A small collection of various kinds of Worms is also exhibited in this Gallery (Wall-cases I.–III.).
ECHINODERMA.
Six table-cases contain the dried Echinoderms arranged in systematic order. The seventh is devoted to preparations, models, and figures illustrative of the structure and life history of various members of the group.
An inspection of that Case and the accompanying woodcuts will make clear the distinctive characters of the Echinoderma. Unlike that of a Crayfish or a Mussel, the body does not appear to be divided into two equal or symmetrical halves, though it really is; this is due to the possession of a number of rays, of which there are ordinarily five. The skin is strengthened by the deposition in it of carbonate of lime, which may be in the form of continuous plates or bars, or of separate scattered spicules. A series of tube-feet or suckers (podia) are generally developed along each ray, and these are supplied by a system of water-vessels peculiar to Starfish and their allies. These rays are often called “ambulacra.”
Fig. 1.
A. Anchor and plate of Synapta. B, C. Tables of Holothuria impatiens; and D. Holothuria atra: from various aspects. E. Spicule from sucker of Stichopus variegatus, magnified about 200 times.
Fig. 2.
Diagram of Water-vessels.
c.c. Circular canal, with p.v., its Polian vesicles; from it a radial canal (v.c.) is given off along the lower surface of each arm; this supplies, by side branches, the suckers, s; connected with each sucker is a contractile swelling or ampulla (a). The circular canal is in connection with the exterior by s.c., the stone-canal, and opens to it by the madreporite (m).
Fig. 3.
Figure of a Starfish (Asterias rubens).
In the ray marked I. the skin has been removed from the upper surface, and the ambulacral ossicles (ao) and the podia (s) are seen in situ; the blind outgrowths (c) from the central stomach (sp) have been dissected out. In II. the gonads (g) are exposed; and in the centre above the stomach the rectal glands (rg) are to be seen. The anus (a) is seen to be subcentral in position.