IV. Embryology. 51 



large ova^ the young oneSj wlien they leave the embryonic membranes, 

 .are very similar to, or almost identical in appearance with, the adalt. 

 This holds, also, for many Crustacea, which are hatched in a very 

 immature condition, with only a small number of appendages (three 

 pairs) . Here the egg is evidently too small to form an animal with 

 .so large a number of legs as is displayed by the adult Crustacean. 

 It seems as if there were a minimum, differing for different animals, 

 for the size of the limbs, and so on. In the larger eggs found in 

 ■other Crustacea, the animals hatch at a more advanced stage of 

 ■development. 



The differences in the conditions of life must be regarded as 

 ■another important factor in determining the differences between adult 

 and larval forms ; they are often intimately related to the cause 

 just mentioned. Many marine animals, which live as adults upon the 

 sea-bottom, are pelagic as larvae, and this has a great effect upon 

 the whole structure (Ch^topoda, Mollusca, Crustacea). Sometimes, 

 for instance, among the Amphibia, the larvee are aquatic, the adults 

 terrestrial, which also entails great differences. The metamorphosis 

 of Insects seems to be connected with the difference of the functions 

 ■of the larva and of the perfect animal, for the latter lives almost 

 exclusively for reproduction, whilst the larva devotes itself to feeding 

 ■and growth. 



In consequence of diversity in mode of life, parts are frequently 

 wanting in the larvse which are present in the adult. On the other 

 hand, they often possess special organs, the so-called provisional 

 larval organs, which are absent from the perfect animal (velum 

 of molluscan larvEe, gills of Tadpoles, pro-legs of Caterpillars). These 

 larval organs may be so large that only a small part of the original 

 larval body develops into the adult, whilst the greater part atrophies 

 (Echinoderma). 



Compared -witli the whole life, the larval period is usually short : as a rule 

 the animal attains the adult form, in most respects, long before it reaches its full 

 size and is sexually mature. Insects are a noteworthy exception to this rule, for 

 the state of perfect insect is usually assumed only when the organism has 

 attained its full size. 



The transition from the larval state to that of the mature 

 .animal is never sudden, as might be supposed from the actual 

 metamorphosis, but is always gradual. The transformation often, 

 however, takes place in relatively short spaces of time ; if a larva 

 has been for a long time in the same condition, it may pass through 

 great changes quickly, so that, to a certain extent, it assumes the 

 adult form at a bound. This is specially marked in the Arthropoda, 

 .since all external changes are connected with the moults; sometimes 

 before an ecdysis, the living part of the skin is loosened from the 

 cuticle and more or less altered, so that the larva, when it strips 

 ■off the old cuticle, suddenly appears in a new and changed form. 



E 2 



