26 CHILDHOOD OF ANIMALS 



many similar cases in marine worms, is very difficult to understand. 

 If the bell -shaped larva, swimming in the water like a transparent 

 jellyfish, represents the far-off ancestor, it baffles the imagination 

 to conceive the stages by which this should have evolved into 

 a creeping worm, by the elongation of the region round its anus. 

 It is much more simple to suppose that the worm developed directly 

 without any floating larva, and that the swimming disk was a 

 secondary development useful, like the wings of a wind-borne seed, to 

 carry the embryo about. If this be correct, the similarity between 

 the Polygordius larva and the larvae of other marine worms, with 

 the larvae of animals belonging to different groups of Invertebrates, 





,o>. 



FIG. 13. Larvae of a Gastropod Mollusc : left-hand figure, a Trochophore ; 

 right-hand figure, a Veliger. (Much enlarged.) 



is, so to say, a mere accident, due to the similar lives the larvae lead, 

 and with little bearing on the ancestral relationships of these groups. 

 The large class of Molluscs contains animals of many different 

 types, such as oysters and mussels, whelks, snails and slugs, cuttle- 

 fish and squids. The period of youth is passed under many different 

 conditions, and especially in those that live on land or in fresh 

 water there are cases which we can see, by comparison with 

 their nearest relations, to be special adaptations to special circum- 

 stances. But there are two successive types of larvae found in so 

 many different molluscs that it seems as if they were at one time 

 stages in the life -history of all molluscs. The first is a trochophore 

 (Fig. 13), very like the trochophore of marine worms, and which 

 grows from the egg in the same way. It is more globular than 

 top -shaped, and the ciliated band, or velum, is nearer the upper 

 pole, so that the part in front of the mouth is smaller in proporfion 

 than in the worm-larva. This rapidly changes into the second type 

 of larva, called the veliger, and peculiar to molluscs. The velum is 

 drawn out into branches or lobes, and the portion in front of it ceases 

 to grow, so that it becomes a mere swimming apparatus carried 



