10 ON THE STRUCTURE OF CERATELLA FUSCA (GRAY). 



represents fairly well the macroscopic characters of the skeleton but conveys the idea 

 of one which has been much water-worn and has lost many of its smaller branches. 

 The branching is much richer and closer than is represented by him and the 

 projecting hydrophores are more distinct and regular. At the same time there is a 

 certain amount of difference in the growth of various colonies some of which are 

 more bushy in appearance than others. That secured by Mr. Gabriel has the 

 branches more distinctly arranged in one place than is the case with the others. 

 The whole colony may be described as follows : The colony arises from a much- 

 branching root-like base encrusting foreign objects. The root-branches unite to 

 form a strong stem common to the colony which is flattened in the same plane as 

 that in which the branches of the colony, generally, expand. The common stem 

 may have the appearance of being formed of intertwined branches and from it arise 

 irregularly larger or smaller branches the former being more or less flattened in the 

 same plane with itself. 



From the larger arise irregularly on either side smaller branches which again 

 branch until a somewhat bush-like or fan-shaped colony is formed the whole being 

 more or less flattened in the plane of the main stem. Except the main stem close 

 to the root branches and the latter all the branches may bear bracket-like 

 projections the hydrophores which are arranged in a roughly spiral manner and 

 are especially abundant on the smaller branches except the growing ends which are 

 somewhat swollen and flattened in a plane at right angles to that in which the 

 branching of the colony takes place. 



In addition to considerable variations in the general form of the colony some 

 appear to have the hydrophores so arranged that they lie along the two opposite sides 

 only of the branches whilst in others they are arranged all around. The more 

 bush-like the growth is, to the greater extent does the latter obtain and vice versa. 

 Usually there is only one main stem arising from the root-branches but at times one 

 or more smaller ones may arise independently. (Fig. 1.) 



The skeleton when examined by the lens has much the same appearance as that 

 of Hydractinia and differs considerably from that of Clathrozoon and more still from 

 that of the polysiphonic stem characteristic of certain species of Plumularia. It forms 

 in the larger branches a meshwork of chitinous tissues so similar at first sight to 

 that of certain homy sponges that as Mr. Bale says " a portion broken off and 

 examined separately might well be mistaken for sponge tissues." This however is 

 only true with certain limitations for, if the soft parts are present, the distinction 

 between the two is readily seen as is also the case when the hydrophores are present. 

 Apart from this also nothing comparable to the layer of cells secreting the sponge 

 skeleton is present nor are the chitin fibres of the two similar in their minute 

 structure. 



