RESPIRATORY ORGANS 



28l 



epithelium which is partially 

 ciliated. The inter -branchial 

 septa are much thicker than in 

 Elasmobranchs, and include not 

 only the walls of adjacent sacs 

 and the branchial muscles, but 

 also contain cavernous peri- 

 branchial lymph-sinuses. The 

 cartilaginous branchial skeleton 

 is situated wholly external to 

 the gill - sacs, the so - called 

 branchial arches lying between 

 the external apertures of the 

 sacs, and directly beneath the 

 superficial skin, or, in other 

 words, on the outer margins of 

 the inter -branchial septa, and 

 not on the inner, as is invari- 

 ably the case with tlie branchial 

 arches of Fishes. 



In the Hag -Fish (Myxine} 

 (Fig. 163), there are usually 

 six, very rarely seven, pairs 

 of gill-sacs, all of which open 

 directly into the pharynx, and 

 not into a branchial canal as in 

 the Lampreys. On the other 

 hand, Myxine is unique in 

 having the outer extremities 

 of its gill-sacs produced into 

 a corresponding number of 

 tubular canals which, after a 

 longer or shorter course ob- 

 liquely backwards and out- 

 wards, unite to form on each 

 side a ventrally - situated ex- 

 ternal aperture (Fig. 103). 

 In the same genus a short 

 canal, or oesophageo- cutaneous 

 duct, passes from the pharynx 



