DE VELOPMENT. 
3i9 
bursse. 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 
Psolus 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 echinoderms were origin¬ 
ally derived from some form or forms 
with a two-sided symmetry; and it is 
certainly curious what a close resemblance brood-pouch op a sea-urchin (enlarged 5 times), 
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. 
T. A. BATHER. 
