THE HYDROMEDUSAE 



which its coelenteron originally communicated with that of the 

 colony from which it was budded. 



The radial canals are generally four in number, and are then 

 perradial ; but four interradial canals are also developed in some 

 cases (Cladonema). Six are normally presented by Clavatella 

 ( = Eleutheria). In Lar ( = Willsia) six are also present, which 

 bifurcate twice; there are thus twenty -four openings into the 

 circular canal. 



The generative cells (gonads) lie in the wall of the manubrium, 

 between the ectoderm and the mesogloea, or in the ectoderm itself ; 

 they rarely reach on to the subumbrella (Nemopsis). They are 



Fio. 25. 



Diagrams of the medusoids of two species of " Sarsia," the one budding medusoids from the 

 manubrium, the other from the ends of the radial canals. (After Allinan.) 



cylindrically arranged (Sarsia), or are broken up into four or eight 

 bands. In Lar they are six in number, and lie on the walls of the 

 six-rayed gastric cavity in the manubrium. The sexes are separate. 

 FORMATION OF THE MEDUSOID BY GEMMATION. A medusoid 

 of the type indicated above is either biidded (a) from a hydroid 

 (Syncoryne), or from a blastostyle (Tubularia), or from the 

 hydrocaulus (Bougainvillea), or, with the intermediation of a 

 short stem, from the hydrorhiza (most Perigonimus), or (b) from a 

 medusoid (Sarsia), either from the manubrium (Fig. 25), or 

 from the margin of the bell, at the end of the perradial canals 

 (Codonium). Although in many cases medusoids have not been 

 traced to hydroids, no medusoid of this group has been found 

 to develop directly from the ovum. 



