NURSING ECHINODERMS. 
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bundles of from four to six diverging spines about one-eighth 
of an inch in length, and these support and stretch a strong 
membrane quite free from the surface of the perisome, having 
an open space beneath it, like the canvas of a marquee. At 
the centre of the dorsal surface this membrane terminates at a 
large aperture, about three-fifths of an inch in diameter, sur- 
rounded by a beautiful valvular apparatus which may be best 
described in Sir Wyville Thomson’s own words. He says, (i These 
valves do not essentially differ from the ordinary radiating 
supports of the marsupial tent ; a stout calcareous rod arises 
from the end of the double chain of ossicles which form the 
floor of the ambulacral groove. From the outer aspect of this 
support three or four spines diverge in the ordinary way under 
the tent-cover ; but from its inner aspect six or eight slender 
spines rise in one plane with a special membrane stretched 
Fig. 6. 
Hymencister nobilis, Wyv. Thomson. The marsupial chamber with the valves 
closed. (Twice the natural size.) 
between them. When the valves are raised and the pentagonal 
chamber beneath them open, these spines separate from one 
another, and, like the ribs of a fan, spread out the membrane in 
a crescentic form (fig. 5) ; and when the valves close, the spines 
approximate and are drawn downwards, the five valves forming 
together a very regular, low, five-sided pyramid (fig. 6). 
Looking down into the chamber when the valves are raised, the 
