THE GONOSOME. 



39 



In comparing the two classes of bodies with the view of determining their homological 

 relations, their composition out of the two membranes ectoderm and endoderm must be carefully 

 kept in mind. 



Fig. 13. 



1_... 



Diagrammatic sections ui' (ilianerocodonic and adelocodumc gouu[)liores. 



A, Phauerocodonic, and B, adelocodonic gonophore. 



a, Ectotheca; b, mesotheca or umbrella; c, endotheca, or ectodermal layer of manu- 

 brum ; d, spadix, or axile tube of manubrium ; e, ova ; f, radiating gastrovascular canals ; 

 g, circular gastrovascular canal seen in transverse section j h, marginal tentacle ; i, ocellus 

 in bulbous base of tentacle ; k, velum ; I, peduncle of gonophore ; m, general cavity of 

 ccenosarc ; «, mouth of medusa. 



In both sections the endoderm is distinguished from the ectoderm by giving it a darker 

 shade. 



Commencing with the central parts of a hydroid medusa, and comparing these with the central 

 parts of a sporosac, we shall find that in the medusa (woodcut, fig. 13, A) we have a manubrium 

 ill the form of a more or less elongated tubular body occupying its axis. The walls of the 

 manubrium are composed of two layers, an internal or endodermal layer [d] and an external or 

 ectodermal (c) ; and in all phauerocodonic gonophores or hydroid medusae of the sexual type, these 

 two layers become ultimately more or less separated from one another by the development of 

 the generative elements [e) between them. 



In the sporosac or adelocodonic gonophore also (£) we have a double-walled tubular body, 

 between whose two walls the generative elements are developed exactly as in the medusa ; but 

 while in the medusa this body is in almost every case perforated by a terminal mouth, in the 

 sporosac it is completely closed, so that it assumes, by the increasing volume of the generative 

 elements, the appearance of a sac, filled with ova or spermatozoa, and having a csecal diverticulum 

 (spadix) plunged into the middle of the mass. This caecal diverticulum [cl) is plainly the 

 equivalent of the endodermal portion of the manubrium in the medusa, while the wall (perigonium) 

 of the sac represents more or less completely the structures which in the medusa lie external to 

 the generative elements. 



When the perigonium presents its highest degree of development it consists as we have 

 already seen of three layers. Of these the inner (c) (endotheca) is the equivalent of the ecto- 

 dermal layer of the manubrium in the medusa ; the middle layer {b) (mesotheca) corresponds to 



