XII 



PHYLUM CHORUATA 



383 



be recognised, 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 absorbtion of nutriment the albumen in 

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

 duct in the viviparous forms. Besides this mode of nutrition 

 the yolk-sac communicates with the intestine by a narrow 



FIG. 214. A, embryo of Scyllium with yolk-sac (x4)j B, under-side of head 

 enlarged. br.f. branchial filaments protruding through _ 

 branchial filaments protruding through spiracle ; cd.f. caudal fin ; d.f. dorsal 



gill-clefts; br.f. 



fins ; e. eye ; ex. br. ap. external branchial apertures ; i/ith. mouth ; na. nostrils ; 

 fct.f. pectoral fin; pv.f. pelvic fin; st. stalk of yolk-stalk ; v.f. ventral fin; 

 yk. s. yolk-sac. (After Balfour, slightly altered.) 



duct, through which absorption of its contents is constantly 

 going on. By the time the young fish is ready to be born 

 or hatched, 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. 



In all the most important features of their organisation 

 there is a considerable degree of uniformity among the 

 Elasmobranchii. 



