36 MORPHOLOGY. 



AVhilc it is itself noii-scxual, the blastoclicme always gives origin to special sexual biulsj or 

 gono])hores, which arc borne upon some part of the radiating canals. 



As characteristic examples of the blastocheme, we may adduce the planoblasts of Campanularia 

 ■Johnstoni (woodcuts, fig. 2, g, and fig. 9) — which are medusiform zooids referable to the deep- 

 belled section of Gegenbaur's genus Eucope — and those oi Laomedea dicliotoma and L.genicnlata 

 (woodcut, fig. 10) of authors — medusaj referable to Peron's type of Ohelia. In none of these 

 are sexual elements over directly developed, but instead of the direct formation of ova and 

 spermatozoa, there is produced a new zooid, which no longer presents the complete medusal 

 type, but is formed upon the plan of the adelocodonic gonophore. This zooid (woodcuts, 

 fig. 9, (/, and fig. 10) springs as a bud from the radiating canals of the medusa, and is 

 constructed upon precisely the same plan as that which we meet with in the gonophore of 

 Clava or Ilydracliivia, except that the perigonium would seem to be simple. It has an axile 

 spadix (woodcut, fig. 10, c), whose cavity is in direct comnumication with that of the radiating 

 canal {h) from which it springs. Immediately investing the spadix are the generative elements 

 (c,/) ova or spermatozoa; while these are themselves surrounded and confined by a true 

 perigonium [d^, which becomes at last ruptured for the liberation of its contents. 



The zooidal nature of these buds is nowhere more distinct than in the genus JgJatira, Per., 

 a form not yet traced to a polypoid trophosome. Here the generative elements are produced in 

 eight sac-like processes which surround the base of the manubrium, which is itself borne on the 

 extremity of a stalk dependent from the summit of the umbrella. These sacs are undoubtedly 

 true buds, and are entirely homologous with the gonophores of Clava ; and it is plain that they 

 are developed from the proximal extremities of the radiating canals, just where these canals pass 

 off' from the manubrium in order to run along the sides of its stalk before reaching the umbrella.^ 



In some blastochemes the sexual bud extends over a greater length of the radiating canal, 

 and presents, in consequence, a less defined and individualised appearance than in the instances 

 just mentioned, so as to lead one at first to hesitate as to the propriety of regarding it as a true 

 zooid. Such, for example, is its ciiaracter in the medusaj referable to the types of Thatnnantias, 

 Tima, and Melicerta, in all of which, while the generative buds are situated as in Obelia, on the 

 radiating canals, they occupy Mith their extended base so much of the canal as to be readily 

 mistaken for mere organs — ovaries or spermaries. Notwithstanding this, however, they are con- 

 structed upon essentially the same i)lan as the others, and offer no exception to the view here 

 taken. 



In Tima, indeed (woodcut, fig. 11), we have an extreme case of this extension of the base of the 

 generative buds, which here present themselves in the form of four long, flattened, sinuous frill- 

 like bands, each attached by one edge along the whole length of a radiating canal. When a section 

 is made from the free to the attached edge of this band (woodcut, fig. 12), the generative elements 



vvliile the view of the structure of Slabheria, Forbes, which would make this medusii to be a blasto- 

 cheme, having ocelli instead of litliocysts, is also, as will be shown below, based on an error. Though 

 it is thus very rare to find a blastocheme without litliocysts, the absence of ocelli in the gonoclicme 

 is quite common. 



' See Leuckart's description of Atjlaura Peronii (' Wiegmaun's Archiv,' 185G, Erster Band, S. 

 10). Tjeuckait recognises in the generative sacs oi Aylaura the significance of true zooids, though he 

 refrains from extending this view to the generative sacs of other medusse. 



