IV 



COELENTEEATA 



63 



enclosed in a special case, the gonangium. This gonangium is 

 secreted by a special outer layer of ectoderm, the mantle layer, 

 which breaks away from the inner ectoderm covering the medusa- 

 bud. In Halecium a set of endoderm tubes, like the radial canals, 

 grow out from the blastostyle and ramify in the mantle. 



Gotte interprets the series of forms which he describes, as steps 

 in the building up of the medusa out of what was originally nothing 

 but a lateral swelling on a hydroid person (as the member of a Coelen- 



FIG. 41. Four transverse sections through the developing medusa of Podoeoryne carnea 

 to show formation of circular canal and endoderm lamella. (After Gotte.) 



A, through base of young gonophore showing four taeniolae. B, througli older bud showing four 

 separate radial canals. C, through base of older bud showing the fringes growing out from the radial 

 canals which form the endoderm lamella. D, through upper part of older bud showing circular canal. 

 In this figure the roof of the umbrella-cavity is grazed. Letters as in previous figure ; en.l, endoderm 

 lamella ; t, taeniola. 



terate colony is termed). This swelling is caused by the genital 

 cells, and is therefore similar to the swelling produced by the 

 ovary or testis of Hydra. The majority of zoologists, however, read 

 the series in the opposite direction and, as it seems to us, with 

 infinitely more justice. They regard the " gonophores " as degenerate 

 forms of medusae, which once were perfectly developed and became 

 free, but have ceased to be detached, and so the structures which a 

 free medusa uses for swimming have become functionless in them. 

 How else can the umbrella-cavity of Clava, which never opens and 

 becomes completely resorbed, be interpreted ? 



