Abnormal Specimen ofAntedon rosacea. 197 



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 recognized 

 without ditficulty 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 onej so also is the central plexus. 



The interesting question now arises — What was the mode 

 of origin of the supernumerary disk? In answer to it two 

 hypotheses may, I think, be advanced : — 



1. That the supernumerary disk originated as a bud from 

 the normal disk. 



2. That it is the result of incomplete evisceration. 



In favour of the former hypothesis is the intercommunica- 

 tion of the body-cavities of the two disks — a condition of 

 things one would expect to find in a budding organism. 

 Against it is the entire absence of arms, skeleton, and axial 

 nervous system in the supernumerary disk. The compara- 

 tively large size attained by the supernumerary disk and the 

 fact that the remaining systems of organs had attained their 

 adult condition add importance to this objection. A still 

 weightier objection lies in the fact that, so far as I know, the 

 formation of a bud has never been observed in any Echino- 

 derm. 



I am indebted to Prof. Marshall for the second iiypothesis, 

 and it appears to me to explain the facts most conclusively. 



Though Antedon rosacea has never been proved to evisce- 

 rate spontaneously, eviscerated specimens frequently occur in 

 dredgings ; and the experiments of Prof. Marshall * and 

 Mr L>endy f have shown that evisceration may be and otten 

 is followed by complete regeneration of the visceral mass. 



* "On the Nervous System of Antedon rosacea" Quart. Journ. Micr. 

 8ci. xxiv. (1884) pp. o07-o48. 



t " On the lie<renerati(in of the Visceral Ma-^s of Anferlun munrcn.''' 



