A NEW FAMILY OF HYDROIDEA. 129 



of the outer grooves, and has nothing to do with any layer lining the latter internally. 

 That it is connected in development with the soft structures in these there can be 

 little doubt if only on account of its relationship to the protective zooids to which 

 they give rise. 



The extension of this layer beyond the mouth of the hydrothecae so as to form, as it 

 were, an operculmn for these, is a curious feature, and one not met with, so far as I am 

 aware, in any other hydroid. This particular part is very flexible, being capable of being 

 thrown into folds (Fig. 16), or even of being pulled back within the outer rim of the 

 hydrotheca by the retreating zooid. Taken altogether the skeleton, though in certain 

 respects showing a resemblance to theHydractiniidae and Ceratelladae, differs essentially 

 from that of these forms in the two important points dealt with above. 



In dealing with the soft structures we find a most curious combination of 

 characters, each one characteristic of hydroid forms, belonging to groups perfectly 

 distinct from one another. 



The network of coenosarcal tubes resembles, to a certain extent, that of the 

 Hydractiuiidse, Ceratelladae and Hydrocorallinas, but even here we have to note the 

 entire absence of an external continuous layer, characteristic certainly of the first 

 and third, and, probably, also of the second group. 



The alimentary polypes are sessile, and distinctly " claviform," that is, have 

 tubular bodies with a conical hypostome and a single circlet of simple solid tentacles, 

 in which points they resemble the genus Clava. Those of the Ceratelladae*, on the 

 other hand, have scattered capitate tentacles, and in the Hydractiniidse they are 

 provided with a strongly developed hydrocope. 



The gastrozoids again resemble those of the Hydrocorallinae, and differ from all 

 others amongst the Hydrozoa in being connected with several of the cceuosarcal 

 tubes. 



The defensive zooids, or dactylozooids, resemble more than anything else, certain 

 individuals characteristic of the Pluniulariidae, to which the name of " machopolype " 

 has been applied. They consist of a solid stalk bearing a number of nematocysts at 

 its free end, each zooid being enclosed in a distinct protective case or nematophore. 



The structure of the gastrozooids thus calls to mind the genus Clava, that of the 

 dactylozooids the family Plumulariidae, and the relationship of the gastrozooids to 

 the crenosarcal tubes, the sub-order Hydrocorallinae. This combination of characters, 

 together with the nature of the skeleton, serves to render the Hydroceratinidae distinct 

 from any family of Hydroidea yet known. 



* Bale, loc. cit. 



