XL] THE ALIMENTARY SYSTEM. 459 



Fishes. It may have a silvery metallic lustre, as often in the 

 last-named class. 



It may, as in Birds, form great cells which communicate 

 with the lungs, and thence become inflated with air. 



The peritoneum may be incomplete, being reduced, as it 

 were by local atrophy, to shreds and patches, as in many 

 cartilaginous Fishes, e.g. the Sturgeon. 



Instead of an omentum passing post-axially as in Mam- 

 mals, two such structures may extend pre-axially (i.e. towards 

 the head) from the oelvic region, as in many Lizards. 



FIG. 390. DIAGRAM OF A TKANSVERSE SECTION OF A LIZARD, showing the 

 stomach, spleen, lesser omentum, and falciform hepatic ligaments, all in the.r 

 typical or median position. 



(After Pittard.) 



ns, neural spine ; c, neural canal ; p, outer surface of body ; /', peritoneum lining 

 the body-cavity and investing the viscera ; a, the aorta ; s, the spleen ; g, the 

 stomach ; /t, the gall-duct ; f t the falciform ligament of the liver ; r t the pleuro- 

 peritoneal cavity. 



The whole intestine (i.e. both the great and small intes- 

 tines) may be suspended by one continuous mesentery, even 

 in man's own class, as in the Shrew and the Elephant. 



17. In the DEVELOPMENT of the alimentary system the 

 first notable modification is the splitting of the laminae ven- 

 trales, as already mentioned. When -by the ventral union of 

 the inner laminae the alimentary canal is formed, this canal 

 is at first a straight tube closed at both ends, and extending 

 along the ventral aspect of the axial skeleton. 



After a time the tube, towards the middle of the body, bends 

 away from this axis in a sharp ventral curve, while that part 

 of it on the pre-axial side (side next the head) of the curve 

 dilates and becomes the stomach. The part on the post- 

 axial side of the curve becomes the transverse colon, while 

 the intervening loop becomes greatly lengthened and exces- 

 sively convoluted to form the small intestine and ascending 

 colon, a little bud-like offshoot laying the foundation of the 

 caecum. As we have seen, this proximity of the transverse 



