232 



supporting apparatus for the muscular system. (Comp. e. g. fig. 34, p. 88). It 

 is of considerable interest to trace this unusual development of a quite 

 indifferent element into a structure of the greatest perfection. 



Reduced larval forms, so common in the other classes of the Echino- 

 derms, are only rarely met with in Echinoids, in fact were hitherto quite 

 unknown. I have had the good fortune of discovering two most interesting 

 cases of reduction — not counting the Echinopluteus transversus — , in 

 which all the arms, excepting the postoral ones, have disappeared. One 

 of them, the larva of Peronella Lesueuri, is still recognizable as an Echino- 

 pluteus, the postoral (and sometimes also the posterodorsal) arms still 

 remaining, and a larval skeleton, rudimentary but still distinctly referable 

 to the normal skeletal type, being developed. The vibratile band has dis- 

 appeared, the larva being covered only by a uniform ciUation; only in 

 some exceptional cases a rudimentary band is developed (p. 116; fig. 48). 



In the other case, Heliocidaris erythrogramma, the reduction has gone so 

 far that there is not the slightest trace of the Pluteus-shape left. This 

 larva recalls the barrel-shaped larvae of Comatulids and Dendrochirotes, 

 but differs markedly from them in having only one, not very distinctly 

 differentiated ciliated ring. 



The existence in Echinoid-larvse of a pupa-stage corresponding to that 

 occurring in Holothurians and Ophiurans has been emphasized by Cas- 

 well Grave, who has observed transverse ciliated rings on the newly 

 metamorphosed sea-urchin of Mellita testudinata. (Op. cit. Fig. 10, p. 178). 

 I have not made any corresponding observations and shall therefore refrain 

 from commenting on the possible existence in Echinoids in general of such 

 a pupa-stage. Whether the Heliocidaris erythrogramma-larva is to be 

 regarded as an indication in that direction I shall leave undecided. 



The few Crinoid-larvse known being all of the vermiform type there 

 is no reason to mention them in the present connection. \Yith the greatest 

 expectations we may look forwards to the future discovery of some typical 

 pelagic Crinoid-larva ; the study of its relations to the other four main 

 types of Echinoderm-larvae will be of extreme interest. The existence of 

 such a pelagic Pelmatozoan-larva would seem almost beyond doubt, 

 though possibly not in any of the recent Crinoids. But these latter repre- 

 sent — in spite of the enormous development of the Comatuhd-type — 

 only a diminutive fraction of the whole group of the Pelmatozoa. It would 

 hardly seem too bold to fancy that at least some of the fossil Pelmato- 

 zoans, especially of the numerous Cystideans, had truly pelagic larvae. It 

 is very sad that we have no hope of learning anything about them, it 

 being in the highest degree unlikely that such larvffi should have been 

 preserved in a fossil state. Fritsch, it is true, has described an organism 



