IgI5.] J. Rircui1E : Hydroids of the Indian Museum. 549 
and it frequently assumes the deep helmet-shape shown in section 
in text-fig. 2A. It contains a large nucleolus. The cytoplasm is 
connected by a long delicate nerve fibre with the ganglionic mass 
on the floor of the cell. Occasionally, however, the connection 
becomes a comparatively broad protoplasmic strand resembling the 
peduncular attachment of some cnidoblasts. 
The internal structure of a microcnide differs much from that 
of a macrocnide The former contains only the filament, which 
proceeds directly from the operculum at the distal pole of the cyst 
in a loose descending spiral of about three small coils. These ap- 
pear to encircle a central pillar of delicate consistency which may, 
however, be simply one of those phenomena of refraction which 
render so difficult the exact observation of the contents of nemato- 
cysts. In a discharged microcnide (text-fig. 2B) three points 
strike one as characteristic: the shortness of the simple filament, 
the length of which is only twice that of the nematocyst; the 
directness with which the filament projects from the nematocyst, 
for it invariably lies in line with the .ong axis of the nematocyst 
and is straight, except for a very regular curve towards the tip ; 
and, lastly, the openness of the spiral of minute barbules, which 
performs only about ten revolutions in its contra-clockwise ascent. 
Hydranth Body. 
The ectoderm of the hydranth-body consists of a layer of ir- 
regular epithelial cells, between which lie small interstitial cells. 
The bases of the epithelial cells are produced into longitudinal 
muscle fibres which rest upon the mesogloea. The ectoderm ave- 
rages in thickness some 7°5,, but especially in the lower prolonga- 
tion of the hydranth is arranged in slight horizontal ridges. The 
cells contain large rounded nuclei, and in parts a large number of 
cnidoblasts, but only some of the latter possess cnidocils and are 
functional, the remainder being under process of formation. ‘The 
cuticle, if present, is of extreme delicacy, and no perisarc is se- 
creted. At certain stages of development, however ,—when a new 
basal bulb is being formed (see p. 555)—a number of hydranth cells 
take part, along with the cells of the basal bulb, in the secretion 
of a thick coating of hyaline mucus. In portions of this secretion 
masses of diatoms and other debris become entangled, and it is 
interesting to note that in this condition the diatoms appear to 
have continued a symbiotic existence, for the greater number show 
evidence, in their well-preserved protoplasm, of recent active 
metabolism. A similar state of symbiosis has been noted by 
Schaudinn in the case of the diatoms and algae which surround 
the body of Haleremita (Schaudinn, 1894, p. 226). 
In the neighbourhood of the tentacle-zone and around the 
margin of the mouth the ectoderm is thickened. In the latter 
region it contains a close array of functional cnidoblasts, similar 
to the macrocnides of the tentacles. In the tentacle-zone, how- 
ever, the majority of the cnidoblasts—macro- and micro-cnides— 
lie some distance below the surface and are in process of formation. 
