328 LIFE BY THE SEASHORE. 



our first hasty conclusion that these minute transparent 

 creatures are really simple really represent the primitive 

 pelagic ancestors. The larvae must have means of keeping 

 themselves afloat, and these means are often wonderfully 

 elaborate; they often have curious spines and processes, 

 whose object seems to be to prevent them being engulfed by 

 a narrow-mouthed foe, but which are too complicated in 

 structure for us to believe that they could occur in a truly 

 primitive animal. These are common in Crustacean larvse, 

 and well shown in the accompanying figure. Another diffi- 

 culty is that in Echinoderms, where the occurrence of simple 

 pelagic larvae is so striking a characteristic, the larvse of 

 the different classes differ from one another markedly. For 

 example, we have seen that morphologically the brittle-stars 



and starfishes are 

 nearly related, but 

 nevertheless the larvae 

 in the two classes 

 show marked differences. This at 

 once introduces a difficulty in regard 

 to ancestry, if we suppose that the 

 larvse represent ancestral forms. 



Pro. Oi-Zoea of a crab(TWa . . 



Note the long seems impossible to doubt that while 



the adult starfish and brittle-stars 

 have been diverging, the larvse 

 have also been diverging along different lines. That is, 

 the common ancestors of starfish and brittle-stars must 

 have had larvse quite different from the larvse either of 

 existing starfish or existing brittle-stars, and if we endeavour 

 to discover the characters of those original larvse by studying 

 the common characters of starfish larva and brittle-star 

 larva, we find that this original larva becomes pretty vague. 

 Generally we may say that just as the apparent simplicity 

 of pelagic animals when closely studied becomes adaptive 

 rather than primitive, so the simplicity of the pelagic larvaa 

 of shore animals when closely examined no longer appears 

 to be due entirely to " ancestral reminiscence," but acquires 

 an adaptive significance. 



This rather subtle argument would perhaps have little 

 force against the theory of the pelagic origin of shore 



