xiii PHYLUM CHORDATA 175 



A still more important difference from Elasmobranchs is the 

 possession of only a single external branchial aperture (&/. a p. ). 

 owing to the fact that a fold of skin, the oferculum (op.}, extends 

 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 urino-genital 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. /.) ; the caudal fin (c. /.) is of the ordinary heterocercal type 

 in the adult Callorhynchus, but in the young (Fig. 805) the 

 extremity of the tail proper is not upturned, and the fin-rays are 

 arranged symmetrically above and below it, producing the form 

 of tail-fin called diphycercal. In Chimsera the dorsal lobe of the 

 tail may be produced into a long whip-like filament (c./'.). The 

 pectoral (pct.f.) and pelvic (pv.f.) fins are both large, especially 

 the former. 



In the male there is a horizontal slit (B, a. d.') situated a little 

 in front of the pelvic fins ; it leads into a shallow glandular pouch, 

 from which can be protruded a peculiar and indeed unique 

 apparatus, the anterior clasper (A, a. cL), consisting of a plate 

 covered with recurved dermal teeth, to which is added, in Callo- 

 rhynchus, a plate rolled upon itself to form an incomplete tube. 

 The use of this apparatus is not known ; as it lies in the line of 

 the hypothetical continuous lateral fin, and as the cartilages which 

 support it articulate with the pelvic girdle, it seems possible that 

 it may have arisen as a portion of the lateral fin, which has 

 atrophied in all other Craniata. If this be so, it must be looked 

 upon as a third or intermediate paired appendage. A rudiment 

 of the pouch occurs in the female, although the clasper itself is 

 absent. The male possesses, in addition, a pair of the ordinary 

 pterygopodia or posterior claspers (ptg& and is further dis- 

 tinguished by the presence of a little knocker-like structure, the 

 frontal -clasper (fr. <?/.), on the dorsal surface of the head. In 

 Harriotta the claspers are poorly developed, and the frontal clasper 

 is absent. 



The lateral line (I. /.) is an open groove, and there are numerous 

 sensory pits, arranged in curved lines, on the head. The skin is 

 smooth and silvery, and bears for the most part no exoskeletal 

 structures. There are, however, delicate, recurved dermal teeth 

 on the anterior and frontal claspers, and the first dorsal fin is 

 supported by an immense bony spine or derma! defence (sp.\ In 

 the young, moreover, there is a double row of small dermal teeth 

 along the back. 



Endoskeleton. The -vertebral column consists of a persistent 



