12 ON THE STRUCTURE OF CERATELLA. FUSCA (GRAY). 



(1). Hydroid zooids (Figs. 2, 4, 6, 7, 9, 13).— These may be fomul on all the 

 branches, large and small except at the growing ends. Often also they are absent on 

 the larger main branches and are always fewer in number on these than on the 

 smaller ones (Fig. 4). Each zooid, in a well preserved spirit specimen, reaches a 

 length of about 1-4 m.m. and in the general form of the body resembles, as Mr. Bale 

 says, those of Coryne. The body (Figs. 4 and (3) is somewhat elongate with a 

 terminal almost conical portion, at the apex of which lies the mouth opening. The 

 basal portion which is seated upon the hydrophore is broad, this is followed 

 by a slightly contracted portion, then comes a slightly swollen part which gradually 

 diminishes in size towards the moiitli end. Over the siirface are scattered irregularly 

 the capitate tentacles from 10-14 or perhaps even more in number (Figs. 4 and 6), 

 and one or two of these are frequently placed close to or upon the basal region. 



The minute structure is generally that which is typical of hydroid forms and is 

 represented in Figs. 2 and 7. The ectoderm {Ect.) is unilaminar over the general 

 surface the cells being in close apposition each with a large niicleus and except in 

 the capitate ends of the tentacles there are no thread cells present. At the ends of 

 the tentacles the ectoderm is swollen out and apparently forms a mass of cells in 

 which large nematocysts are present with barbed threads (Fig. 7). The nematocysts 

 here and elsewhere seem to be all of the one size. The ectoderm lies on a thin layer 

 of mesogloea scarcely visible in extended zooids but more prominent in retracted 

 ones. There cannot be detected any fibrous muscular elements such as form so 

 distinct a feature around the basal region of the zooids in, for example, Clathrozoon. 

 The endoderm consists of large vacuolate cells each one subtending the base of, as a 

 general rule, at least three ectoderm cells. The protoplasm appears to be always 

 concentrated at the inner ends where the ni;clei arc placed and where in preserved 

 specimens the cell outlines are completely lost. In zooids which axe feeding 

 (Fig. 7) this inner end of the cell is filled with minute food particles the digestion 

 being evidently, in part at least, intracellular. 



The ectoderm is continuous at the base with the layer which covers externally 

 the whole colony whilst the endoderm is continuous with that of two or more of the 

 ccsnosarc tubes. 



(2.) Gonophorcs. — I have only been able so far to find the male gonophores 

 which are present on three colonies secured at Coogee on the New South Wales 

 coast. Each of these colonies carries numbers of minute somewhat pear-shaped 

 structures which are only from one-third to one-quarter of the length of the hydroid 

 zooids and which are seen when rendered transparent or cut into sections to be 

 medusoid in nature. 



