252 Proceedings of the Eoyal Irish Academy. 



The extraordinary power of regeneration as seen in this group is of great 

 ph)-siological interest. Xormally they are attached to rocks or corals ; but 

 even shells — e.g., Margaritifera margaritifera — may form a basis of support. 

 Eidley records the case of a colony in the " Alert " Collection which had been 

 broken from its attachment, and in which the coenenchyma had quite over- 

 grown the fractured part, which had continued its existence as a free 

 colony, floating in the ocean. A similar case has been recorded by the 

 writer for Ids hippuris (Journ. Linn. Soc. ZooL, vol. xxxn^L, pp. 421-433, 

 pi. 43). 



These large Juncellid colonies also form bases of attachment for numerous 

 kinds of animals. Ophiuroids and crinoids are constantly found attached to 

 them, but equally common and more permanent are acorn-shells and bivalves. 

 The former settle down in the larval stage, bore their way through the 

 coenenchyma, and remain attached for life to the axis. The Alcyonarian 

 colony responds to the stimidus, and continues to develop coenenchyma at 

 the fractured part, so that eventually the acom-shell is quite overgrown by 

 pol_\-p-bearing coenenchyma, leaving only a small oval aperture, by means of 

 which the acom-shell derives its food — a characteristic form of commensalism. 



Of more economic interest, however, is the case of Pteria mocroptera, 

 which is eagerly sought for on account of its pearl-bearing proclivities. 



While examining the marine fauna of the Mergui Archipelago, one of the 

 most striking phenomena encountered was the fact that on nearly eveiy 

 colony of Juticdh f/cmmacea obtaine<I there were abundant specimens of this 

 oyster. Some idea of the strength of these colonies may be gathered from 

 the fact that on one individual colony there were over a hundred oysters. 

 The greater number of these were almost full-grown, and each of them 

 weighed on an average more than the colony itaelf. The byssus was usually 

 overgrown by coenenchyma; but the great rate of growth of the shell itself 

 precluded the possibility of the Alcyonarian keeping pace with it. 



Rfprcdudion. — A large proportion of the colonies examined contained 

 enormous spherical reproductive bodies. Serial sections of some of these 

 were made; and Professor Hickson also kindly sent me some sections prepared 

 by him. These bodies consisted of two kinds : — 



1 1) Ova with a large nucleus and a distinct nucleolus almost identical 

 with the figures given by von Koch. 



(2) Spennathecae or sperm sacs in which it was possible to trace 

 spermatogenesis almost up to the stage of fully formed spermatozoa. 



No trace of segmentation of ova was discernible ; and it is more than 

 probable that this does not take place within the parent body. 



It is also worthy of note that the ova and spennathecae occurred in 



