XXIX GILLS 383 



proteids into peptones, starch into sugar, and breaking up 

 fats. Opening into the cloaca is a small finger-like rectal 

 gland {ret. gl), the function of which is uncertain. 



In addition to these glands the inner surface of the 

 stomach and intestine is dotted all over with microscopic 

 apertures, leading into minute tubular glands sunk in the 

 mucous membrane. These are the gastric and intestinal 

 glands : the former secrete gastric juice, which digests pro- 

 teids ; the latter intestinal juice, which probably acts upon all 

 classes of food. Thus as compared with the animals pre- 

 viously studied, the dogfish, in common with other Verte- 

 brates, shows an extraordinary differentiation of digestive 

 glands and fluids. 



There is another characteristic vertebrate organ in close 

 connection with the enteric canal and called the s/>leen 

 (spl). It is an irregular dark-red, gland-like body, of con- 

 siderable size, attached by peritoneum to the stomach. It 

 has no duct, and its chief function is probably the manufac- 

 ture of leucocytes and the disposal of worn-out red blood 

 corpuscles. Other ductless glands are the thyroid in the 

 throat ; the thymus in connection with the dorsal ends of 

 the branchial arches ; and the supra- and inter-renal bodies in 

 relation with the kidneys. 



The respiratory organs or gills consist of five pairs of 

 pouches, each opening by one of the internal branchial 

 apertures (Figs. 99, .\ and b. Int. br. ap and 103) into the 

 pharynx, and by one of the external branchial apertures 

 {Ext. br. ap) on the exterior. The walls of the pouches are 

 supported by the visceral arches {Br. A) and branchial rays 

 {Br. R, Br. R'), and are lined with mucous membrane 

 raised into horizontal ridges, the branchial filaments {Br. Fit), 

 which are abundantly supplied with blood-vessels, and are 

 the actual organs of respiration. As the fish swims, water 



