). The bilocular heart of fishes is entirely branchial ; it is often pre ceded by a sinus venosus, and is always succeeded by a bulbul arteriosus, which often presents numerous internal valves in its course. The venous blood is entirely sent through the gills, and the branchial veins, after giving branches to the anterior parts, unite to form the aorta which sends the arterialised blood through the rest of the system without the aid of a systemic heart. The respiration is effected by the transmission of water through the mouth or through distinct spiracula, and over the surface of the branchia, which are internal in the adult, and are often preceded by external branchke in the young. The lungs are always rudimentary, when present, sometimes in form of a shut single air-bag, sometimes divided or ramified, and most generally communicating by a ductus pneumaticus with the intestine or stomach, or oesophagus, but seldom employed for respiration. Fishes are oviparous and have the sexes separate ; the ovaries are continuous with the oviducts in osseous fishes, and de tached from them in the plagiostome chon dropterygii, and impregnation sometimes takes place internally and sometimes after the ova are separated from the body. (See PiscEs.) 20. Amphibia, cold and red-blooded, verte brated, oviparous animals, with three cavities of the heart, with a naked skin, and breathing, in the young state, by gills. These animals com mence their career like fishes with one auricle and one ventricle, which send the whole of the blood through the branchia, and they have at this period also double concave bodies of the vertebra, as in fishes. Many retain the gills through life, accompanied with pulmonic cavi, ties, from which the arterialised blood is sent to a small left auricle. These animals are termed amphibia from the metamorphosis to a terres trial from an aquatic life seen in most of the species. Their skeleton is imperfectly con solidated, their ribs very short or wanting, their pelvic arch free or nearly so, and their atlantal and sacral extremities often very imperfectly developed or partly deficient. Their toes are destitute of claws, as their skin is of scales, and the respiration through their naked, highly sensitive, and secreting surface compensates for the imperfect development or limited use of their lungs, especially during submersion or hybernation. Some reside constantly in the water, others occasionally, and others continue on land. The male organ of intromission is rarely developed, and impregnation of the ova is generally effected externally. The genital organs are double and symmetrically developed in both sexes. The perennibranchiate amphi bia, especially the axolotl, have been shown by Weber to possess a double auricle like the caducibranchiate species. (See AMP11 IBIA.) 21. Reptilia, cold and red-blooded, ovipa rous, vertebrated animals, with two auricles and one ventricle, not breathing by gills in their young state, covered with scales, and with the means of internal impregnation. These animals, whether aquatic or terrestrial, breathe only by means of lungs, and their pulmonic respiration and the left auricle of the heart are 2 greater than in the amphibia. Their bones are more consolidated than in the lower vertebrate, their pel vic arch, when developed, is more firmly attached to the vertebral column, the centres of oss,fication, especially of the cranial bones, generally remain detached, the extremities are for the most part more competely developed, and the toes are generally provided with claws. Their cerebellum is remarkably small, their muscular irritability languid, and they have great tenacity of life. This ventricle, which receives both the venous and arterialised blood, is more or less divided by an ascending imperfect septum. The thoracic and abdominal cavities are not separated by a muscular diaphragm, and the lungs extend backwards over the abdominal viscera. Their organs of generation are double in both sexes, and symmetrically developed on the two sides of the body. The two portions of the corpus are often detached and bifid ; the chorion of the ova is generally thin or coriaceous, seldom calcified or hard, and the instincts of the parent generally extend to the protection of the young. (See REPTILIA.) 22. Ayes, warm and red-blooded, ovipa rous, vertebrated animals, with four cavities of the heart, covered with feathers, and with their arms organized for flight. Their bones are the most compact and dense in texture, the most extensively anchylosed, and generally contain air admitted from the cells of the lungs. Their tympanic bone is moveable, they have horny mandibles in place of teeth, their coracoid bones reach the sternum, the sternal ribs are ossified, and they want the tarsal bones. Their diaphragm never forms a complete partition between the thoracic and abdominal cavities. The hemispheres of the brain are without con volutions, the optic lobes are large and hollow, the cerebellum is large and sulcated, and the posterior enlargement of the spinal chord of great size. The great irritability of their mus cular system corresponds with the great extent of their respiration, the high development of their nervous system, the rapidity of their -cir culation, and the increased energy of all their functions. Their alimentary canal is furnished with a crop, a glandular infundibulum, a giz zard, and generally with two cceca-coli, as seen in the annexed diagram (fig. 47), showing the common form of these parts in a gallinaceous bird. In these gallinaceous birds the oesopha gus (a) sends out at a right angle with its course a large crop (b), with a contracted neck, and supplied with glandular follicles. Beneath this is the infundibulum or glandular stomach (c), with numerous large follicles placed between the mucous and muscular coats, and this opens into the large muscular gizzard (d), provided externally with two strong digastric muscles (e). The cardiac and py loric orifices of the gizzard are close to each other (f), and towards the lower part of the small intestine a minute ccecum often indi cates the original entrance of the yolk-bag. The two long cceca-coli (g) commence by nar row entrances (h), and the short colon ends in a common cloaca (/) for the genital and urinary secretions.