128 A NEW FAMILY OF HYDIiOIDEA. 



defensive polypes are shown arising from the outermost layer of coenosarc. The hard 

 skeleton parts represented in figure 11 occupy the spaces between the tubes in figure 3, 

 the polypes corresponding in position to the large circular openings leading down 

 into the hydrothecae. The general relationship of the polypes to the skeleton and the 

 coenosarcal tubes is represented diagrammatically in figure 12. 



Affinities of the Hydroceratinidce. 



Clathrozoon differs in important points from any hydroid hitherto described, 

 and it is somewhat difficult to determine its affinities, more especially in the 

 absence, as yet, of any knowledge of its reproductive elements. 



At first glance it would appear to be related to the Ceratelladae, but this is simply 

 in consequence of the superficial resemblance in the skeleton of the two groups ; 

 each consists of a branching mass of entwined perisarcal tubes, and here the 

 resemblance ends, for, whilst the branching network of tubes may be compared with that 

 constituting the skeleton of the Hydractiniidae and the Ceratelladas, there are present two 

 structures entirely wanting in the members of these two families, (1) the hydrothecas, and 

 (2) the external layer of thin perisarc, with its projecting cylindrical tubes. The first 

 of these structures are quite unlike those of any other hydroid in their simple 

 shape, the thickness of their walls, the relation of these to the surrounding 

 perisarc tubes, and the number of openings leading through the wall into the 

 enclosed space. At the same time, if the cavity in the scoop-like projection, formed 

 of a small network of perisarcal tubes in the Ceratelladae, were to become deeper 

 and to penetrate the substance of the branch, and if the branches of the network 

 bounding the cavities thus produced were to " run together " and thus give rise to a 

 tubular structure, in which apertures were left, we should have produced a structure 

 similar to the hydrothecae of Clathrozoon. The second of these structures is, 

 apparently, not present in any other hydroid, and the manner of its origin is difficult 

 to conceive. There is no such external layer of ccenosarc investing the colony as is 

 present in the Hydractiniidae, and which might be supposed to secrete such a layer. 

 On the contrary, the surface of the colony shows a series of much branching tortuous 

 perisarc tubes, each incomplete externally, and the whole covered in by this thin 

 continuous layer, which does not give the idea of having been formed as a separate 

 outer covering for each most external tube, though it must apparently have been pro- 

 duced in this way. The little cylinders arising from it enclose each, it is important to 

 notice, a defensive zooid, which arises directly from the cosuosarcal tube immediately 

 below. In one or two of the sections cut transversely near to the end of a growing 

 branch, the perisarc tubes appear to have a thin innermost lining distinguishable 

 from the rest of the perisarc ; possibly this may be comparable to this outer layer, 

 but at all events the latter, as shown in sections, passes continuously from lip to lip 



