ABNORMAL SPECIMEN OF ANTEDON ROSACEA. 265 



of the supernumerary disk i 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 intercommunication 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 comparatively large size attained by the supermimerary 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 Echinoderm. 



I am indebted to Prof. Marshall for the second hypothesis, and it 

 appears to me to explain the facts most conclusively. 



Though Antedon rosacea has never been proved to eviscerate spon- 

 taneously, eviscerated specimens frequently occur in dredgings ; and 

 the experiments of Prof. Marshall* and Mr. Dendyt have shown that 

 evisceration may be and often is followed by complete regeneration of 

 the visceral mass. 



These facts seem to me to make more than probable the supposition 

 that at an earlier period the specimen had suffered evisceration with- 

 out the visceral mass being completely detached. By the continuity 

 of the ambulacral grooves of two of the arms of the normal disk with 

 one of the grooves of the supernumerary disk a supply of food would 

 be ensured to the latter without seriously curtailing that of the former 

 during regeneration. In the paper just cited Mr. Dendy has shown 

 in how short a time the visceral mass may be regenerated, twenty-one 

 days being a sufficient length of time for regeneration to become so 

 complete that "there is little to distinguish a regenerated specimen of 

 this date from a normal Antedon except the small size of the visceral 

 mass and the want of pigment upon it." 



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

 Sci. xxiv. (1884) pp. 507-548. 



+ "On the Regeneration of the Visceral Mass of Antedon rosaceaa," 

 Studies from the Biological Laboratories of the Owens College, i. (1886) 

 pp. 209-312. 



