H addon — On Parasitic Larva of Halcampa. 479* 



sea-anemones, and in our species the adult has twelve rudimentary 

 mesenteries in addition to the twelve primaries [cf . Proc. Roy. Dub. 

 Soe. (N.S.) v., 1886, p. 12, fig. 4]. The same occurs in H. arenacea, 

 Haddon 1 ; but according to K. Hertwig, there are only the twelve 

 primaries in H. davits, Quoy et Gaimard. 



The brothers Hertwig * first insisted upon the systematic im- 

 portance of the disposition of the muscular bands on the mesenteries. 

 A comparison of the diagrams on PI. XL will demonstrate the fact 

 that the eight strong mesenteries of the larval Halcampa perfectly 

 corresponds with the eight mesenteries of Edwardsia. The Hert- 

 wigs have further shown that the normal Hexactina pass through 

 a stage in which there are eight strong and four weak mesenteries 

 (PL XL, figs. 10, 11) ; but it will be seen that these mesenteries do 

 not correspond with those of the larval Halcampa and adult 

 Edwardsia on the one hand, or with those of the Octactinise on the 

 other. 



The inequality in the development of the septa of the adult 

 Halcampa was first pointed out by R. Hertwig 2 (Actiniaria, " Chal- 

 lenger " Reports, Zoology, vi., 1882, p. 95). He found that four 

 were somewhat smaller than the eight others. I have quoted (loc. cit. 

 pp. 7, 8, footnote) an observation of Dixon's confirming this, and 

 Strethill Wright found the same in his larval form. He says : — 

 " Eight septa were continued downwards to the lower extremity of 

 the body, and had their free edges bordered by a convoluted cili- 

 ated band, furnished with cnidee, or thread cells ; the intersepta 

 {i.e. the four smaller mesenteries) bore no convoluted bands." 



On a future occasion I propose to give a detailed account of the 

 anatomy of Halcampa chrysanthellum ; for the present I would 

 merely state that I find that, in the adult, the generative organs 

 only occur on six mesenteries. These correspond with the eight 

 strong mesenteries mentioned above, less the dorsal pair. The 

 axial, or directive mesenteries, which support the siphonoglyphe, 

 are here considered as the ventral, and the opposite pair as the 

 dorsal. 



The Hertwigs also pointed out that the Actinidae (larval forms) 



i First Report " On the Marine Fauna of the South-west of Ireland— Actinozoa, 

 Froc. E. Irish Acad. (2) iv. (Sci.), 1886, p. 616. 



2 Die Actinien (Studien zur Blattertheorie), 0. and E. Hertwig, Jena, 1879. 



SCIEN. PROC. R.D.S. — VOL. V., PT. VI. 2 K 



