GENERAL FACTS ABOUT SEA FISH 41 



opens, and the little teeth develop, just at the right moment 

 for the fish to start out foraging on its own account. It is 

 now in what is known as the " post-larval " stage. As both 

 of these terms will necessarily recur constantly in the following 

 pages, it is quite important to have a clear understanding of 

 what they mean. 



Some of the larval and post-larval forms of fishes differ 

 from the adult not less than the caterpillar and chrysalis differ 

 from the perfect butterfly. Mcintosh, for instance, describes 

 the larval stage of the grey gurnard as "one of the most 

 grotesque little animals which one meets with ; its long, 

 angular snout, large greenish eyes, huge pectoral fins, and 

 numerous little spines, all adding to its unique appearance. 

 The huge pectoral fins form a drapery for the entire body 

 when folded back, only the tip of the tail extending beyond 

 them." 



Even after the post-larval stage the young fishes often 

 differ considerably from what they will be when full-grown. 

 Thus, whereas the adult coal-fish has only a small barbel and 

 its lower jaw only slighter longer than the upper, young 

 examples have the barbel comparatively long and the lower 

 jaw conspicuously protruding. It has also been remarked, 

 with much evidence in support of the view, that characters 

 acquired comparatively late in the evolutionary history of a 

 species are generally developed late in the post-larval stage. 

 Conversely, it may be inferred that characters present in the 

 larval form or new-born young (as the fold of skin in the tail 

 of the young Indian sting-ray), and lost at a later stage, indicate 

 features which were possessed by some ancestor of the species, 

 but which it was found advantageous to drop. This theory 

 must not, however, be pressed too far. For instance, it is 

 thought that the protruding lower jaw indicates predatory 

 habits. Now, the larval stages of practically all bony fishes 

 show a protruding lower jaw. This does not, however, 

 indicate that the ancestors of all bony fishes were in conse- 



