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 (Pl. 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 body to which allusion has already been 
made, and which proved to be a supernumerary disk (figs. 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 mouth, 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 difficulty 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, fp. (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 through 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 
vresents 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 
