CHAP, x DIGESTIVE ORGANS 447 



tained in the coelome, to the dorsal wall of which it is 

 suspended by a median incomplete mesentery (Figs, in 

 and 117, mes) . The greater part of the canal is developed 

 from the enteron of the embryo, and is consequently 

 lined by endoderm ; the oral cavity is formed from the 

 stomodaeum, and the cloaca from the proctodaeum 

 (p. 204). Outside the enteric epithelium are connective- 

 tissue and muscular layers, the latter formed of un- 

 striped fibres : it is generally characteristic of verte- 

 brates that the voluntary muscles are striped, the 

 involuntary unstriped (compare pp. in and 112). 



The immense liver, divided into two lobes (/. Ir, r. Ir), 

 is situated below the stomach along the whole length of 

 the abdomen,, to the front wall of which it is attached by 

 a fold of peritoneum. It discharges its secretion, the 

 bile, into the anterior end of the intestine by a tube, the 

 bile-duct, which gives off a blind offshoot terminating 

 in a. gall-bladder (p. 69). 



The pancreas (pan) lies against the anterior end of the 

 intestine, with which it communicates independently by 

 the pancreatic duct. Opening into the hinder part of 

 the intestine or rectum is a small finger-like rectal gland 

 (ret. gl), the function of which is not known. In addition 

 to these there are, as in all Vertebrates, minute tubular 

 gastric glands sunk in the mucous membrane of the 

 stomach (p. 131). 



The spleen (spl) is an irregular, dark-red, gland-like 

 body, of considerable size, attached by peritoneum to 

 the stomach (compare pp. 23 and 98). 



Other so-called " ductless glands " which are also repre- 

 sented in the frog, giving rise to internal secretions the 

 functions of which are imperfectly understood are the 

 thyroid in the throat (Fig. 122) and the thymus in connection 

 with the dorsal ends of the branchial arches ; there are also 

 suprarenal and interrenal bodies in the neighbourhood of the 

 kidneys, corresponding to the adrenals of the frog (p. 145). 



F F* 



