THE HYDROMEDUSAE 23 



The HYDROID is generally colonial, and has a trumpet-shaped 

 liypostome. The tentacles form a single circlet ; they have a solid 

 endodermal axis, and are filiform ; they are rarely webbed (Cam- 

 panularia flexuosa). The hydrorhiza is generally well developed and 

 gives origin to simple (Lafoea) or branching (Obelia) hydrocauli. 

 The tubes of the hydrocaulus are generally distinct from one another 

 (monosiphonic) ; but several hydrocauli, each bearing hydroids or 

 branches, may be closely bound into a compound or polysiphonic 

 stem with greater or less confluence of the ectoderm (Aglaophenia). 

 The hydrocaulus may be strengthened by the apposition of a second 

 hydrocaulus which bears no hydroids, the perisarc of the two being 

 continuous, except for occasional points at which the ectoderm of 

 the two becomes confluent (Anisicola halecioides Jickeli, 31). 



Very exceptionally, numerous hydrocauli may be clustered 

 round a central hydrocaulus ; of these the central one gives origin 

 to the hydroids and to the smaller branches ; the accessory ones 

 communicate occasionally with each other and with the central 

 one, and give origin to the nematophores (Plumularia procumbens). 

 A still more complex and unique condition, but one apparently 

 deducible from the last instance, is presented by Clathrozoon ; the 

 accessory hydrocauli, surrounded by perisarc, anastomose so freely 

 that each stem appears to be made up of a mass of irregularly 

 branching tubes, several of which communicate with the coelenteron 

 of each hydroid. This arrangement at first sight suggests an 

 Alcyonarian or a perforate Madreporarian rather than a Hydro- 

 medusan (Spencer, 32). 



A perisarc is always present as a complete investment of 

 hydrorhiza and hydrocaulus ; it generally has the same substance 

 and structure as in Anthomedusae. It almost invariably ex- 

 pands at the base of each hydroid as a rigid hydrotheca, of variable 

 form in different genera, into which the entire hydroid can with- 

 draw itself (Figs. 30, 31); this is occasionally absent (Cam- 

 panopsis). 



The edge of the hydrotheca is often toothed ; on the retraction 

 of the hydroid, these teeth may fold inwards to form a conical 

 operculum which closes the orifice of the hydrotheca (Calycella). 

 There is sometimes only a single plate serving as operculum 

 (Diphasia), or none at all (Halecium). 



The hydrocaulus may carry only one hydrotheca (Clytia) ; 

 more commonly it bears a large number ; it may give off branches 

 (rami, pinnae), on which the hydroids are borne ; or the rami 

 may bear branchlets (ramuli, hydrocladia) to which the hydroids 

 are limited. Hydroids are occasionally borne on a blastostyle 

 which carries also a medusoid (female Halecium). 



The hydroids and hydrotheca are often confined to one plane 

 on the branch, lying either on one side only of the hydrocaulus 



