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- 

 thurice 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 disc of the body. It was supposed 

 formerly that this stomach was in all of them a blind sac, as 

 TlEDEMANN has described it in Asterias aurantiaca (Astropecten 

 aurantiacus MUELL. and TE.) Afterwards MECKEL detected in 

 Comatulce a second opening of the intestinal canal, that lies on the 

 same surface of the body with the mouth 1 . Only lately it has 

 become apparent from MUELLER'S investigations, that in most of 

 the proper Asterice 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 Comatulce, 

 on the same surface as the mouth, but opposite to it, on the back 

 of the disc. In the Opliiurce 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 Asterice proper, the intestinal canal has 

 csecal appendages, which divide into branches, and fill the rays of 

 the body ; in those species whose intestinal canal is a blind sac, 

 the appendages proceed laterally' from the stomach, at whose base 

 on the dorsal surface there are usually two cgecal appendages in 

 addition. In those Asterise which have an anal opening, the 

 stomach is divided by a circular fold from a second compartment, 

 to which the cgecal appendages of the rays are attached ; to this 

 succeeds a third compartment, the rectum, a short straight tube, 

 which has also csecal appendages ; sometimes they are placed round 

 the intestine in rays, like the appendages of the rays, and fill up 



1 Archiv filr die Physiologic viu. 1823. s. 470 477. The same observation was 

 made by LEUCKART and HEUSTNGER ; see the not very clear description by the last- 

 named in MECKEL'S Archiv f. Anat. u. Phy*iol. 1826. s. 317324. 



