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vent appears at the posterior extremity of the body, and the 
young assumes nearly the form of the adult.” By this time the 
young animals having acquired a considerable increase of size, 
their nursery apparently becomes rather small for them and they 
begin to jostle each other on the floor of the breeding-pouch, 
and very soon to push each other out between the spines 
which fringe the pouch. At this period the test of the young 
Urchin is about inch long, but its intestine is found to be 
Fig. 4. 
Hemiaster , sp. Arrangement of the eggs in one of the marsupial cavities. 
(Five times the natural size.) 
already full of dark sand and to follow nearly the same course 
that is characteristic of the adult. 
In the Asteridea, or true starfishes, an example of the marsupial 
development of the young has already been recorded by Sars in the 
case of Pteraster militarise a northern species, in which this 
process takes place in a brood-chamber formed very much in the 
same fashion as in the P solus already described, by the stalked 
paxilli, or calcareous plates of the dorsal surface. Professor 
Wyville Thomson describes a similar example in a species of 
Archaster , named by him provisionally A. excavatus , and allied 
