THE DIGESTIVE AND RESPIRATORY SYSTEM 305 



is essentially pharyngeal and consists primarily of a series of 

 lateral pockets in the walls of the pharynx, which break 

 through to the exterior and form slits. These openings are 

 metameric in arrangement and are paired, each pair corre- 

 sponding to a single metamere as expressed in the associated 

 organs, but show considerable reduction in number from those 

 found in Amphioxus, the functional slits in most cases being 

 limited to five pairs. In two especially primitive genera, how- 



FIG. 85. Respiratory organs. 



(a) Cyclostome. (b) Selachian, (c) Teleost. (d) Selachian embryo, (e) Am' 

 phiuma larva. (f) Cryptobranchus larva [from a Japanese print]. 

 n, nostril; s, spiracle; g, gill-slits. 



ever, Hexanchus and Heptanchus, there are respectively six 

 and seven, facts which suggest that the number at present rep- 

 resents a reduction from a previously more extensive series, 

 the reduction being from behind forwards. Upon the pharyn- 

 geal side of these slits there develops a series of soft organs in 

 the form of fringes or tubes, which consist of localized elabora- 

 tions of the pharyngeal wall, the gills or branchice. These 

 are profusely vascular and are supplied with a rich capillary 

 network developed between two sets of branchial arteries, the 





