DEVELOPMENT. 



3i9 



bursas. In a sea-urchin {Hemiaster philippi) there are depressions between the 



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



Psoitis ephippifer (shown on p. 314), they 



live on the back of the mother under 



some large mushroom-like plates. Some 



star-fish, 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. 



The direct development from the 

 egg to the adult in these protected forms, 

 seems to show that the elaborate shapes 

 of the various larvae have been developed 

 secondarily for the special purpose of 

 transporting the young and aiding in the 

 dispersal of the species, and, therefore, 

 that they are not relics of any ancestral 

 forms. There can, however, be little 

 doubt that the echinoclerms were origin- 

 ally derived from some form or forms 

 with a two-sided symmetry; and it is 

 certainly curious what a close resemblance 

 their assumed primitive larval form 

 presents to the larva of Balanoglossus, the worm-like animal described on p. 573 

 of the last volume, and considered by many authorities to be in the ancestral line 

 of the Vertebrata. 



F. A. BATHER. 



brood-pouch of a sea-urchin (enlarged 5 times). 



