196 Mr. H. C. Chadwick on an 



been forwarded to the Zoological Laboratory of the Owens 

 College by the authorities of the Zoological Station at Naples, 

 my attention was arrested by one to the disk of which a small 

 rounded body was attached. A cursory examination at once 

 showed the specimen to be one of very exceptional interest, 

 and my thanks are due to Prof. Milnes Marshall for per- 

 mission to examine and describe it. 



The disk (PI. VIII. figs. 1 and 2), which measured 7'5 

 millim. in diameter, bore the usual number of well-developed 

 arms, and with the exception of the displacement of one of 

 the ambulacral grooves, to be more fully described later on, 

 was in all respects quite normal. On its oro-lateral border, 

 however, it bore the tjody to which allusion has already been 

 made, and which proved to be a supernumerary disk (tigs. 1, 

 2, and 3, s.d.). Roughly spherical in shape and about 3 millim. 

 in diameter, it was attached to the normal disk by a sort of 

 stalk, which gradually narrowed from the oral to the aboral 

 surface. Near the centre of its oral surface was a well- 

 developed moutli, fringed with tentacles, from which five 

 ambulacral grooves radiated, just as do those of the disk of a 

 normal Antedon. Of these, four could with little difl&culty be 

 traced outwards to the aboral aspect. 



The remaining one (figs. 1 and 3, x) ran along the stalk of 

 attachment to the normal disk and joined the ambulacral 

 grooves of the pair of arms nearest to it, immediately after 

 crossing the line of junction of the two disks. On the aboral 

 surface the anus appeared as a minute crescent-shaped aper- 

 ture (figs. 2 and 4, a). Close to it was a minute scarcely 

 distinguishable pore, another rather larger aperture appearing 

 on the summit of the funnel-shaped projection, f.p. (figs. 2 

 and 5). The nature and connexion of these will appear 

 later on. 



Minute Anatomy. — Having carefully noted and drawn the 

 external characters of the specimen, I decalcified it by immer- 

 sion for twenty-four hours in a 10 per cent, solution of nitric 

 acid, and, after staining in borax carmine, I was fortunate 

 enough to obtain an unbroken series of sections by means of 

 the rocking microtome. From a very careful study of these 

 I find that the body-cavities of the two disks communicate 

 freely with each other tlirough the stalk or isthmus of tissue 

 which unites them, their alimentary canals, on the other hand, 

 being quite distinct. The alimentary canal of the super- 

 numerary disk (figs. 3 and 4, g') is well developed and con- 

 tains food. The ambulacral system is also well marked and 

 presents a feature of special interest. The minute pore close 

 to the anus, to which I have already alluded, opens into a 

 canal -like space (fig. 4, c), which traverses the body- wall for 



