COMPARATIVE ANATOMY 183 there are remarkable exceptions; in frugiv- orotis mice they are only three times the length of the body, while in the carnivorous seal they are twenty-eight times, the anomaly being prob- ably explicable by the greater development of the ca3cum in the former. In the elephant the total length is about 60 feet, in the camel 130, in man from 25 to 30; the intestine is 10 times the length of the body in the horse, and 28 times in the sheep; in an ornithorhyn- chus 17^ inches long, the intestinal canal was 5f feet, ending hi a cloaca as in birds. The liver is large in cetacea, smaller in herbivora, and least in carnivora. There seems to be no general law for the presence or absence of the gall bladder ; it is said that all the mammalia in which it is absent, except the porpoise, are vegetable-feeders. The spleen, pancreas, peri- toneum, and other appendages of the digestive cavity, are much like those of man, from which they differ principally in shape ; the biliary and pancreatic fluids are received into the duode- num. Absorption is effected in the lowest animals by simple membranous surfaces, with- out the aid of vessels or tubes ; the latter are gradually added, and in man and the higher animals like cylindrical processes in immense numbers are developed in the intestinal mucous membrane, the mlli, by which the nutrient materials are absorbed and conveyed into the circulation. In all vertebrates there are special vessels, the lacteals, in the coats of the intes- tine, for the absorption of chyle, which convey it to the thoracic duct, from which it is poured into the venous blood near the heart ; in fishes they are destitute of complete valves, which exist in reptiles, and in both these classes their convoluted arrangement supplies the place of glands; the fluid contained is limpid in the fish and milky in the reptile ; in birds, glands appear in connection with the lymphatics, but not with the lacteals, and the valves are more numerous and distinct ; in mammals the chyle assumes more of the characters of blood, the valves are increased in number, the glands are numerous in the mesentery, and the thoracic duct becomes a distinct vessel, occasionally double. External Covering. The skin of fishes is generally covered with scales, varying from rough grains to large flattened tubercles or thick plates, and thin lamella overlapping each other like the tiles on a roof; they are held by folds of the skin, very much like the nails, which they resemble except in being more cal- careous. Carpenter considers the scales of fishes as developed in the dermis, and those of rep- tiles as mere epidermic appendages, like nails, hoofs, feathers, and hairs. They are orna- mented with the most beautiful and varied colors, presenting all the metallic reflections. The most recent chemical researches have es- tablished that fish scales have the constitution and many of the peculiarities of structure and growth of bone ; the arrangement of the la- cunas and their forms are of importance in studying the resemblances of allied fishes; they are composed chiefly of phosphate of lime, and contain a little magnesia and fluorine. The order of development of the scales in osseous fishes is not well ascertained ; but in young garpikes (lepidosteus), according to Agassiz, a row of scales is first formed along the middle line of the body ; as the age ad- vances, other rows appear above and below the median line, and the scales are crowded to- gether and of a rhomboidal form toward the tail; the same is true of the sturgeon and other ganoid fishes. From the study of fossil species, Agassiz was led to recognize and to classify fishes by the structure of the scales ; he found that all living species resembling the ancient types had scales of a peculiar structure. A common scale is composed of successive layers of horny or bony matter, the oldest layer being the lowest; over this bony layer in fossil fishes is a covering of enamel-like substance as hard as that of the teeth ; by this character the affinities of many modern species have been determined. The sharks and rays have scales which consist only of enamel, forming rough points, known in the former as shagreen ; these he called placoids. Finding singular coincidences between the structure of scales and the general form and internal organization, he united the sharks and rays into one order, characterized by a carti- laginous skeleton, by a separation of the ver- tebral column from any upper or lower ap- pendages, by teeth without sockets, attached to the jaw by skin and movable, and by the gills being covered only by strips of skin, form- ing as many openings on the sides or under part of the head as there are gills. The fishes whose scales are covered by enamel he called ganoids, embracing among living species the sturgeon and the garpike of North America, and among fossils some of the most remark- able forms, well described in the works of Hugh Miller ; the scales in this order are ex- tremely hard, and more or less smooth, and with the reptilian character of vertebra united by ball-and-socket joint. These two orders were numerous at the remote geological epoch before reptiles, and consisted of many genera, with species of great size ; the types of these ancient fishes are now reduced to a very few genera. These first created vertebrates he considers, in his "Essay on Classification," as classes of the animal kingdom distinct from fishes proper. In ordinary bony fishes he found two types of scales: in one they consisted of simple layers with regular outlines, and in the other the edges were serrated posteriorly, the serrations becoming more numerous and the surface more rough as the scales increased in size ; the former he called cycloids, the latter ctenoids. Ctenoids have bony spines in the anterior portion of the dorsal fin, and serra- tions or spines in the opercular bones; this order includes the perch family, and others with spiny dorsals and rough scales ; the flat fishes (pleuronectes), characterized by their
