THE GONOSOME. 



35 



5. The Blasfocheme {non-sexual Medusa). 



It has been already stated that the blastocheme presents, hke the phanerodoconic gonophore, 

 the form of a completely developed gyranophthalmic medusa. It differs, however, from the gono- 

 phore, not only in never producing generative elements directly, and in being thus, properly 



Fig. 9. 



Fig. 10. 



Sexual zooid (sporosac budding from a radiating canal 

 in the blastocheme oC Obelia geniciilnta. 



o, Portion of the umbrella, and b. radiating canal of 

 Jledusa of Campanularia the blastocheme ; c, spadix of sporosac ; d, perigouium 



Johjtsioiii, shortly after libera- consisting here of a single membrane ; e, ovum with 



tion from the gonangium, il- germinal vesicle and spot ; f^ ovum with numerous 



lustrating the peculiarities of germinal spots in the germinal vesicle, 



the blastocheme. 



a, liithocyst; b, incipient 

 tentacle; y, incipient sporosac, 

 formed as a bud upon the ra- 

 diating canal. 



speaking, non-sexual, but also in certain points of structure ; for it is almost universally charac- 

 terised by the absence of ocelli, and by the presence of peculiar capsules called lit/ioci/sfs, which are 

 attached to the margin of the umbrella, and enclose one or more transparent refractile corpuscles.' 



' An exceptional condition ia this respect is presented by a few medusje referable to the type of 

 the blastocheme. Thaumantias, as limited by Gegenbaur, has ocelli instead of lithocysts, and the 

 same is the case, according to Agassiz, in Staurophora, Brandt, and in Laodicea, Lesson ; while in 

 Melicertum, Oken, there are neither lithocysts nor ocelli. 



In Tiaropsis diademata, Agas., a well-defined pigment spot has been described by Agassiz as existing 

 in the base of the lythocyst, a statement which I can confirm by my own observations on a species of 

 Tiaropsis captured in the Firth of Forth. As will be afterwards seen, however, I do not regard the 

 pigment spot of Tiaropsis as representing a true ocellus. According to Strethill Wright, a medusa, 

 which has been described by him under the name of Goodsirea {' Edin. N. Phil. Journ.,' July, 1859J, 

 is a true gonocheme, and yet it has lithocysts instead of ocelli. As to Oceania octona, Fleming, and 

 O. turrita, Forbes — raedusse belonging to the type of the gonocheme — the statement of Forbes, 

 that thev have a lithocvst imbedded in the base of the tentacle, is founded on an error of interpretation 



