264 HERBERT C. CHADWICK. 



tion of the two disks. On the aboral surface the anus appeared as 

 a minute crescent-shaped aperture (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 immersion 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 communi- 

 nicate 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 supernumerary disk (figs. 

 3 and 4, g') is well developed and contains 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 

 a distance equal to the thickness of seventeen sections, and again 

 communicates with the exterior through the funnel-shaped projection 

 already described (figs. 2 and 5, f.p.). That this canal was a modified 

 ambulacral groove is shown by the epithelial cells which line it. They 

 are precisely similar to those which line the ordinary ambulacral 

 grooves ; and further evidence in the same direction is afforded by 

 the presence in its walls of numbers of the deeply staining proble- 

 matical bodies which are invariably seen in sections of the ambulacral 

 grooves of this species. Beneath the epithelium of the ambulacral 

 grooves the nerve-band can be recognised without difficulty in most 

 sections. The circular water-vessel (fig. 5, c.w.v.) and radial water- 

 vessels are also present, and from the former a considerable number 

 of water-tubes (fig. 5, w.t.) depend into the body-cavity. Water-pores 

 traverse the body-wall in all the sections and are abundant on the 

 interambulacral area, marked with an asterisk in fig. 1 (see also fig. 3, 

 w.p.). The skeletal and axial nervous systems present in the normal 

 disk are entirely absent in the supernumerary one ; so also is the 

 central plexus. 



The interesting question now arises — What was the mode of origin 



