3i8 



E CHINODERMS. 



illustration on p. 317, in which the letters have the same meaning. This larval 

 form is called a Platens, on account of its frequent changes of shape, as it swims 

 about with its arms constantly moving. It will be noticed that through the whole 

 of its development it retains a two-sided symmetry, such that if cut down the 

 middle it would be divided into two precisely similar halves. This is very different 



from the five-rayed symmetry of the 

 \'J sea-urchin, and the difficulties arise 

 both in this class and in the others 

 when we try to discover how the 

 iive-rayed form was produced from 

 the two-sided one. 



From this larva only the stomach 

 and the water- vascular system are 

 continued into the sea-urchin, whose 

 prickly body is now being formed 

 around the stomach of the larva ; 

 and it is in just those two systems, 

 especially in the madreporite and in 

 the intestine, that we note in the 

 adult the traces of the primitive 

 bilateral symmetry. When the little 

 ho(\y of the sea-urchin, which at first 

 is like a flat box, has become pro- 

 vided with a mouth of its own, and 

 with a circlet of comparatively large 

 spines, then the parts not necessary 

 to the new structure disappear. The 

 calcareous skeleton of the larva is 

 absorbed, and the lime salts thus set 

 free help to build up the test of the 

 agf ' t; \m\ > -^ sea-urchin. The arms sink in, and 



\\\ at last the outer larva remains as 



nothing more than a skin over the 

 test of the urchin. The mode of life 

 of the little sea-urchin, about one 

 millimetre in diameter, is now com- 

 pletely altered. It is no longer 

 carried .about through the water, but crawls by means of its tube-feet and its spines, 

 as shown in the above illustration. We cannot here follow the further changes 

 that it undergoes ; but a study of those later stages is of great importance. For 

 by means of such study Agassiz has shown that many supposed genera are nothing 

 more than undeveloped forms of well-known species, and he has thus been able 

 to work out the relations of specie:; and genera to one another. It is not, however, 

 all echinoderms that pass through these curious larval stages, for in many species 

 the young are developed in the shelter of the mother. We have already seen this 

 to be the case witli many brittle-stars, which are protected in the so-called genital 



TOUNG sea-urchin {Strongylocentrotus). a, From below ; 

 b. From above (enlarged '20 times). 



