parasitic on Hydromedusce at St. Andrews. 257 



In the slightly older larvse the general structure and dis- 

 position of the parts is precisely as above, except that, of 

 course, the size is larger, the tentacles longer, and that new- 

 tentacles are making their appearance in the lateral entocoels. 

 I am not yet in a position to state whether there is any parti- 

 cular order in their development. 



In a paper (3) which will shortly appear in the Transactions 

 of the Koyal Irish Academy, I am pointing out the great 

 importance of the earlier stage in the development, onto- 

 genetic and phylogenetic, of the Actinice, and it is highly 

 satisfactory to find another example of it in these forms. 



We now come to the question. To what genus and species 

 can we refer these interesting little parasites ? At first sight 

 they bear an unmistakable resemblance to the larva of Jlal- 

 campa chrysanthellum, both externally and anatomically ; so 

 we may confidently consider them to be closely allied. The 

 presence of twelve mesenteries precludes their being Ed- 

 wardsid^. The genera Peachta and Ilyanthus are usually 

 associated with Halcampa ; of the second of these I can say 

 nothing, as it has never been anatomically described, besides, 

 it is quite a rare form, and our larvse are decidedly common. 

 We then narrow the problem to the genera Halcampa and 

 PeacMa. Only two British species of Halcampa are known — 

 H. chrysanthellum of North-European distribution (4), and 

 H. arenaria, discovered and described by me (5) from the 

 south-west of Ireland ; of this only two specimens have been 

 obtained. 



I have made sections through three parasitic larva3 of //. 

 chrysanthellum, and am therefore well acquainted with it, and 

 although they agree perfectly with the St.-Andrevvs forms in 

 their general appearance and structure, still there are distinc- 

 tions in their histology which cannot now be gone into. One 

 point, however, deserves special mention, and that is, the ap- 

 pearance of the longitudinal retractor muscle of the larger 

 tentacles. In the earlier stages of K chrysanthellum the 

 muscle is relatively more extended than in the adult, but the 

 more distal plications are of considerable length, and decidedly 

 recall tl\ose of the adult. In a later stage this is naturally 

 still more marked. The corresponding muscle in the St.- 

 Andrews specimens is much less conspicuous, it extends for a 

 greater distance across the mesentery, consequently the plica- 

 tions are more numerous j but, on the other hand, they are 

 very much shorter and simpler than in ZT. chrysanthellum. 



In transverse sections through Peachia hastata I find that 

 the retractor muscles are very greatly developed, forming a 

 regular close-set fringe to the border of the mesentery, and 



