LARVAL SHARKS 



217 



an exceedingly slow one ; Pristiurus emerges from the 

 egg in about nine months, Scyllium in about seven. And 

 in consequence of the large amount of yolk stored in 

 the yolk sac, the young shark, as in Fig. 289, has fully 

 acquired its adult outward characters by the time the yolk 

 is exhausted and its sac absorbed. 



In Fig. 284 is figured a stage in the development of 

 Pristiurus which may be regarded as either embryonic 

 or larval ; the form of the larva is well established ; gill 

 clefts, muscle-plates, mouth, and sense organs are present ; 

 but, on the other hand, unpaired fin and anus are lacking. 

 There is shown the abrupt constriction, characteristic of 

 Elasmobranchs, which separates the animal from the yolk 

 sac, a construction which in later stages becomes narrow 

 and tubular. The relatively larger size of the yolk sac 

 in later stages is, of course, the result of the bulkier elabo- 

 ration of the yolk material. 



The youngest stage (Fig. 284) shows prominently the 

 great enlargement of the anterior end of the embryo, a 

 marked cephalic flexure, large optic capsule, and irregular 

 gill slits of graded sizes ; a tubular tail end, bulbous at 

 the terminal, where the neurenteric canal occurs ; as yet 

 the nasal pits are in close proximity to the mouth. In the 

 next stage (Fig. 285), the elongated trunk has its unpaired 

 fin, the neurenteric canal disappearing ; the beginnings 

 of the pectoral fins are noticeable ; gill clefts are of more 

 uniform size ; and the anal region is indicated. In the 

 stage of Fig. 286, further advances are seen in the con- 

 stricting off of the unpaired fins, the appearance of the 

 ventral and the continued growth of the pectoral fins ; 

 in the reduced foremost gill slit (spiracle) ; in the jaw 

 region, and, in fact, in the entire shaping of the head ; 

 in the appearance of the lateral line. In the ventral head 



