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. 



