DISTRIBUTION OF SHORE ANIMALS. 327 



of the class to which the adult belongs; thus the very 

 young mollusc is like a young 

 worm, and is without such struc- 

 tures as shell, foot, mantle, etc., 

 which are characteristic of the 

 adults. Adult Echinoderms are 

 radially symmetrical, but the 

 larvse are bilaterally symmetrical ; 

 we might go on to give many 

 other examples, but these may 

 serve to make the point clear. 

 There can be no reasonable doubt 

 that in some cases these simple FIG. 91. Naupiius of Peneus, a 

 larva, display what has been ff^J^fyyy^ 



aptly called " ancestral remi- Crustacea, and is very common 

 niscence"; that is, they display ^the^surface of the sea. After 



ancestral features, which the 



adults have lost. Thus the long tail of the megalopa stage 

 of the crab shows that crabs had long-tailed ancestors ; the 

 shelled larvse (veligers) of the common sea-slugs show that 

 these are descended from ancestors with shells. Can we, 

 then, say generally that the occurrence of pelagic larvse in the 

 life-history of littoral forms shows that these all had pelagic 

 ancestors 1 It would seem that such a view had much plausi- 

 bility, and yet there is a good deal to be said against it. 



In the first place, when we study pelagic animals closely, 

 we find that while they often appear at first sight to be 

 extraordinarily simple and primitive, yet close examination 

 shows that they must have had complex and specialised 

 ancestors. Thus there are a great number of pelagic 

 molluscs, often without shell, sometimes without foot or 

 mantle, delicate and transparent in texture, simple, as one 

 might say, in structure, and yet closer study shows that 

 they are apparently descended from littoral forms with 

 distinct shell, foot, and mantle. The same thing happens 

 in other groups, and leads us to the conclusion that pelagic 

 animals in general are often, apparently as an adaptation to 

 their peculiar habitat, simple, delicate, and transparent 

 creatures, but this simplicity is adaptive and not primitive. 



If armed with this deduction we return to the pelagic 

 larvse of littoral animals, we shall find some reason to doubt 



