ON THE STRUCTURE OF CERATELLA FUSCA (gRAY). 11 



The mesliwork is extremely irregular in the larger branches as is shown in the 

 sections represented in figs. 9 and 11. There seem to be two not clearly 

 distinguishable portions present (1) large and strong fibres (2) smaller ones which 

 form connecting bars and often have the appearance of thin web-like plates which 

 are especially well developed on the hydrophores. Fig. 5 represents a medium-sized 

 branch, from which the soft parts have been removed by potash, seen by reflected 

 light. At times the fibres seem to run for some distance parallel to the long axis of 

 the branch though this is much more strongly marked in some branches than in the 

 one figured. There are no definite hydrothecre present within which the zooids can 

 be completely retracted but the branches are studded sometimes only at the sides 

 sometimes all over with little bracket-like projections which give a characteristic 

 •serrate appearance to the branches, very different however from that which is 

 produced by the hydrothec<E of the Sertularians. Mr. Bale has aptly suggested that 

 the term " hydrophores " (originally applied by AUmau to the calyces of Halecium) 

 should be used to describe these structures which form merely supports for the 

 hydroid zooids. Each one may perhaps be best likened to a very concave scallop 

 shell with ribs formed by the strong fibres continuous with those of the branch whilst 

 the spaces between them are filled up by a thin fenestrated web of chitin. In some 

 hydrophores the ribs are more strongly marked than in others and project as small 

 points around the margin. (Figs. 5 and 6.) The growing ends of the smaller 

 branches usually contain two or three longitudinal fibres connected by transverse 

 bars, often somewhat web-like, and each of these growing ends is clearly 

 distinguishable (1) by its being flattened in a plane at right angles to that in which 

 general growth takes place and (2) by the entire absence of hydrophores and zooids. 

 It may be added that the longitudinal arrangement of the fibres in the branches is 

 more clearly noticeable in specimens with the soft parts present than in those in 

 which the latter are absent since the soft parts conceal from view largely the 

 connecting and the deeper-lying fibres and thus prevent to a large extent the network 

 structure from being seen. The whole skeleton also in living specimens is 

 completely enclosed by the soft parts though only a thin layer of tissue is present 

 over the external surface. (Figs. 9, 10 and 11 E.) There is no protective covering 

 for the reproductive structures. 



Soft Parts. 



The structure of these has, as yet, been only briefly described once and that by 

 Mr. Bale, whose short account refers only to the external form of the hydroid zooids. 

 He pointed out, as stated before, that these possessed irregularly scattered capitate 

 tentacles and that hence they differed considerably from those of Hydractiuia. 



Amongst the specimens from the Australian Museum is one collected by 

 Mr. Whitelegge on Lord Howe Island with the zooids fully expanded and others 

 with the soft parts well preserved. 



