WYVILLE THOMSON^ ON ASXEKACANTHION VIOLACEUS. 101 



valuable compilatious on Comparative Physiology^ have 

 suggested a correspondence between the provisional append- 

 ages of the Echinoderms and the temporary vascular appa- 

 ratus of the vertebrate embryo. This view I believe to be 

 correct, and capable of more accurate definition^ as will be 

 seen in the sequel. 



A series of careful observations which I have had an 

 opportunity of making during the last few months upon 

 a common littoral species of Aster acanthion, agree in almost 

 every particular with the observations of Sars, which, in 

 this as in all the other investigations of that most distin- 

 guished naturalist, are singularly clear and faithful. 



I was foi-tunate, however, in selecting for study a species 

 in which the provisional absorbent and respiratory vessels are 

 much more fully developed than in Echinaster sanguino- 

 lentus. 



The whole organism seems to be paler and more trans- 

 parent, and the relations and development of the internal 

 organs are accordingly more easily traced. My results are, 

 to a certain extent, at variance with those of some later 

 writers, and particularly with those of Dr. W. Busch. I 

 must state, however, that the plasticity of the tissue of 

 which these temporary embryonic appendages in the Echino- 

 derms are formed seems to be almost infinite. So capri- 

 cious are the variations in a structure essentially the same in 

 all, that it is impossible to anticipate its form in any parti- 

 cular case from the analogy of even the most closely allied 

 species. 



Early in December of the present winter I procured several 

 specimens of Astei'cicanthion violaceus (L.) (PL VII), in the 

 peculiar pregnant condition so graphically described by Sars. 

 The disc was raised into a hump, and the rays drawn closely 

 together at the base, to form over the mouth of the star-fish 

 the " marsupium'' for the protection of the young, diverg- 

 ing at the tips, to attach themselves to a stone or to the 

 glass wall of their prison. All the eggs or embryos in 

 a single marsupium were nearly at the same stage of deve- 

 lopment. In the least advanced the eggs were undergoing 

 the later stages of yelk-segmentation, while in others this 

 process had been completed. In other individuals the 

 embryos were partially or fully formed, while in the most 

 mature the outline of the five-rayed star was perfect, and 

 the echinoderm structure well marked by the development 

 of the oral ring of the ambulacral system and of the rudi- 

 ments of the vertebral (ambulacral) and dorsal calcareous 

 plates. 



