XIII 



PHYLUM CHORDATA 



189 



I' 



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

 Chimsera 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 supphed 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 {hr. ap), 

 owing to the fact that a fold of skin, the opercuhcm {op.), extends 



Flu. S47.— A, Chimaera monstrosa ; B, Callorhynchus antarcticus. a. d. anterior 

 clasper ; a. d.' pouch for its reception ; Or. ap. branchial aperture ; c. f. caudal fin ; c. f.' its 

 whip-like prolongation ; d. f. 1, d. /. S, dorsal fins ; fr. cl. frontal clasper ; I. f., I. f.' labial 

 folds ; I. I. lateral lino ; na. ap. nasal aperture ; o'p. operculum ; pet. f. pectoral fin ; ptfj. 

 pterygopodia ; pv. f. pelvic fin ; t. teeth ; tc. tactile liap ; v. f. ventral fin. (A, after Cuvier.) 



backwards from the region of the hyold 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. 2) and a small ventral 

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



