VELELLA. 113 



Attached to its outer surface are numerous groups of little oval bodies (figs. 5, 6, 7), the 

 smallest of whicli are simple csecal diverticula of the ectoderm and endoderm of the peduncle. 

 These, as they increase in size, undergo all those changes which have been described as 

 characterising the development of a medusiform zooid, so that in such bodies of less 

 than ruoth of an incli in length, one is able to distinguish a circular aperture in the free 

 truncated end of the calyx, and four wide, longitudinal canals in its walls. Along the 

 sides of these canals many peculiar, reddish-yellow vesicles, of about Tj^^^th of an inch 

 in diameter, containing two or three small granules, are discernible, and round tlircad- cells 

 appear in the outer wall of the calyx.' 



On the 23d of August, 1850, while traversing the South Atlantic Ocean, I found in the 

 vessel in which a full-grown Velella that I was examining was contained, a number of free 

 bodies more or less like these medusiform buds (fig. 8). The smallest were not more than 

 jijth of an inch in length, and were motionless. They very closely resembled the still 

 attached zooids, but there was an indication of four lobes around the mouth of the calyx, 

 and a small ovate sac containing much dark pigment, depended from the summit of the 

 roof of the calyx into its cavity. From these every intermediate stage could be found to 

 little "Medusa" one sixtieth of an inch in length, or thereabouts, which propelled 

 themselves by the vigorous contractions of their bell-shaped calyces (fig. 8 a). 



The circular mouth of the deep nectosac is surrounded by a narrow membranous 

 valve, and externally to this by four rounded lobes, whose surface, like that of the calyx, is 

 here and there beset with thread-cells. Four longitudinal canals traverse the calyx close to 

 its inner surface, and end below in ca3cal extremities, lines of the brownish-yellow vesicles 

 marking their course. An oval sac of -^^-jCw of an inch in length depends from the summit of 

 the cavity. It is closed at its extremity, and near its base is coloured by much dark pigment. 

 No trace of their original attachment is visible in these free gonophores. 



The substance of the above observations, accompanied by illustrative figures, was 

 published in Miiller's 'Archiv.'^ for 1851. They were, in the main, confirmed by 

 M. Vogt, in his ' Recherches,' published in 1854, but in the meanwhile Professor Gegenbaur 

 had indicated the ultimate fate of the Velella-zooids. This naturalist observed free-swimming 

 medusiform bodies perfectly similar to those just described ; and in the larger ones he found 

 two tentacles, with greatly enlarged ends, developed from opposite sides of the rim, at points 

 corresponding with the ends of two of the longitudinal canals. In these the yellow vesicles 

 formed eight series, and indications of the development of four more longitudinal canals, in the 

 intervals of the old ones, from the common cavity were discoverable. 



In fact, Gegenbaur adduces reasons for beheving that, eventually, sixteen such canals 

 are developed, and that the zooid becomes the " Medusa," which he has termed Chrysomitra 

 striata (PI. XII, fig. 13). 



The calyx of CUrysomitra is about three lines high, and has the same diameter at the 

 mouth. In the centre it is about a line thick, becoming gradually thinner towards the edge. 



' Vogt gives a view of one of the medusiform buds, like my figure, but he has missed the four 

 cauals. He states, further, that "the yellow cells lie in the interior of the canals;" but this, as 

 Gegenbaur has pointed out, is certainly incorrect. 



''■ ' Ueber die sexual-organe der Diphyiden und Physophoriden.' 



15 



