LARVAL TELE OS TS 



223 



shark larva of Fig. 286: dorsal, caudal, and anal regions are 

 outlined in the unpaired fin ; a pectoral fin of a fin-fold 

 character, PF, has appeared ; the spiracle, SP, is becom- 

 ing established. The mouth region is more clearly indi- 

 cated in this stage, S, but may better be seen in ventral 

 view in a slightly later larva ; here (Fig. 299) the posterior 

 lip is constricted off from the yolk region, and the anterior 

 lip is budding off near the median line a pair of the tactile 

 barbels ; the dermal fold (operculum) enclosing the gills 

 is in a condition very similar to that of Ceratodus in 

 Fig. 293. A larva of the fourth day (Fig. 300) shows 

 well-marked advances : the snout is elongated ; the opercle 

 is enclosing the gills, which are now seen to protrude as 

 external branchial ; the pectoral fin elongates and is tend- 

 ing to protrude its fin axis ; body segments and heart are 

 encroaching into the region of the now elongate yolk sac ; 

 the lateral line has been formed. In a larva of four weeks 

 (Fig. 301), the essential outlines of the sturgeon may be 

 recognized, although the head appears of strikingly larger 

 proportions : barbels, nares, mouth, operculum, and spiracle 

 are as in the adult ; fins, of the mature outlines, are want- 

 ing in all save basal supports ; yolk material has long since 

 been exhausted. A very late larva (Fig. 302), supposed to 

 be twelve months old, differs outwardly from the sexually 

 mature form in but its colouring and dermal plates : those 

 of the regular rows are of great size, conspicuous in their 

 abrupt spines and well-roughened borders ; and those of the 

 remaining trunk integument are remarkably prominent ; the 

 tail of the larva shows clearly its palaeoniscoid character. 



V. Larval Te leasts 



The metamorphoses of the newly hatched Teleost 

 must finally be reviewed ; they are certainly the most 



