214 DEVELOPMENT OF FISHES 



larval life may be said to begin when the following con- 

 ditions have been fulfilled : the outward form of the larva 

 must be well defined, separating it from the mass of yolk, 

 its motions must be active, it must possess a continuous 

 vertical fin fold passing dorsally from the head region 

 to the body terminal, and thence ventrally as far as the 

 yolk region ; and the following structures, characteristic 

 in outward appearance, must also be established, the sense 

 organs, — eye, ear and nose, — mouth and anus, and one 

 or more gill clefts. 



Among the different groups of fishes the larval changes 

 are brought about in widely different ways. These larval 

 peculiarities appear at first of far-reaching significance, 

 but may ultimately be attributed, the writer believes, 

 to changed environmental conditions, wherein one proc- 

 ess may be lengthened, another shortened. So too the 

 changes from one stage to another may occur with sur- 

 prising abruptness. As a rule, it may be said the larval 

 stage is of longest duration in (I) the Cyclostomes, and 

 thence diminished in length in (II) Sharks, (III) Lung- 

 fishes, (IV) Ganoids, and (V) Teleosts ; in the last-named 

 group, a very much curtailed {i.e. precocious) larval life 

 many often occur. 



I. Larval Cyclostomes 



The Cyclostome larva is represented in a stage as 

 early as that of Fig. 212: its form is here retort-shaped; 

 the yolk material is concentrated in the ventral region 

 immediately in front of the blastopore (the anus i*), but 

 is distributed in addition in the cells of other body regions. 

 In the section of a slightly older larva (Fig. 215), in which 

 the mouth is all but established, the form outline has 

 become regular, the bulk of the yolk, Y, restricted to the 



