ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
53 
layer is only well developed at the base of the hydrantli and on many 
points of the hydrorhiza ; in section a large number of nuclei become 
visible, the arrangement of which indicates the presence of several layers 
of cells with invisible contours. In some parts this layer is closely 
packed with nematocysts ; all these cells are of the same kind — they 
have an ovoid form, and the filament has several strong spines. It is 
to be noted that these capsules have no cnidocils. 
The author does not absolutely deny the presence of nerve or sensory 
cells in this hydroid, but he states that he was unable to find them. On 
the whole, the ectoderm of the hydranths and of the pseudonematophores 
gives evident signs of atrophy due to parasitism. The endoderm of the 
hydranth is formed of cells of very large size lying on circular muscular 
fibres ; they form a syncytium on the inner surface, but on the outer the 
boundaries of the cells can be distinguished ; the fusion on the inner 
surface is due to the fact that two or more adjoining cells send out 
pseudopodia which touch and fuse. There is thus formed a digestive 
protoplasmic layer, filled with nutrient and other particles, and entirely 
covering the internal surface of the gastric cavity. 
The structure of the tentacle is interesting, as it presents an inter- 
mediate stage between the hollow tentacles of some and the solid tentacles 
of other hydroids. In fact', by supposing the cells of the internal 
epithelium of the tentacles of Hydra to become so large as to touch at 
either end of the tentacle, we get the arrangement which obtains in 
Monobrachium. The membrane proper is feebly developed, and is 
scarcely visible on the hydrorhiza; it gives rise to protuberances or 
crests among the ectodermal, and more particularly among the endo- 
dermal cells, in such a way that, if all the cells are removed, the surface 
of the membrane shows the contours of the bases of the cells. This 
appearance, however, is only seen in the median part of the hydranth. 
The membrane appears to be simple. 
The gonophores are placed on short peduncles, and contain a medusa 
which becomes almost completely developed ; there are two and not four 
sexual sacs. The author describes in detail the histological character 
of the medusoid body. The spot at which the genital products aro 
matured is not that at which they appear. In the female, at any rate, 
the embryonic cells of the hydrorhiza of the female colonies are the 
female elements, and the same is probably true of the males. The 
author was unable to discover which germinal layer gave rise to the 
genital products. The sexual products are differentiated in the endoderm 
of the hydrorhiza, and are matured in the ventral epithelium of the 
radial canals. There is, then, in Monobrachium , a very important 
migration of sexual cells, notwithstanding the fact that this hydroid 
possesses an almost completely formed medusa. The embryonic cells 
pass along the endoderm from the hydrorhiza into the blastostyle, and 
then into the ventral epithelium of the radial canals, whence they reach 
the genital sacs by perforating the membrana propria. 
Hydroids of Plymouth.* — Mr. G. C. Bourne gives a list of fifty-five 
species of Hydroids found at Plymouth. Among them is Haloihema 
lankesteri g. et sp. n., which is closely allied to Halecium , but is dis- 
* Journ. Marine Biol. Assoc., i. (1890) pp. 391-8 (1 pi.). 
