26 



AMERICAN HYDROIDS. 



metrical, as indicated above. It has occurred to the writer that this eccentricity of the aperture 

 of the diaphragm in the Sertularida' may be accounted for in much the same way as the lack of 

 radial symmetry. A nearly vertical position of the hydranth seems to be the most favorable, 

 and where the hydrotheca is sessile the base of the hydranth is forced away from the hydrocaulus 

 by the eccentricity of the aperture of the diaphragm, and it is thus made possible for the 

 hydranth to assume a vertical position without the tentacles being impeded in their action by 

 coming in too close contact with the hydrocaulus. 



78 



THE DIAPHRAfiM OF THE SERTt'l.ARln.E. 



Longitudinal sections of lower purls of hydtothcc;!- (highly magnified i. 



Fig. 74.Sertularia pumila; section taken in the median plane of a pair of hydrothec;e. <l, d, the diaphragm in section. 



Fig. 7.i. Same species; section taken in a plane at right angles to the last. Lettering as before. 



Fig. ~6.Hydrallmaniafalcatn. Lettering as before. 



Fig. 77. Thttiaria rubttxta. Lettering as before. 



Fig. 7s. Diphasia rosaaa: section similar to that represented in tig. 74. Lettering as before. 



GONOSOME. 



The structures which have to do with the reproduction of the species of the Sertularida' 

 include nothing aside from the gonangia and their contents, there being nothing to represent the 

 corbula? and phylactocarps so conspicuous in the Plumularida'. 1 The absence of these has been 

 offset in the Sertularida' \>y a much greater variety of gonangia and gonophores, especially the 

 former, which often exhibit ornamentations of surprising beauty. 



Gonophores, These structures are fundamentally the same as have been described under 

 the Plumularidae, but are often more conspicuous and highly specialized. Perhaps the most 

 common form is that exhibited by X< r1nl<tri<i pinnilii (tigs, sl-83). The blastostyle originates in 

 the usual manner as a sort of hernia-like diverticulum from the stem or branch, containing the 

 ectoderm and endoderm in their normal relations, and bearing a number of ova in the endoderm 

 (see tigs. 79, 84, <n>). Weismann says that there are three cell layers external to the ova, and homol- 

 ogizes these layers with those of a medusa, claiming that the gonophore is merely a degraded 

 medusa (see also Part I, p. 30). The distal end of the blastostyle is expanded into a round, more 

 or less obconical plug ("Deckenplatte" of authors), composed of large, loosely aggregated cells 

 principally from the ectoderm, although the endoderm is also involved in its formation. At a 

 later stage the blastostyle is crowded to one side by a rapidly growing gonophore, which is 

 formed as an outgrowth from the proximal end of the blastostyle, and grows to such a size that 

 it sometimes almost tills the cavity of the gonangium from top to bottom. When fully developed 

 (see fig. 79, ov), the ova are seen to have taken up their position in the ectoderm, having pierced 



'See Part I, pp. 31-35. 



