ON AN ABNORMAL SEA-URCHIN. 5D 
5. Note on an imperfectly developed specimen of the Sea- 
Urchin (Eehinus esculentus). By H. C. CHapwiok, 
Aol Se 
[Received October 30, 1913: Read February 17, 1914.} 
(Text-figures 1-4.) 
The subject of this note was collected and handed to me 
for description by Prof. W. J. Dakin, who found it upon the 
ruined breakwater at Port Erin during the autumn of 1912. 
Prof. Dakin’s attention was attracted to it by two well-marked 
depressions in the test, which were evident while the animal 
was living and covered with spines. The depressions are in inter- 
ambulacra 1 and 4; and, denuded of its spines, the test presents 
the appearance of a lump of plastic material which has been 
pinched by the thumb and forefinger. The spines of the entire 
test were distinctly larger and more densely crowded than those 
of a normal specimen of the same size from the same locality. 
The apical system presents two abnormalities. Genital 4 is 
of normal shape, but consists of two distinct plates united by 
suture. Two closely approximated but distinct pores occupy the 
position of the normal one on genital 1. 
As shown. in text-fig. 1, the composition of the first seven 
or eight plates of both series in ambulacrum II is irregular, 
that of the first four especially so. The plates numbered 5 
and 6 respectively in series @ ie to be composed of two 
primaries, the adoral of number 5 being imperforate. In the 
case of the elements which I have interpreted as plates 7 and 8, 
I conclude that dp’ is the demi-plate of plate 7; and but for the 
complete reversal of the position of the pores of the pair borne by 
the small plate lying between it and the aboral primary of plate 6, 
this might have been regarded as the adoral primary of the 
same plate. dp” is probably the demi-plate of plate 8. The first 
recognisable plate in series 6 is most probably number 5. 
Number 6 1s clearly defined, but there are no traces of sutures 
to indicate its composition. The position of the single pore-pair 
suggests that it is that of a demi-plate. Number 7 appears 
to consist of two primaries without a demi-plate. ‘The peripode 
of the element which I have interpreted as the aboral primary 
lies at right angles to the normal position and encloses only 
one pore. It is, however, possible that the pore-pair which 
lies avorally and to the right of this may be an element of 
plate 7, and that the large ellipsoid tubercle occupies the position 
of the pore-pair of the demi-plate. Should this view be correct, 
the next. plate, number 8, is practically a normal one. 
Proceeding now in the direction of the apical pole, all the 
* Communicated by Dr. F. A. Barner, F.R.S., F.Z.S. 
Proc, Zoom. Soe—_ 1914, No. V- +s) 
