CNIDARIA 47 



that of the polyp (Fig. 17 o), and the manubrium of the 

 former to the oral cone (peristome, hjpostome) of the latter. 

 The cavity of the medusa-bell would be represented by a 

 concavity of the peripheral part of the peristome, which 

 exists in many hydroid polyps, while the polyp's crown 

 of tentacles would be equivalent to the marginal tentacles 

 of the medusa (Fig. 17 t). According to this interpretation, 

 the aboral part of the polyp, broadened and flattened, would 

 be metamorphosed into the exumbrella of the medusa, 

 while the gastral cavity of the latter is differentiated into 

 a central stomach cavity and a peripheral intestine (Kranz- 

 darm), consisting of radial canals and circular canal, together 

 with the vascular lamella lying between them (Fig. 17). 

 The velum, produced by a fold of the ectoderm, would be a 

 new structure, not present in the polyp. To the differentia- 

 tions which characterize the medusa belong the greater 

 development of the musculature and the nervous system 

 (double nerve-ring of the margin of the bell) and the 

 evolution of sensory organs. 



In many Hydroids, paii passu with an acceleration of sexual maturity 

 accomplished by a dislocation of the germarium (Weismann, No. 50), the 

 sexual persons have lost the power of free locomotion, and have been 

 metamorphosed into sessile medusoid buds (gonophores). They must 

 be regarded as reduced medusas (v. Koch, No. 34), in which the marginal 

 tentacles, the sensory bodies, and the velum — and often the opening of 

 the bell also — have disappeared, while the peripheral intestine has under- 

 gone considerable reduction (Fig. 18). According to the degree of de- 

 generation, there may be distinguished with Weismann (No. 49) the 

 following five stages: — (1) viedusoids with canals in the bell, but with- 

 out marginal tentacles, usually also destitute of velum and sensory 

 bodies, manubrium without mouth, usually becoming detached at 

 maturity (Pennaria) ; (2) sessile medicsoids, bell usually without canals 

 or with incomplete ones, but with bell-cavity and bell-mouth (Tubu- 

 laria) ; (3) sessile gonophores, the wall of whose bell — still retaining the 

 entodermic lamella and two layers of ectoderm, but without canals or 

 bell-mouth — immediately encloses the manubrium (Clava, Hydractinia) ; 

 (4) sessile gonophores, the medusa-layers of whose wall are incomplete 

 (female Campanularia) ; (5) sporophores, i.e. sessile gonophores without 

 any trace of medusoid structure (Cordylophora). 



It is still questionable whether the sexual organs of Hydra are related 

 to the last of these groups, according to which Hydra would be an 

 extremely modified form, or whether we are not perhaps to regard 



