THE HYDROMEDUSAE 23 



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

 hf/postome. The tentades form a single circlet ; they have a solid 

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

 panularia Hexuosa). The hi/drorhiza is generally well developed and 

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

 The tubes of the hijdrocaulns 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 continence of the ectoderm (Aglaophenia). 

 The hydrocaulus may be strengthened by the apposition of a second 

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

 contiiuious, 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 hj'droid. This arrangement at first sight suggests an 

 Alcj'onarian or a perforate Madreporarian rather than a Hydro- 

 medusan (Spencer, 32). 



A ])erisarc is always present as a complete investment of 

 hydrorhiza and hydrocaulus ; it generally has the same substance 

 and structure as in Anthomedu.sae. It almost invariably ex- 

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

 form in difi'erent 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 hydi'otheca (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 h3^drotheca are often confined to one plane 

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



