128 CLASS IV. 
effected by means of the sea-water in which these animals live. 
When parts are multiple, the number five prevails as remarkably 
in this class, as did the number four in the preceding: and the 
quintuple organisation is often obvious, as in star-fishes and sea- 
urchins, in the external form of the body. This is round or some- 
what pentagonal in the sea-urchins; flat and spread out in rays in 
the star-fishes, with the mouth on the inferior surface. The Holo- 
thurie have, on the contrary, a cylindrical body. 
In the star-fishes, of which the body is flat, the mouth conducts 
to a wide stomach that fills the dise of the body. It was supposed 
formerly that this stomach was in all of them a blind sac, as 
TImDEMANN has described it in Astertas aurantiaca (Astropecten 
aurantiacus MurELL. and Tr.) Afterwards MecKet detected in 
Comatule a second opening of the intestinal canal, that lies on the 
same surface of the body with the mouth’. Only lately it has 
become apparent from MUELLER’S investigations, that in most of 
the proper Asterte an anus exists, and that the structure in Ast. 
aurantiaca is to be considered as the exception rather than the 
rule. But this second opening does not lie, as in the Comatule, 
on the same surface as the mouth, but opposite to it, on the back 
of the disc. In the Ophiure and Euryale it is wanting: conse- 
quently these, with some star-fishes (the genera Astropecten, Cteno- 
discus, and Luidia of the moderns), and some Crinoids are the 
only Echinoderms in which the intestinal canal forms a blind sac, 
as in the Anthozoa. In the Asterte proper, the intestinal canal has 
excal appendages, which divide into branches, and fill the rays of 
the body; in those species whose intestinal canal is a blind sae, 
the appendages proceed laterally from the stomach, at whose base 
on the dorsal surface there are usually two cecal appendages in 
addition. In those Asterize which have an anal opening, the 
stomach is divided by a circular fold from a second compartment, 
to which the cecal appendages of the rays are attached; to this 
succeeds a third compartment, the rectum, a short straight tube, 
which has also cecal appendages; sometimes they are placed round 
the intestine in rays, like the appendages of the rays, and fill up 
1 Archiv fiir die Physiologie vit. 1823. s. 470—477. The same observation was 
made by Levuckart and H&USINGER ; see the not very clear description by the last- 
named in Mroxer’s Archiv f. Anat. u. Physiol. 1826. s. 317 —324. 
