294 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



Spotted Dogfish {Scyllium — Fig. 121, A) or the Spiny Dogfish (Acanihtas 

 —Fig. 121, B). 



The form of the body is that of an elongated spindle, gradually 

 tapering off towards the posterior end. The crescentic mouth is situated 

 on the ventral side of the head : the anus or cloacal opening is situated 

 ventrally, in Scyllium about the middle of the length of the body, the tail 

 region behind it being about as long as the head and trunk regions lying 

 in front of it. The body is prolonged outwards in the form of flattened 

 projections — the flns — and these are distinguished as unpaired fins lying 

 in the median plane of the body, and paired fins which project laterally. 



Kcl Kcl .ixcS/l 



A, Scyllium; B, Acanthias. a.f. Anal fin; d} and d'^^ dorsal fins; olf, oliactory organ; 

 p.f, pectoral fin ; pL, pelvic fin ; u.c.l, spiracle; v,cM and u.c.VIj second and sixth visceral clefts. 



Of the latter there are two pairs, an anterior — the pectoral -fins i^.f) — and 

 a posterior pair, situated at the sides of the cloacal opening — the pelvic 

 fins {j)l). The median fins are in the young embryo in the form of a 

 continuous fin-fold extending along a great part of the dorsal edge of 

 the body, round the tip of the tail, and then along the ventral edge nearly 

 to the anal opening. With further development large stretches of this 

 fin-fold disappear while the intervening portions, increasing greatly in 

 size, persist as the unpaired fins of the adult. Of these we distinguish 

 two dorsal fins {3), an anal fin (a./ — absent in Acanthias), and a caudal 

 or tail fin. The last mentioned, which is better developed in Acanthias 

 than in Scyllium, is unsymmetrical or heterocercal, this asymmetry afford- 

 ing a conspicuous difference in appearance between these shark-like fishes 



