792 LIFE : OUTLINES OF GENERAL BIOLOGY 



degenerate types. Thus we see that the larva cannot be an entirely 

 novel interpolation; it must show some of the hereditary racial 

 characters, however little marked; and it is sometimes plainly 

 recapitulative of an ancestral stage. On the other hand, the details 

 of a larva are peculiarly adaptive to environmental conditions, 

 exigencies, and opportunities. 



The question now rises: What are the advantages of larval 

 stages — advantages which have secured the survival of those types 

 that varied in the direction of this staccato phase of life-history? 

 A first advantage may be diffusion or scattering, for mobile larvae 

 often .spread in all directions. This abates overcrowding in the 

 vicinity of the birthplace ; and is of particular advantage when the 



Fig. 136. 



Diagram of the Trochosphere Larva of a Sea-Worm, adapted for pelagic life. 

 A.SP, apical spot, with some nerve cells and projecting cilia; PR, pre- 

 oral ring of locomotor cilia; S, a similar post-oral ring; M, mouth; A, 

 anus; the food-canal, lined with endoderm, twists through the body- 

 cavity; R, the beginning of reproductive cells. After Hatschek. 



parents are fixed animals, such as cri. oids and brachiopods. This 

 spreading may also lead to the discovery of more suitable haunts, 

 to cross-fertilisation when maturity is reached, and to a lessening of 

 the risks of being devoured en masse. 



Another advantage is that the larva is often in a better position 

 than would be a fully formed miniature to secure a foothold, to 

 accumulate capital in the form of tissues and reserves, and to grow 

 in relative safety. This is well illustrated by caterpillars ; they make 

 butterflies possible. Sometimes, moreover, the insinuation of a 

 prolonged larval stage may circumvent the dangers of a difficult 

 season, such as winter. And, again, when the race has made a big 

 and difficult change in habitat, such as leaving water for dry land, 

 a larval period in the old haunt, as in the case of frogs and land- 

 crabs, may save the situation during tender youth. These are some 

 of the reasons why there are so many larvae in spring. 



