XIII 



PHYLUM CHORDATA 



189 



and has the appearance of a second or external lower jaw. In 

 Chimera the snout is blunt, in Harriotta long and pointed ; in 

 Callorhynchus it is produced into a rostrum, from the end of which 

 depends a large cutaneous flap (B, tc) abundantly supplied with 

 nerves and evidently serving as an important tactile organ. 



A still more important difference from Elasmobranchs is the 

 possession of only a single external branchial aperture (br. ap.), 

 owing to the fact that a fold of skin, the opcrculwn (op,), extends 



fr.cl 



Km. S47. A, Chimacra monstrosa ; B, Callorhynchus antarcticus. a. d. anterior 

 clasper ; a. d.' pouch for its reception ; br. ap. branchial aperture ; <. /. caudal fin ; c. /.' its 

 whip like prolongation ; d. /. 1, d. /. 2, dorsal fins ; //. d. frontal clasper; l.f., I. /.' labial 

 folds ; 1. I. lateral line ; na. ap. nasal aperture ; op. operculum ; pet. f. pectoral fin ; ptg. 

 ptei-ygopodia ; pe.f. pelvic fin ; t. teeth ; tc, tactile flap ; v.f. ventral fin. (A, after Cuvier.) 



backwards from the region of the hyoid arch and covers the true 

 gill-slits, which thus come to open into a common chamber situated 

 beneath the operculum and communicating with the exterior by 

 a single secondary branchial aperture placed just anterior to the 

 shoulder-girdle. Equally characteristic is the circumstance that 

 the urinogenital aperture is distinct from and behind the anus, 

 there being no cloaca. 



There are two large dorsal fins (d.f.l } d.f. ( ^ and a small ventral 

 (v. /.) ; the caudal fin (c./.) is of the ordinary heterocercal type 



