NURSING ECHINODERMS. 
55 
tissue of the perisom ; the consequence of this arrangement is 
that when the plates are fitted together edge to edge, cloister- 
like spaces are left between their supporting columns. In these 
spaces the eggs are hatched, and the eggs or the young in their 
early stages are exposed by removing the plates (fig. 3).” This 
therefore is a true special marsupium or incubatory cavity, and 
as it occupies the greater part of the dorsal surface and its 
passages advance close to the edge of the mouth where the ovarial 
aperture is situated, the eggs pass at once into the protective 
cavities, without any exposure to external dangers. As the 
young animals increase in size, the surface of the marsupium 
Fig. 3. 
Psolus ephippifer , Wyv. Thomson. About three times the natural size. 
becomes more and more convex, and the plates enclosing it, 
which at first fitted accurately together, separate more and more 
from each other, until at length they are wide enough apart to 
permit the escape of the young animals. All the young in the 
same marsupium appear to be nearly of an age. In the male 
the middle of the hack is occupied by a saddle like that of the 
female, but the plates composing it are not supported upon stalks, 
and consequently no brood-chamber is produced. 
Among the Echinoidea Desmosticha, or Sea Urchins with 
