24 CHILDHOOD OF ANIMALS 



change from the bilateral symmetry of the larva to the radial 

 symmetry of the adult is itself lop-sided and unsymmetrical. 



There can be no doubt but that the greater part of this strange 

 life -history of the echinoderms, which seems more like the fantastic 

 changes of a pantomime than the orderly, deliberate processes of 

 nature, does not represent ancestral evolution. The early stages 

 up to the development of the dipleurula quite possibly recall the 

 structure of some remote and primitive marine creature from 

 which not only the echinoderms but other marine creatures may have 

 descended, for larvae of a similar type are found in the life-history 

 of many other animals. But the later stages and the curious mode 

 of transformation into the adult occur only inside the group itself. 



Polygordius is a small worm which lives in the sand farther out 

 than the lowest tide-mark, rather in the way that an earthworm 

 lives in the garden soil. It is a bilaterally symmetrical, ringed 

 creature with the mouth nearly at the anterior end, with only the 

 portion containing the brain and a pair of sensitive tentacles in 

 front of it. It swallows quantities of sand, passes these through 

 its digestive canal, absorbing any contained food material. 

 The eggs are small, are shed into the water and soon grow into a 

 cup-shaped larva very like the early larva of echinoderms. In 

 the same way, the aperture of the cup narrows and a mouth breaks 

 through. The larva, however, then changes in a different way. It 

 becomes shaped like a top, with a tuft of sensitive bristles repre- 

 senting the upper pole of the top, the narrowed original aperture, 

 which becomes the anus, being at the lower pole, and the mouth 

 just below the widest part of the body. A band of long cilia, called 

 the velum, passes round the circumference of the widest part of 

 the body, just above the horizon on which the mouth is placed. 

 This larva, which has been named a trochophore and which is totally 

 unlike the parent worm, swims about, feeds and grows, and then 

 suddenly begins to change (Fig. 12). The region round the anus 

 grows out into the long- jointed body of the worm, which hangs down 

 from the floating bell-shaped larva like a tail, and becomes the 

 greater part of the adult worm, soon growing to many times 

 the original size of the larva. The mouth of the larva remains as the 

 mouth of the adult, and the upper half of the larva becomes the 

 region in front of the mouth containing the brain, whilst the ring 

 of cilia disappears. The worm drops to the bottom and begins to 

 be a wriggling burrower in the sand. 



The case of Polygordius, which I have taken as an example of 



