82 ZOOLOGY SECT. 



the gall-bladder (g. />.), in which the bile is stored. We thus have 

 one or more hepatic ducts conveying the bile from the liver and 

 meeting with a cystic duct from the gall-bladder, while from the 

 junction a common bile duct leads into the intestine. 



Another important and characteristic organ in the abdomen of 

 Craniata is the spleen (spl.\ a gland-like organ of variable size 

 and shape, attached to the stomach by a fold of peritoneum, but 

 having no duct. It is formed of a pulpy substance containing 

 numerous red blood-corpuscles, many of them in process of dis- 

 integration : dispersed through the pulp are masses of leucocytes 

 which multiply and pass into the veins. 



Two other ductless glands are formed in connection with the 

 enteric canal. The thyroid (tJid.) is developed as an outpushing 

 of the floor of the pharynx which becomes shut off and forms, in 

 the adult, a gland-like organ of considerable size. Its final posi- 

 tion varies considerably in the different classes. It has been com- 



/ 



pared with the endostyle of Tunicata, which, as will be remembered, 

 is an open groove on the ventral side of the pharynx. 



The thymus is developed from the epithelium of the dorsal ends 

 of the gill-clefts : in the adult it may take the form of a number 

 of separate gland-like bodies lying above the gills, or may be 

 situated in the neck or even in the thorax. The functions of both 

 thyroid and thymus are very imperfectly understood. 



The whole intra-abdominal portion of the enteric canal as well 

 as the liver, pancreas, spleen, and indeed all the abdominal viscera, 

 are supported by folds of peritoneum, called by the general name 

 of mesentery (Fig. 715, C, mes,) and having the usual relation to the 

 parietal and visceral layers of peritoneum. 



Two kinds of respiratory organs are found in Craniata : 

 water-breathing organs or gills, and air-breathing organs or lungs. 



Gills arise as a series of paired pouches of the pharynx which 

 extend outwards or towards the surface of the body and finally 

 open on the exterior by the gill-slits already noticed. Each 

 gill-pouch thus communicates with the pharynx by an internal 

 (Fig. 715, B, i. br. a.), with the outside water by an external bran- 

 chial aperture (e. br. a), and is separated from its predecessor and 

 from its successor in the series by stout fibrous partitions, the 

 inter-branchial septa (Fig. 726, i. br. s). The mucous membrane 

 forming the anterior and posterior walls of the pouches is raised 

 up into a number of horizontal ridges, the branchial filaments 

 (br.f.), which are abundantly supplied with blood. A current of 

 water entering at the mouth passes into the pharynx, thence by 

 the internal gill-slits into the gill-pouches, and finally makes its 

 way out by the external gill-slits, bathing the branchial filaments 

 as it goes. The exchange of carbonic acid for oxygen takes place 

 in the blood-vessels of the branchial filaments, which are, therefore, 



