148 CLASS IV. 



fourteen to one-and-twenty in Echinaster Solaris; finally, in Asterias 

 helianthus the rays are found up to thirty and more. The greater 

 the number that any species possesses, the less is it constant. On the 

 dorsal surface is placed a calcareous star-formed plate between two 

 rays of the disc (verruca dorsi, tubercule madreporiforme, Madrepore- 

 plate), which in Ophiura is wanting, and in Euryale lies on the 

 oral surface. AGASSIZ who endeavours, with great acuteness, to prove 

 a lateral symmetry in the Echinodermata, considers the ray that is 

 opposite to this plate to be the first ray of the body. A tortuous 

 tube proceeds from this dorsal plate downwards as far as the mouth, 

 and is filled internally with a calcareous matter (see above, p. 130). 



This tube was named byTiEDEMANN (Anat. der Roehren-HolotJi., &c. 53, 

 54) Stone-canal or Sand-canal; it terminates, becoming narrower, in the 

 circular vessel surrounding the mouth and filled with watery fluid ; see 

 above, p. 131. SIEBOLD has closely investigated the calcareous balk, 

 consisting of several joints and internally hollow, which occupies this 

 canal and described its complicated structure ; MUELLER'S Archiv. 1836, 

 B. 291, &c. [Also SHARPEY, in TODD'S Cyclop, of Anat. and Phys. n. 

 PP- 35> &c., describes in the interior of the jointed calcareous tube a lamina 

 attached longitudinally, which passes inwardly a certain way and then 

 separates into two which are rolled in opposite directions, something after 

 the manner of the inferior turbinated bone of the ox.] 



The Sea-stars can bend their rays towards each other, which is 

 serviceable in moving through narrow fissures arid between stones. 

 They do not swim, but creep by means of their tentacles with 

 mouth downwards. They feed principally upon Molluscs. Though 

 the genus Asterias of LAMAKCK, by the exclusion of Comatula, 

 Ophiura and Euryale, be much more narrowly limited than the 

 same genus in the Systema Naturae, of LINNAEUS, still the species are 

 too numerous and the forms too various not to be regarded rather 

 as a natural group which ought to be divided into several genera 

 or sub-genera. This has been done by LINCK, and more lately 

 especially by AGASSIZ and MUELLER and TEOSCHELL, to whose works 

 we refer. The primary division of the group by MUELLER and 

 TROSCHELL is founded on the Tentacles, which in most of them are 

 placed in two rows in every furrow, but in others in four rows. 



A. Ventral furrows, with two rows of tentacles. 



Astropecten LiNCK. (Astropecten and Ctenodiscus MUELL. and 

 TROSCH.), Luidia FORBES. 



