DICORYNE CONFERTA. 227 



adherent to its wall. In the male (fig. C)) the spadix is siiiTounded by a contiiuious mass of 

 spermatozoa. In the female (figs. 3, 4) the place of the spermatozoa is taken by ova, which are 

 invariably two in number, and are situated one on each side of the spadix, and in such a position 

 that only one is visible when the zooid is viewed at right angles to the plane of the two tentacles 

 (fig. 3). 



The endoderm of these tentacles is composed of large transverse cells, giving the usual 

 septate appearance to the axis of the tentacle, and by the careful employment of the compres- 

 sorium this structure may be seen to be continued for some distance into the walls of the 

 spadix (fig. 7), where, however, thft cells become less regular, while the cavity of the s])adix is 

 immediately surrounded by a layer of endodermal cells filled with reddish-brown granules. The 

 walls of the sac contain imbedded thread-cells (fig. 7). 



The ovum presents a well-defined germinal vesicle, in which a germinal spot is visible, or 

 maybe easily rendered so by slight compression. Each ovum is invested by a proper membrane, 

 which presents the remarkable and unique character of possessing considerable thickness and 

 being richly set with thread-cell-like bodies (fig. 7). 



It is plain, then, that the plauoblast of Bicoryne is a free sporosac, consisting of a simple 

 endotheca traversed by a spadix. To the base of the spadix the endotheca still adheres for some 

 distance, the generative elements not intervening here between spadix and endotheca so as to 

 separate them from one another. It is from the extreme end of this part that the two tentacles 

 are given off. 



The plauoblast of Bicoryne admits of a very instructive comparison with an ordinary 

 racdusiform gonophore. It is, in fact, a medusa in which the place of the umbrella and its canals 

 is taken by two tentacles, the manubrium of the medusa being represented by the rest of the 

 plauoblast. It will be recollected that the two tentacles are turned forward, while the sporosac is 

 still invested by its ectotheca, and that they then hold exactly the place of an umbrella 

 (mesotheca) between endotheca and ectotheca. They ai'c, in fact, the radiating canals of the 

 medusa reduced to two and developed as free tubes, instead of being immersed in the walls of 

 an umbrella. It will also be borne in mind that in the medusae of Ohelia, the umbrella with 

 its canals is frequently inverted and thrown back, so as to assume the position of the tentacles 

 in the plauoblast of Bicoryne, after this body has broken through the ectotheca and become 

 free. The free sporosac of Bicoryne is thus a medusa reduced to the condition of a manu- 

 brium and two opposite radiating canals. In order, indeed, to convert it into an ordinary medusa, 

 little more is necessary than to suppose the number of the tentacles increased to four by 

 the STOimetrlcal development of two others, their extremities connected by a circular canal, and 

 their sides by a continuous muscular membrane (umbrella) inflected at its free margin so as to 

 form a velum. 



It will be noted that the planoblast of Bicoryne is one of the three forms of locomotive 

 zooids which occur among the Hydroida, these three forms being the natatory medusa {Syncoryne, 

 Cladonema, Corymorplia, &c.), the ambulatory medusa {Eleitfheria, Clavatella), and the natatory 

 sporosac {Bicoryne). 



The planoblast of Bicoryne conferta always swims with its body in a vertical position, 

 carrying the posterior or tentacular extremity uppermost, and maintaining all the time a constant 

 rotation on its longer or vertical axis. 



30 



