THE AFFINITIES OF THE ECHINODERMATA. 507 



metry is often hardly discernible (e.g., CalycophoridcB)\ in the 

 latter — the medusoid — it is very marked, and especially char- 

 acterizes the arrangement of the gastro-vascular canals, which 

 are offshoots of the alimentary cavity, and, if they became 

 shut off therefrom, would answer to the enteroccele of the 

 Echinoderm. 



Suppose that, from a hydranth such as that of a Diphyes, 

 a medusoid were developed, and that, instead of projecting 

 from the exterior of the body, it remained hypodermic, spread- 

 ing out between the ectoderm and the endoderm of the hy- 

 droid, and consequently superinducing a very marked radial 

 symmetry upon it. The resulting form would give us a 

 Ccelenterate which would be a close analogue of an Echino- 

 derm. 



In a certain sense, an Actinozoon may be fairly regarded 

 as such a combination of a hydroid with its medusoid ; and, 

 hence, it must be conceded that the parallel between the gas- 

 tro-vascular system of the Ctenophora and the ambulacral 

 system of the Echinoderm s, instituted by the elder Agassiz, 

 was well worthy of consideration. Shut off the gastro-vascu- 

 lar canals of a Cydippe from the alimentary canal, and they 

 become an enteroccele, of which the prolongations along the 

 stomach may be compared with the peritoneal sacs, and those 

 beneath the paddles with the ambulacral vessels of the Echino- 

 derm. 



But there is a long step between the admission of the force 

 of these analogies, and the conclusion that the Echinoderms 

 and the Ccelenterata are so closely allied as to be properly 

 associated in one natural assemblage of " Radiate " animals. 

 On the contrary, the Echinoderm, by its Echinopaedium stage, 

 shows an advance in organization far beyond anything known 

 in the Ccelenterata ; and in the highly-characteristic mode of 

 development of its enteroccele (the elucidation of which in 

 the " Star-fishes," by Prof. A. Agassiz, is the most important 

 advance in our knowledge of the Echinoderms made since 

 the time of Muller), the Echinoderm agrees with the higher, 

 and not with the lower, Metazoa. 



Echinodermata abound in the fossil state. Calcareous 

 plates, referred to the Holothuridea, occur in the Mesozoic 

 rocks, but are not known earlier. The Star-fishes are met 

 with in the older Palasozoic strata, under forms very similar 

 to some of those which now exist. The Echinidea abound 

 from the Upper Silurian (Paloechinus) onward. The Palaso- 



