26 THE HYDROMEDUSAE 



pigmented and provided with cnidoblasts (Octorchis). They are 

 often placed opposite to the marginal funnels (subumbral papillae), 

 which lie on the subumbral surface above the velum ; these are 

 conical prominences with an excretory pore at the apex, through 

 which fluid has been seen to be ejected from the circular canal 

 (Octorchis). These three structures are shown in Fig. 34. 



The sense organs are ocelli, otocysts (marginal vesicles), and 

 cordyli (marginal clubs) ; their arrangement has been utilised for 

 purposes of classification. 



The ocelli are generally placed on the tentacle bulbs, but may 

 occur also at the bases of the marginal cirrhi, or of the cordyli; they 

 may be dotted, in number several hundreds, along the margin of 

 the bell (Orchistoma). They are rarely provided with a lens. 



The otocysts are placed at the base of the velum ; they are at 

 least eight in number, and are then adradial in position (Obelia). 

 They are often numerous, and may be reckoned by hundreds 

 (Polycanna). The cordyli are indefinitely scattered, and are 

 generally numerous 



The gastric cavity is simple, and is excavated in the bell. The 

 radial canals are often four in number, and perradial in position 

 (Eucope) ; to these four interradial canals may be added (Meli- 

 certum) ; fresh canals may be added till the number amounts to 

 about 200 (Orchistoma). The canals occasionally branch after 

 leaving the gastric cavity ; in this case only the perradial canals 

 may reach the circular canal, the branches ending blindly (Stauro- 

 discus); or the branches also may open into the circular canal 

 (Berenice). 



The generative cells in the medusoid lie on the subumbral wall 

 of the radial canals, rarely reaching back to the gastric cavity or 

 on to the manubrium. They form either one central, or two 

 lateral flat bands along the course of the canal (Fig. 33) ; or in many 

 cases lie in special pouches on the canals (Fig. 32). They occur 

 on the four perradial canals, or on both perradial and interradial 

 canals ; in species with branching radial canals they may lie on the 

 branches also. As in the Anthomedusae, they are placed between 

 the ectoderm and the mesogloea, or in the ectoderm itself. The 

 sexes are separate. 



The method of FORMATION OF THE MEDUSOID is of the type 

 already described in Anthomedusae. The medusoid may be budded 

 from the hydrocaulus (Campanulina) or, more commonly, from a 

 blastostyle. Although even fewer medusoids have been traced to 

 their hydroids in this group than in the Anthomedusae, no 

 medusoid has been observed to develop directly from the ovum, 

 and it is probable that a regular alternation of generations or 

 metagenesis is the invariable rule. 



POLYMORPHIC MODIFICATIONS OF THE MEDUSOID. The simpli- 



