3286 



ECHINODERMS 



be divided into two precisely similar halves. This is very different from the five- 

 rayed symmetry of the 'sea urchin, and the difficulties arise both in this class 

 and in the others when we try to discover how the five-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 body of the sea urchin, which at first is like a flat box, has become 

 provided 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 sea urchin. The arms 

 sink in, and at last the outer larva re- 

 mains as nothing more than a skin over 

 the test of the urchin. The mode of life 

 of the little sea urchin, about one milli- 

 metre in diameter, is now completely 

 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 illustration on p. 3285. 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 species and 

 genera to one another. It is not, how- 

 ever, 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 with many brittle 

 stars, which are protected in the so-called 



genital bursae. In a sea urchin (Hemiasfer philippf) there are depressions be- 

 tween the ambulacra, which are called brood pouches; for in these the young de- 

 velop from the egg, covered over by the spines of the parent, as in the annexed 

 illustration. In some holothurians the young are attached to the body of the 

 parent, as in Cladodactyla crocea; but in others, as in Psolus ephippifer (shown on 

 p 3281), they live on the back of the mother under some large mushroom-like 

 plates. Some starfish, too, such as Pararchaster, have a kind of tent of plates in 

 the middle of the disc, where the young grow up as in a nursery. 



BROOD POUCH OF A SEA URCHIN. 



(Enlarged 5 times.) 



