310 COMPARATIVE ANATOMY 



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. 10). The region where one 

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

 the mesentery?- and this serves not only to support the alimentary 

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

 vessels, lymph-vessels, and nerves. With the lengthening of the 

 alimentary canal during development, the mesentery may give rise 

 to a more or less complicated system of folds in which the viscera 

 are enveloped. 



The most anterior section of the primitive alimentary tract 

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

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

 other, are developed in the embryo from the mucous membrane 

 and eventually unite with the ectoderm, apertures being formed 

 to the exterior (Fig. 226, A). In the septa between the channels 

 thus formed, the visceral arches are situated (cf. Fig. 63), and 

 along the septa certain vessels arise by means of which a continual 

 interchange of gases can take place between the blaod and the 

 air contained in the water passing through the sacs. In this 

 manner the gills or branchiae arise. Even in the Amniofca, 

 although gills are not developed, that portion of the cavity of 

 the mouth and pharynx which lies behind the internal nostrils 

 serves as a common passage for air and food unless a proper 

 palate is formed (Fig. 226, B, c). 



With the formation of a secondary palate (p. 132), the primi- 

 tive mouth-cavity becomes divided into an upper respiratory, 

 and a lower nutritive portion that is, into a nasal and a secondary 

 or definitive mouth- cavity. The separation, however, is never a 

 complete one, the passage being common to both cavities for a 

 certain region (pharynx), which in Mammals is partially separated 

 from the mouth by a muscular fold, the velum palati, or free edge 

 of the soft palate (Fig. 226, D). 2 



The alimentary canal of Vertebrates is typically divisible into 

 the following principal sections (Fig. 227) : mouth- or oral-cavity, 

 pharynx, guild or ORSOphagu^ stomach, and intestine, the last men- 

 tioned being usually differentiated into a small and a large 

 intestine. The small intestine is in most cases the longest section 

 of the alimentary tract ; the bile and pancreatic ducts open into 

 its anterior portion (duodenum). 



The large intestine communicates with a cloaca, which also 

 receives the urinary and genital ducts, or it may open independently 

 to the exterior. The small intestine may be further differentiated 



1 In Muraenoids, Dipnoans and Lepidosteus, a ventral mesentery is also present, 

 but in Lepidosteus it only extends for a short distance along the hinder part of 

 the gut. 



2 A membranous velum palati exists in Crocodiles. A median, finger-like 

 process of the soft palate, the uvula, is well developed only in Man and some 

 Apes. 



