REPORT ON CORALS — HYDROCORALLIN-E. 29 



zooids, the absence of all trace of mesenteries, the apparent septa present in the tentacles, 

 the presence of the nematocysts of the form peculiar to Hydrozoa, and in fact every item 

 of histological structure, point irresistibly to the same conclusion. Professor Agassiz 

 considered the Millepores to be allied to the Hydractihise, and Glaus remarks on their 

 resemblance in some points to the Corynidse. Both Hydractinia and Podocoryne 

 resemble MiUepora in having a ccenosarc which forms a continuous encrusting layer ; 

 and in essential structure the ccenosarc of these two genera seems closely to resemble 

 that of MiUepora. Mr Carter 1 has described a species of Hydractinia from the Guinea 

 coast, H. calcarea, which has a hard calcareous ccenosteum. The genus Podocoryne 

 (Sars) has a " hydrophyton consisting of a continuous adherent expansion formed by 

 adnate inosculating canals, the deeper part, with its component canals, invested by a 

 chitinous perisarc, while a layer of naked ccenosarc spreads over the free surface." In 

 MiUepora the canals are not adnate, being separated by the stout trabecular of calcareous 

 matter which here take the place of the chitinous perisarc. The layer of naked ccenosarc 

 on the surface is probably homologous with the layer in the ccenosarc of MiUepora 

 described in the present paper as the superficial layer of the ectoderm. The structure 

 of the ccenosarc of Hydractinia is essentially similar to that of Podocoryne. Distinctive 

 features in the ccenosarc of MiUepora are the presence in it of the pore-like excavations 

 into which the zooids are retracted, the presence of large main branching canals, and the 

 formation of successive superposed layers of ccenosarc, and consequent formation of lines 

 of growth and tabulae in the calcareous skeleton. In having zooids of two kinds, 

 mouth-bearing and mouthless, the Millepores resemble Hydractinia echinata, which 

 bears likewise alimentary (gastrozooids) and spiral mouthless zooids (dactylozooids). In 

 the form of the zooids,- however, and shape and arrangement of the tentacles, and in 

 the nature of the nematocysts, 2 MiUepora seems to resemble such a form as Gemmaria 

 iniplexa. The real affinities of MiUepora amongst the Hydroids cannot, however, be 

 determined until the mode of reproduction is discovered. 



It is a remarkable fact that the ccenosteum of MiUepora seems undoubtedly to be 

 generated by the ectoderm. It is thus- not homogenous with the corallum of Anthozoa, 

 which is developed from the mesoderm, as arjpears certain in the latest accounts of 

 the matter from M. Lacaze-Duthiers' 3 researches on Astroldes calycularis, and from 

 those of Kowalewsky 4 on Astrcea and on Ahyonium digitatum. I have, for this reason, 



1 H. J. Carter, F.R.S., On the Close relationship of Hydractinia Parheria and Utromatopora. with Descriptions of 

 new species of the former, both recent and fossil, Ann. and Mag. Nat. Hist., vol. xix. p. 44, 4 ser., 1877. 



2 It would seem that a classification and nomenclature of the various forms of thread-cells is much needed, since 

 these forms appear to be of classificatorv value in the Ccelenterata. Certain forms are peculiar to Hydroids, e.g., others 

 to Alcyonaria. 



3 H. de Lacaze-Duthiers, Developpement des polypes et de leur polypier, Comptes Rend us, 1873, t. lxxvii. 

 (Hoffman und Schwalbe, Jahresberieht, 1875). 



4 A. Kowalewsky, Unteisuchungen iiber die Entwicklung der C'celenteraten, Nachrichten der kaiserlicben 

 Gesellschaft der Freunde der Naturerkenntniss, der Anthropologic und Ethnographie, Moskau, 1873. (Ibid.) 



