282 R. E. Lloyd : A new venits of Hyd raids. [VOL. I, 



soaking it freely in fresh xylol for about six hours it was imbedded 

 in paraffin and cut into sections. »Sections so obtained seemed 

 quite as good as others, treated in the usual way. Owing to the 

 heat of Calcutta, high-melting paraffin had to be employed in mak- 

 ing these sections. Portions of the colony were placed in paraffin 

 of 55° C. melting point for half an hour ; this interval of time was 

 found to be long enough for complete penetration. 



The Hydranth — 



Bach hydranth is a club-shaped body measuring about "75 mm. 

 in length in the contracted state. In internal structure it 

 differs remarkably from other hydroid colonies, but it seems 

 difficult to arrive at any other conclusion than that the form in 

 question is a hydranth and part of a hydroid colony. It resembles 

 the genus Protohydra (2) and the parasitic Hvdrichthys mirus (3) 

 in that it is entirely devoid of tentacles or any trace thereof ; but 

 apart from this, the internal structure, as seen in sections, shows 

 some most unusual features. 



The ectoderm is relativel^^ thin and, owing perhaps to want of 

 proper fixation, does not show much structure. The appearance 

 it presents in section is that of a somewhat irregular layer of 

 protoplasm, containing a single series of nuclei (plate xvii, figs, i 

 and 4). This layer is easily distinguished from the mass of endo- 

 derm cells, which show peculiar structural features. Careful search 

 failed to demonstrate the presence of nematocysts in this ectoderm. 



The endoderm is, in these contracted specimens, verj^ much 

 lobulated, so that the central cavity, which can be clearly made 

 out both in optical and actual section, usually" takes a sinuous 

 course. The opening of the central cavity at the distal end of 

 the hydranth can be clearly seen, and there is usually a slight 

 external depression at its site. The endoderm cells, which make 

 up the bulk of the hydranth, are of a peculiar structure : the}^ 

 are ovoid or spherical and have well-deffiied outlines. After 

 staining with haematoxylin a nucleus cannot be demonstrated in 

 them, but each cell contains a large number of small spherical 

 granules, arranged round the periphery with great regularity. 

 These granules take the stain exactly like nuclei, and they arc 

 probably composed of chromatin and perform the functions of 

 nuclei. These large cells do not actuall^^ line the central cavity, 

 but are separated therefrom by a pavement epithelium — a single 

 delicate layer of flat cells in which nuclei are easily demonstrable. 

 This epithelium not only lines the central cavity but is continued 

 outwards through the mass of the spherical cells and joins the 

 peripheral ectoderm. Where it lines the central cavity, this 

 epithelium is composed of one layer of cells, but where it passes 

 out to join the ectoderm it is, like a mesentery, composed of two 

 layers. This is clearly seen on examining favourable sections un- 

 der a T2-inch objective (fig. 4). As shown in tigs, t to 3, the 

 endoderm of the hydranth is divided by this epithelium into two 

 separate parts. 



