ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



.lish (St-ylliiiniFig. 121, A) or the Spiny Dogfish (Acanthias 

 . j i . B). 



Tlu- form of the body is that of an elongated spindle, gradually 



taperin- off towards the posterior end. The crescentic mouth is situated 



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



vent rally, in Scyllimn about the middle of the length of the body, the tail 



>i'hind 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 



ions the fins and these' are distinguished as unpaired fins lying 



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



FIG. 121. 



turn; I'-, Acanthias. a.f, Anal fin; d l and d*, dorsal fins; olf, olfactory organ ; 

 ;.il tin ; pi., pelvic iin ; v.c.l, spiracle; v.c.Il and v.c.Vl, second and sixth visceral clefts. 



( )t the latter there are two pairs, an anterior the pectoral fins (p.f) and 



rior pair, situated at the sides of the cloacal opening the pelvic 



fins (/>/). 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 ihe anal opening. With further development large stretches of this 



tin fold disappear while the intervening portions, increasing greatly in 



.1-- the unpaired fins of the adult. Of these we distinguish 



tw.i dorsal fins (rf), an anal fin (a.f absent in Acanthias}, and a caudal 



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



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



n>].i( uous difference in appearance between these shark-like fishes 



