366 CONTRIBUTIONS TO THE NATURAL HISTORY 



The extraordinary metamorphoses of star-fishes and Echini, which Professor Miiller 

 first ol)served, ought not to be neglected in the study of Beroid Medusae; for the re- 

 markable resemblance between the singular transparent frame which protects the growing 

 embryo of the star-fish and the body of Beroid Medusae cannot be overlooked by an 

 attentive observer ; and the fact, that the parts of that external frame present numeric 

 combinations which are unusual among Echinoderms, but which correspond to those of 

 the Beroid Medusa;, will be an inducement to institute, at some future day, a close com- 

 parison between their structure and that of the Beroids. The ciliated appendages which 

 hang downwards in those animals resemble closely the vertical rows of locomotive 

 fringes with their chymiferous tubes, as observed in Beroid Medusae. And it is inter- 

 esting to find hat in Echinoderms there is a metamorphosis going on in the embryo, 

 recalling the structure of the inferior class of Acalephae in a manner very similar to 

 the analogy which exists between the embryos of Medusae and Polypi. For whether 

 we compare the Strobila in its earliest conditions, or the young buds of Hydroid Polypi 

 when producing Medusae, the analogy of this earliest state of development of Acalephae 

 with Polyps is unmistakable, and I have no doubt that the external frame of the young 

 Asterias and Echini, which Professor Miiller has so beautifully illustrated, will be found 

 to bear the closest resemblance to the structure of Beroid Medusae, as soon as an actual 

 comparison can be instituted with reference to the analogy of their structure, which is 

 far more difficult to trace from descriptions and figures, however accurate these may be ; 

 but Professor Miiller's attention seems not to have been attracted by this remarkable 

 analogy, which he might have traced so fully when studying these embryos. So much, 

 however, may be said already, that the general arrangement of the ciliated lobes of 

 Plutus corresponds to the ambulacral rows of Pleurobrachia and Bolina, and that the 

 tubes which accompany them compare closely with the vertical chymiferous tubes of the 

 same Medusae. Notwithstanding my efforts to observe the various developments of 

 Asterias, I have, up to this time, been able to trace their growth only in the peculiar 

 forms first described by Sars. 



