236 COMPARATIVE ANATOMY 



free surface by pavement epithelium, and, dorsally to the alimentary 

 canal, is reflected round the entire body-cavity, converting the 

 latter into a large lymph-sinus. A parietal layer, lining the body- 

 cavity, and a visceral layer, reflected over the viscera, can thus be 

 distinguished in the peritoneum (Fig. 7). The part where one 

 passes into the other, which is thus primitively double, is called 

 the mesentery} and this serves not only to support the alimentai-y 

 •canal from the dorsal body-wall, but also to conduct the vessels 

 and nerves passing from the region of the vertebral column to the 

 viscera. With the lengthening of the alimentary canal during 

 development, the mesentery may give rise to a more or less com- 

 plicated system of folds in which the viscera are enveloped. 



The most anterior section of the primitive alimentary tract 

 of the Ichthyopsida functions as a respiratory cavity as well as a 

 food-passage. A row of sac-like outgrowths, lying one behind the 

 other, are developed from the mucous membrane and eventually 

 (Unite with the ectoderm, apertures being formed to the exterior 

 (Fig. 189, A). Between the channels thus' formed, the visceral 

 arches (p. 69) are situated, and along the latter certain vessels 

 are formed by means of which a continual interchange of gases can 

 take place between the blood and the air contained in the water 

 ]")assing through the sacs. In this manner the gills or hranchice 

 {p. 273) arise. Even in the Amniota, although gills are not 

 developed, the larger portion of the cavities of the mouth and 

 pharynx lying behind the internal nostrils serves as a common air- 

 and food-passage until a proper palate (pp. 92, 202) is formed (Fig. 

 189, C). 



With the formation of a definite palate (most Amniota), the 

 primitive mouth-cavity becomes divided into an upper respiratory, 

 a,nd a lower nutritive piortion — that is, into a nasal and a sceondury 

 or definitive mouth-cavity. The separation, however, is not a com- 

 plete one, the passage being common to both cavities for a certain 

 region (Fig. 189, D). This region, in all Vertebrates, is called 

 the pharynx, and in Mammals it is partially separated from the 

 mouth by a fibrous and muscular fold, the velum palati, or free 

 edge of the soft palate? 



The alimentary canal of Vertebrates is typically divisible into 

 the following principal sections (Fig. 190) : — Mouth or oral 

 cavity, pharynx, gxdlct or asopihagu^s, stomach, small intestine, and 

 large intestine. The large intestine may communicate with a 

 cloaca, into which the urinary and genital ducts also open, or it 

 may open directly to the exterior. The small intestine may be 

 further differentiated into duodenum, jejeunum and ileum, and the 

 large intestine into colon and rectum. A blind-gut or ewcum is 



^ In Mui;unoids, ])ipno;ins, ;ind Lepidosteus, a rent ml mesentery is also present, 

 Ijnt in Lepiilosteus it only cxtuiids for a short distance along the hinder part of 

 the gut. 



- A meiubranous x'clum palati exists in Crocodiles. 



