xxix DEVELOPMENT 399 



be recognized, but the proportions are different, and the 

 head presents several peculiarities. The gill-filaments 

 (br. f) are so long as to project through the external 

 branchial apertures and the spiracle (br. f), in the form of 

 long threads, abundantly supplied with blood-vessels, and 

 apparently serving for the absorption of nutriment the 

 albumen in the egg-shell in the case of Scyllium, secretions 

 of the oviduct in the viviparous forms. Besides this mode 

 of nutrition the yolk-sac communicates with the intestine by 

 a narrow duct (st), through which absorption of its contents is 

 constantly going on. By the time the young fish is ready 

 to be hatched or born the greater part of the yolk-sac has 

 been drawn into the ccelome, a mere vestige of it still 

 dangling from the ventral surface of the body. 



