222 



LECTURE X. 



is developed, which afterwards acquires a shell, about Y\,th of a line 

 in diameter, and effervescing with acids : a discoid foot, like that of 

 a gastropod, is formed, which supports an operculum ; and a respira- 

 tory cavity is established like that of a Pectinibranchian : in short, an 

 unmistakeable young univalve mollusk is the result of the develop- 

 ment of the impregnated ova in the tubular sac — hence called 

 "molluskigerous" — which Prof. Miiller discovered in so many 

 specimens of the Synapta. 



In two cases the molluskigerous sac was found in specimens which 

 also possessed the ordinary generative organs of the Synapta, and this 

 coincidence might have been more common had not the specimens 

 brought for examination consisted only of those fragments of the 

 body which result from the usual spontaneous transverse fission which 

 follows the capture of this Holothurioid. The shell of the embryo so 

 developed bears most resemblance to that of the Natica, and it re- 

 mains to be determined, therefore, how the Mediterranean species of 

 that genus propagate and are developed. 



The analogies that offer the most probable explanation of the occur- 

 rence of a molluskigerous sac in an Echinoderm also possessing the 

 ordinary generative organs of its class, are such instances of pai'asi- 

 tism, for the purposes of development, as occur in the lymneus in- 

 fested by the cercarial larvfe of a distoma, in the bee which hatches 

 the strepsipterous larvEe, and in the living caterpillar which is made 

 the similar nursery of the ichneumons. 



We have here, therefore, probably a generic phenomenon, which 

 relates more immediately to the Gasteropoda than to the Echino- 

 dermata, and I shall return to the latter with a few concluding words 

 on the singular metamorphosis of the Comatulce. The ovicapsules 

 of this genus, which are developed in each of the pinnse of the long 

 and branched rays, at least as far as the fifteenth or twentieth pair, 

 become distended with ova about the month of July, when they escape 

 by a round aperture, but adhere together for a while, in a cluster of 

 about one hundred. They have been next observed dispersed, and 

 attached in the form of flattened oval discs to corallines and sea weed; 

 they then develope an obscurely jointed stem, ending in a clavate 

 head, which soon shows traces of the arms and mouth with its 

 tentacula. When the column presents a distinct division into twenty- 

 four joints, its expanded capital bears five bifurcating arms, which 

 are at first simple, but afterwards acquire the pinnte, and subsequently 

 the dorsal cirri. They further resemble small Comatulas, in having a 

 median mouth and a distinct lateral prominent vent, as well as in 

 their sulphur-coloured arms. These small Pentacrines attain the 

 height of about three-fourths of an inch, and were originally de- 



