188 Royal Society : — 



6ubdi>ide to join the coeuosarcal meshwork. The cavities of the 

 tentacular zooids are circular in transverse section. Both kinds of 

 zooids are provided with strong circular and longitudinal muscles, 

 which form wide conspicuous bands beneath the ectoderm. The 

 alimentary zooids are situate on the summits of the styles of 

 the corailuni, where these are present. In Poh/poi-a, in the 

 retracted condition of the zooids, the styles traverse the axes of 

 the zooids from below for at least two thirds of their length. In 

 Pohipora, Errina, and Acanthopora the zooids of both kinds are 

 retracted within long sacs, the cavities of which communicate with 

 the surrounding network of the ccenosarc by a series of radially 

 disposed canals, which canals in transverse sections of the zooids 

 have at first sight exactly the appearance of a system of mesenteries. 

 In Criiptohelia and the St)/Iaster so closely resembling it the ali- 

 mentarv zooids, lying as they do deep in the calicles, are probably 

 never far protruded. The tentacular zooids are partly retracted 

 between the pseudo-septa, partly doubled down within the calicles 

 when the colony is in the retracted condition. In the other Stifhcs- 

 ters and in Allopora the conditions are much the same. Two 

 kinds of thread-cells are present, large and small : the large are 

 of the slightly curved cylindrical form, and emit a thread with an 

 elongate enlargement upon it near the sac, beset with a spiral of 

 spines : these larger cells are mostly gathered together in nemato- 

 phores, which are disposed irregularly amongst the zooids in Poly- 

 pora, regularly in the intervals between the tentacular zooids at 

 the margins of the caUcles in Cryptohdia and the Stijlaster resem- 

 bling it. The smaller kind of thread-cells are of an ovoid form, 

 slightly flattened on one side ; they occur in the tentacles of the 

 alimentary zooids, and form a closely set covering over the entire 

 external surfaces of the tentacular zooids. Xo three-spined thread- 

 cells, like those occurring in MiUepora, exist in the tStylasteridae. 

 Eeproduction takes place by means of adelocodonic gonopbores, 

 which are produced as buds from the ccenosarcal network without 

 having any other coJinexion with the other zooids. They occupy 

 in the coral! um the ampullae which in Polypora are concealed be- 

 neath the even external surface of the corallum, but in the other 

 genera of Stylasteridse show themselves as rounded prominences 

 on the surface of the coralla, being specially prominent in Errina 

 and Distichopora. The Stylasteridae are all dioecious. Females 

 only of Errina and Cryptohelia* have been examined, and males 

 only of the other genera. The generative elements of Acanthopora 

 were not observed at all. In the males of Polypora the gonophores 

 present the usual structures occurring in Hydroids; they are simple 

 ovoid sacs, \^-ith an axially placed spadix, and resembling in all 

 respects those, e. y., figured by Allman from Laornedea Jlexuosaf. 



* Off Japan last year a small fragment of what, at the time, I determined 

 to be a male Cryptohelia was obtained by the dredge. I unfortunately cannot 

 now refer to the specimen. 



+ ' A Monograph of the Gymnoblastic or Tubularian Hydroids,' by G. J. 

 Allman, M.D. &c., Ray Soc. part 1, p. 65. 



