THE TROPHOSOME. 



25 



mutability of outline {Eudendrium, Cawjmmdana, woodcuts figs. 2 and 13). The tentacles vary 

 ill form, number, and arrangement in the different species, and thus afford excellent characters 

 for the purposes of classification. Their axial tube may be continued throughout {Ilydrd), 

 or it may be more or less obliterated {Coryne, Campanularia). They are usually simple 

 filiform processes, as in Hydra, Clava, Hydradinia, Campanularia, &c. ; but in some cases their 

 extremity presents a knob-like enlargement, as in Co/y«(?, when they are said to be "capitate." 

 Their number may be as low as two {Lar), or it may amount to more than 100 {Corymorplia 

 nutans). They may be arranged in a single circlet {Campanularia)} or in two {Vorticlava), or 

 in several (Stauridiim), or they may be scattered over the body of the hydranth {Coryne), or may 

 be partly verticillate, partly scattered [Pennaria). Finally, they may be all of one kind (filiform, 

 as in Clava, capitate, as in Coryne), or the same hydranth may carry both filiform and capitate 

 tentacles {Stauridium) . 



In a great number of hydroids the hydranth is destitute of any external protection 

 {Tubularia, Coryne, &c.). In others, however, there exists a cup-shaped receptacle, within which 



Fig. 3. 



Portion of the Hydrosoma of Calycella syrinya, showing the operculate hydrotheca;, -Vc. 

 a a The hydranth extended ; b, b, hvdranth retracted ; c, hydrotheca ; d, operculum open for the exit of the hydranth ; 

 <f ope'reulum closed after the retraction of the hydranth ; e, e, hydrocaulus ; /, hydrorhiza ; g. gonangium with its contents 

 previous to the escape of ovum ; g , uonangium after the passage of the ovum into an acrocyst ; h, walls of sporosac ; 

 r, gubernacular sac ; k, remains of blastostylc; m, ovum still contained within the sporosac ; u. acrocyst ; o, ovum after its 

 escape into the acrocyst; p, empty sporosac, still enclosing the spadis. 



^ A single circlet may be absolutely simple or it may result from the close approximation of tuo 

 or more verticOlate series of tentacles. 



