12 ASTEROIDEA. 



different sizes, disposed more or less irregularly, and which furnish excellent specific 

 characters. The lateral and inferior parts are, on the contrary, sustained by a greater 

 number of pieces, much more regularly disposed. They acquire sometimes a very great deve- 

 lopment, as in the Asterias tessellatay Lamk. ; they appear to me always to form three 

 series, one superior, the second altogether lateral, and the other inferior. It is these 

 which unite with the series of pieces I call ambulacra], because it is between them 

 that the tubular feet escape, as in the sea-urchins. The last two series of lateral pieces 

 carry moveable spines ; they are still the analogues of the inter-ambulacral areas of the 

 sea-urchins ; these spines vary, nevertheless, both as regards their figure and the number 

 of rows which they form ; they are always very singular for their resemblance to a grain 

 of corn. As to the ambulacral pieces, they are very regular, very symmetrical, and they re- 

 semble in the median Hne of the inferior part of each ray a kind of spine, which sustains 

 it, and which permits movements between its numerous articulations, as in a species of 

 vertebral column. 



Miiller and Troschel, ^ in their ' System der Asteriden,' state, that the Asteriada are 

 Echinodermata of a stellate, or polygonal, mostly of a pentagonal form. In addition to the 

 tegumentary skeleton, they possess an internal skeleton, which is wanting in all the 

 others. This consists of as many rows of pieces moveably articulated together, as there 

 are lobes in the body, and which always proceed from the circumference of the mouth, 

 and from the under side of the rays. In the Asterias these rays form the floor of the 

 abdominal furrows, and the tegumentary skeleton is supported in such a manner on the 

 sides of the vertebral pieces of the internal skeleton as to form thereby hollow lobes, in 

 which the intestines, or csecal prolongations of the stomach, and a part of the genital 

 organs extend. In the Ophiura the intestines are limited to the naked disc, and the 

 articulated rows of the internal skeleton are everywhere surrounded by the tegumentary 

 skeleton ; so that the abdominal furrows are wanting in this group. 



Professor Agassiz,^ in the ' Catalogue raisonne des Echinides,' in treating of the 

 affinities existing between the diflerent orders of Echinodermata, combats the opinion 

 expressed in the ' System der Asteriden.' " M. J. Miiller affirms," observes M. Agassiz, 

 " in his great work on the Asteriadce, that the character which most clearly distinguishes 

 these animals from other Echinoderms consists in an internal skeleton, a kind of vertebral 

 column, on which the solid plates of the external skeleton are fixed. He affirms, even, 

 that we observe nothing similar in the Echinidse, of which the solid framework is altogether 

 external. But this assertion is erroneous, and the learned anatomist of Berlin appears to 

 me to have completely misunderstood the analogy which exists between the ambulacra 

 of sea-urchins and the grooves on the under side of the rays of star-fishes. This analogy 

 is nevertheless the most complete, for we here remark the same arrangement of the plates, 

 the same openings for the passage of the tubular feet, the same relations with the ocular 



' System der Asteriden,' p. 1. 

 ' ' Annales des Sciences Nalurelles,' 2>""- scrie, tome vi, p. 309. 



