METAZOA COELENTERA. 105 



The hydras budded from the star-shaped root become 

 differentiated into nutritive and reproductive zoons, the 

 latter giving rise to medusa buds. These medusae are 

 free in Obelia, Tima, and Gonionemus (No. 130, show- 

 ing young stages), but in Gonothyraea (No. 131 ; PI. 132, 

 enlarged) they are reduced and always remain attached. 



DISCOPHORA. 



We have shown that the complex larval colonies and 

 specialized adults of the Hydrophora may have arisen 

 from the comparatively simple Aeginopsis. It may be 

 possible also that this same form, or one similar to it, 

 produced along another line of development the Disco- 

 phora. The egg of most Discophora, like that of Aegi- 

 nopsis, develops into a free-swimming, ciliated, solid 

 embryo or parenchymella. This attaches itself and in 

 time becomes a hydra. By a remarkable growth the 

 height increases greatly. The body then begins to divide 

 horizontally, and the saucer-like divisions free themselves 

 as medusae. Thus it is seen that one hydra gives rise by 

 the process of division to several medusae. The latter 

 produce eggs which develop into the hydra form, so that 

 we have an asexual hydroid generation alternating with 

 a sexual medusa generation. The Discophora are repre- 

 sented in the Collection by Cyanea capillata Linn. (No. 

 133, greatly reduced), Aurelia flavidula Per. & Less. 

 (PI. 134), and Aurelia aurita Linn. (No. 135, young 

 medusa; No. 136, adult). It is interesting to note that 

 the adult medusa of Cyanea sometimes attaches itself 

 quite firmly to an object. One observed by Dr. Robert 

 T. Jackson, at Eastport, Maine, in the summer of 1892, 

 was settled so firmly that it required considerable force 

 to separate it from the tub in which it was living. While 

 attached, its resemblance to a hydra was striking. 



The segmentation of the egg of Cyanea is regular and 



