86 



ZOOLOGY 



SECT. 



will be remembered, is an open groove on the ventral side of the 

 pharynx. This view is supported by the condition of the parts in 

 the larval Lamprey (see Cyclostomata). 



The ihymus 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 thymus and 

 thyroid, by virtue of internal secretions which they produce, 

 and which mingle with the blood, control and modify the 

 physiological condition of various organs and tissues with which 

 they have no immediate anatomical connection. 



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. 769, C, 

 mes.) and having the usual 

 relation to the parietal and 

 visceral layers of the peri- 

 toneum. 



i.br. 



FIG. 780. Diagrammatic horizontal section of the 

 pharyngeal region of a Craniate : on the left are 

 shown three gill-pouches (g.p.) with fixed branchial 

 filaments (br. f.) and separated by inter-branchial 

 septa (i. br. s.) ; on the right one hemibranch (hm. 

 br,) and two holobranchs (hi. br.) with free fila- 

 ments, covered by an operculum (op). Ectoderm 

 dotted, endoderm striated, mesoclerm evenly 

 shaded, visceral bars (?. b.) black; ph, pharynx. 



Two kinds of respira- 

 tory organs are found 

 in Craniata : water-breath- 

 ing 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 ex- 

 terior by the gill-slits 



already noticed. Each gill-pouch thus communicates with the 

 pharynx by an internal (Fig. 769, B, i. br. a), with the outside 

 water by an external branchial aperture (e. br. a), and is 

 separated from its predecessor and from its successor in the 

 series by stout fibrous partitions, the interbranchial septa 

 (Fig. 780, 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, the actual 

 organs of respiration. It will be noticed that the respiratory 



