S6 ZOOLOGY 



tentacles are solid. The ectoderm secretes a perisarc which 

 is continued into a cup, the Hydrotheca, surrounding the 

 Hydranth. The groups of nudusiform buds are similarly 

 enclosed in a capsule, the Gonotheca. 



The medusae of this group are all Zeptomedusae, and are 

 characterised hy 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- 

 pa nulakidab each hydranth is stalked; in the Plumulaeidae 

 the hydrothecae are sessile on one side of the branch only ; in the 

 Sertulaeidae they are sessile, and on both sides of the branch. 



Certain tentaculoid structures, termed nematophores, occur 

 in relation to the hydrothecae of the Plumulaeidae. They are 

 sohd, 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 perisarc. By this means considerable masses 



1 A very primitive form of medusa bearing male organs only (spermospores) 

 has been recently described in MiUepora Murrayi. 



