Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/673

655 AND DIGESTION.] have the margins denticulated or serrated. Compressed teeth may be confluent, and form a cutting edge in both jaws, which assume the shape of a parrot s beak. In some the apex is hooked or provided with barbs. Again, some teeth are broad, with flat or convex surface, like molar teeth. With regard to size, the finest teeth are like fine flexible bristles, ciliifonn or setiform ; or, if very short and anchylosed to the bone, they appear only as inconspic uous asperities of the bone. Very fine conical teeth arranged in a band are termed villiform teeth ; when they are coarser, or mixed with coarser teeth, they are cardlike (dents en r&pe or en cardes) ; molar-like teeth of very small size are termed granular. In all fishes the teeth are shed and renewed during the whole course of their life. In fishes which have com pound teeth, as the Dipnoi, Chimaeroids, Scari, Gymno- donts, as well as in those which have apparently perma nent teeth, as in the saw of Pristis, the detrition of the surface is made up by a constant growth of the tooth from its base. When the teeth are implanted in alveoli, they are generally succeeded by others in the vertical direction; but in other forms they succeed one another side by side. In the majority of fishes the new tooth is not developed (as in reptiles and mammals) in a diverticulum of the sac of its predecessor, but, like it, from the free surface of the buc- cal membrane. Generally there are more than one tooth growing which are in various stages of development, des tined each to replace the others in function. This is very conspicuous in sharks, in which the whole phalanx of their numerous teeth is ever marching slowly forwards (or in some backwards), in rotatory progress, over the alveolar border of the jaw, the teeth being successively cast off after having reached the outer margin and fulfilled for a longer or shorter period their special function. 1 Intestines. The intestinal tract is divided into four portions, the oesophagus, the stomach, the small and the- large intestine ; two or more of these divisions may coalesce in fishes and become indistinguishable. But it is char acteristic of the class that the urinary apertures are con stantly situated behind the termination of the intestinal tract. In Branchiostoma the whole intestinal tract is straight, and coated with a ciliated mucous membrane. The liver is represented by a green-coloured coecal diverticulum of the stomachic dilatation. In the Cydostomi the intestinal tract is likewise straight, and without clearly defined divisions. The Pal&icJdhyes show differences in the structure of their intestinal tract as considerable as are found among the Teleostei, but they have this in common that the absorbent surface of their intestine is enlarged by the development of a spiral valve, evidence of the presence of which in extinct Palrmchthyes is still preserved in the fossilized faeces or coprolites, so abundant in some of the older strata. In Chondropterygians (fig. 46) the btomach is divided into a cardiac and a pyloric portion, the former frequently terminating in a blind sac, and the latter varying in length. The pyloric portion is bent both at its origin and its end, and is separated from the short duodenum (called Bursa entiana in these fishes) by a valve ; the ductus hepaticus and ductus pancreaticus enter the duodenum. This is succeeded by the straight intestine, provided with the spiral valve, the coils of which may either be longitudinal and wound verti cally about the axis of the intestine, as in Carcharias, Galeocerdo, Tkalassorhimis, and Zygoma, or they may be transverse to that axis, as in the other genera, The number of gyrations in the latter case varies ; there may many as forty. The short rectum passes into a ICHTHYOLOGY 655 cloaca, which contains also the orifices of the urogenital ducts. Only the beginning and end of the intestinal tract are fixed by mesenterial folds. be The richest materials for our knowledge of the teeth of fishes are contained in Owen s Odontography ; Lond., 1840, 8va. FIG. 46. Siphonal stomach and spiral valve of Basklng-Shark (Setache). (After Home and Owen.) a, (Esophagus; b, cardiac portion of stomach; c, pyloric portion ; d, pouch intermediate between stomach and duodenum, with circular valves at both ends ; e, duodenum ; /, valve of intestine ; &amp;lt;/, ductus hepaticus; h, spleen. The structure of the intestinal tract of Teleosteous fishes is subject to so numerous modifications that we should go beyond the limits of the present article were we to attempt to enter into details. Great differences in this respect may be found even in groups of the same natural families. Frequently the intestinal tract remains of nearly the same width throughout its course, and only the entrance of the various ducts serves as a guide for the distinction of its divisions. An intestine of such uniform width may be straight and short, as in Scomlresocidce, Symbranchida 1, or it may be more or less convoluted and long, as in many Cyprinidce, Doradina, &c. On the whole, carnivorous fishes have a much shorter and simpler intestinal tract than the herbivorous. In the majority of Teleosteans, however, the oesophagus, stomach, duodenum, small intestine, and rectum can be more or less clearly distinguished, even extern illy. There are two predominant forms of the stomach, inter mediate forms, however, being numerous. In the first, the siphonal, it presents the form of a bent tube or canal, one-half of the horse-shoe being the cardiac, the other the pyloric portion. In the second, the csecal, the cardiac division is prolonged into a long descending blind sac, the cardiac and pyloric openings of the stomach lying close together (Chipea, Scomber, Thynnus, &c.). The duodenum always receives the hepatic and pancreatic secretions, and also those of the appendices pyloricse, which, in varying numbers (from 1 to 200), are of very common occurrence in Teleosteans. They vary also in length and width, and whilst the narrowest serve only as secretory organs, the widest are frequently found filled with the same contents as the intestine. Glands. The liver of fishes is distinguished by the great quantity of fluid fat (oil) which it contains. The gall-bladder is but rarely absent; it is attached to the right lobe, or towards the centre ; in some fishes, however, it is detached from the liver and connected with it by the cystic duct only. The bile may be conveyed by one or more hepatic ducts into a common duct which is continued towards the gall-bladder as ductus cysticus, and towards the duodenum as ductus choledochus ; or some of the hepatic ducts enter the gall-bladder directly, or the duodenum directly, without communicating with the common duct. Individual varia tions in this respect are of common occurrence.