TIIH GONOSOME. 



41 



ectoderm traversed in a radiating direction l)y tubular extensions of the endoderin which lines 

 the body-cavity of the hydranth, and this disc would only need to become still further expanded in 

 order to show itself as an unmistakable umbrella, with radiating gastrovascular canals, whde the 

 hypostome or proboscidiform extension of the body, which in these genera advances far in front 

 of the base of the tentacles, would rescnd)le in all essential points the manubrium of the 

 medusa. 



Now, the commencement of such an expansion is evident in the hydranth of many Campanu- 

 laridm, while in certain species, as Laomedmflexuosa and Campanidina acuminata, the ectoderm of 

 the body is actually extended as a thin disc for a considerable distance in the plane of the tentacles, 

 which acquire in consequence the appearance of being connected at their bases by an intervening 

 web (woodcut, fig. 14). 



Ijongitudinal section through tlie liydrantli and hydrotbeca of Laomedea Jlexuosa, showing the u*eb-like membrane by 

 which the bases of the tentacles are connected to one another. 



a. Body of hydranth; b, hypostome, carrying the mouth on its summit; r, c, intertentacular web; rf, hydrotbeca. 



While the portion of the tentacles included in the thickness of the body-wall of the hydranth 

 will thus be the equivalent of the radiating canals of the medusa, their free portion is plaiidy 

 homologous with the free tentacles, which in the medusa hang from the margin of the umbrella 

 at the points corresponding to the entrance of the radiating into the circular canal, and which 

 must be regarded as strictly the continuation of the radiating canals beyond their apparent 



G 



