20 THE HYDROMEDUSAE 



POLYMORPHIC MODIFICATIONS OF THE MEDUSOID. In many 

 cases the gonophore, or bearer of the generative cells, has not the 

 complicated structure of the medusoid, but one far simpler ; the 

 simpler conditions are probably not phases in the evolution of a 

 more complex type, but, contrariwise, have been attained by the 

 reduction of the higher organisation. Every stage in this process 

 of simplification is represented among the Anthomedusae, until in 

 Hydra, the final term of the series, nothing remains of the highly 

 organised medusoid except the generative cells. The following 

 types (Weismann, 10) indicate the gradual abandonment of com- 

 plexity of structure : 



1. The gonophore has the general form of a medusoid, but is 

 never freed. The ocelli are always wanting, the velum and mouth 

 generally, the tentacles sometimes; but the subumbral cavity, 

 the manubrium, and the radial canals are developed (Tubularia). 

 2. The gonophore is arrested at an early stage in the development 

 of the medusoid, corresponding roughly to Fig. 27, III. The ento- 

 codon and subumbral cavity are developed, but the latter never 

 opens to the exterior, and no radial canals are traceable. The manu- 

 brium is only slightly indicated (male Clava). 3. The gonophore 

 develops no entocodon. (a) In some cases the endoderm lamella 

 is nevertheless formed, combined with a few immigrant ectoderm 

 cells (Coryne) ; (b) in other cases the endoderm lamella is not 

 developed, and a section to the centre shows merely ectoderm, 

 generative cells, mesogloea, endoderm lining coelenteron (Fig. 28). 

 This type of gonophore is termed a sporosac, and is very commonly 

 found among Anthomedusae. The endodermal core (sometimes 

 termed the spadix) may be straight (male Eudendrium, in which 

 the sporosac is ampullated), or may be curved round the 

 generative cells (female Eudendrium), or form anastomosing 

 branches (Cordylophora, Fig. 29). 4. The generative cells are 

 developed in the ectoderm of the body of the hydroid, and no 

 trace of a medusoid is recognisable (Hydra, Fig. 3). 



In a few instances a thin and temporary gelatinous capsule 

 invests the gonophore, whether a medusoid (Bougainvillea) or a 

 sporosac (Cordylophora). 



ORIGIN OF THE GENERATIVE CELLS. Approximately parallel 

 to these modifications, and probably correlated with them, is a 

 gradual alteration in position of the spot at which the generative 

 cells are differentiated in various genera ; this is apparently 

 attributable to a necessity for the production and maturation of 

 these cells as early as possible, and may be termed a process of 

 acceleration. In the medusoid the generative cells are both formed, 

 and ripen, in the manubrium ; in the first stage of acceleration they 

 are formed in the entocodon, and ripen in that part of it which 

 ultimately becomes the manubrium (Tubularia). In the second 



