56 ZOOLOGY 
tentacles are solid. The ectoderm secretes a perisare which 
is continued into a cup, the Hydrotheca, surrounding the 
Hydranth. The groups of medusiform buds are similarly 
enclosed in a capsule, the Gonotheca. 
The medusae of this group are all Leptomedusae, and are 
characterised by having four, eight, or more radial canals, 
on which the generative glands are always situated. The 
tentacles on the rim may be few or very numerous.  Ocelli 
are found in two families ; more commonly the sense organs 
take the form of hollow vesicles, the otocysts, in which the 
otolith cells are formed from the ectoderm. 
The hydroids belonging to this order are colonial, the 
colonies being arranged in branching filaments, which have a 
superficial resemblance to some of the branching colonies of 
Polyzoa. Certain families are distinguished by the way in 
which the hydranths are placed on the branch. In the CAm- 
PANULARIDAE each hydranth is stalked; 11 the PLUMULARIDAE 
the hydrothecae are sessile on one side of the branch only; in the 
SERTULARIDAE they are sessile, and on both sides of the branch. 
Certain tentaculoid structures, termed nematophores, occur 
in relation to the hydrothecae of the PLUMULARIDAE. They are 
solid, with an endodermic axis, and knobbed at the end, and 
the knob contains nematocysts and sense cells. Their ectoderm 
has been seen to ingest carmine granules, and they have been 
observed to bend into the hydrotheca and eat up the remains 
of dead hydranths of the same colony. 
The medusae are often rudimentary, and remain attached 
to the blastostyle, as the hydroid individual which gives rise 
to them is called. The free medusae arise from the Cam- 
panularian hydroids; the gonophores of the Plumularians and 
Sertularians do not become detached. 
Order 3. Hydrocorallinae. 
This is a very well-marked order, in which the hydroid 
stage only has hitherto been found.!’ The hydrorhiza in these 
animals deposits a copious secretion of carbonate of lime instead 
of a chitinoid perisare. By this means considerable masses 
1 A very primitive form of medusa bearing male organs only (spermospores) 
has been recently described in Millepora Murray. 
