148 
CLASS IV. 
fourteen to one-and-twenty in Lchinaster 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 madréporiforme, Madrepore- 
plate), which in Ophiuwra is wanting, and in Lwryale lies on the 
oral surface. AGASSIZ who endeavours, with great acuteness, to prove 
a lateral symmetry in the Zchinodermata, 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 by TIEDEMANN (Anat. der Rochren-Holoth., &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, 
s. 291, &e. [Also SHarpeEy, in Topp’s Cyclop. of Anat. and Phys. I. 
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 and 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 Lamarck, by the exclusion of Comatula, 
Ophiura and Euryale, be much more narrowly limited than the 
same genus in the Systema Nature of Linnxvs, 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 TroscHELL, to whose works 
we refer. The primary division of the group by Musrnier 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. 
* Anus none. 
Astropecten LINCK. (Astropecten and Ctenodiscus MUELL. and 
Troscu.), Lidia ForBrs. 
