LARVAL SHARKS 



215 



cavity of the intestine, the only instance of this condition 

 known among fishes (Ceratodus ?), and, with but a single 

 exception (Ichthyophis),* among all other vertebrates. 

 The larval lamprey is by this time a quarter of an inch 

 long, yellowish white in colour ; its movements are slug- 

 gish, rarely more than to cause it to wriggle worm-like 

 from the bottom. A few weeks later it has acquired its 

 brownish grey colour, its fin fold is well marked, and its 

 habit is active ; it now feeds on muddy ooze rich in 

 organic matter. It by this time possesses the essential 

 characters of the well-grown larva, long looked upon 

 as a distinct genus, Ammoccetes. In its larval stage the 

 lamprey appears to live a number of years ; in Petromyzon 

 planeri the adult stage is said to be sometimes deferred 

 until the autumn of the fourth or fifth year. The trans- 

 formation is then a surprisingly sudden one ; the head 

 attains its enlarged size, the mouth its ring-like and suc- 

 torial character, losing its more anterior position, and its 

 lip-like flaps (cf. Fig. 72, C, D) ; teeth are developed in place 

 of the numerous mouth papillae ; gills, formerly simpler 

 in character, opening directly from neck surface to gullet, 

 now enter the branchial chamber, a ventral diverticulum 

 of the gullet ; eyes become prominent, complete their 

 development, and attain the head surface ; unpaired fin, 

 formerly of great extent, is now reduced to its adult 

 position and proportions. 



II. Larval Sharks 



The larval history of Sharks has been summarized in 

 Figs. 284-289 : the younger of these stages (Figs. 284, 

 285, 286) have not as yet escaped from their ^^g mem- 

 branes. The hatching, in fact, of the young shark is 



* The writer has not confirmed Salensky's observation upon the sturgeon. 



