1908.] 



AN ABNORMAL ECHINUS. 



649 



SO that ill the short series of three there are inckided the plate 

 formed immediately prior to the truncation (12), that formed 

 concurrently with the truncation (13), and that immediately 

 succeeding the truncation (14). The letters a and h refer to the 

 plate-rows in an area taken in counter-clockwise rotation. 



The above measurements indicate that, in those areas (4 and 5) 

 which bound the centre of disturbance, the plates formed con- 

 currently with the disturbance are on the whole slightly longer 

 than the corresponding plates in the other ai'eas, while the plates 

 immediately preceding and immediately succeeding the disturb- 

 ance are on the whole shorter. But the depth of the plates in 

 the affected areas is in every case greater than the average depth 

 of the corresponding plates in the normal areas. The details here 

 recorded are of value as showing to what measurable extent 

 definite portions of the test have been affected in the effort of the 

 organism to adapt itself to highly abnormal conditions. This 

 phase of regulation will be referred to later. 



The following measurements give some idea of the proportions 

 of the test in various directions : — Height = 55 mm. ; long axis 

 = 95 m^m. ; short axis = 89 mm. ; circumference at ambitus = 

 293 mm. 



(b) Detailed Description — Ahiormcdities in Plates. 



Examination of the elements which make up the test reveals 

 additional features of interest. The numbers of the plates in the 

 various interambulacral series are : — 



Table II. 



Area 



1 



2 



3 



4 



5 



Series 



a h 

 23 24 



a 

 25 



b 

 26 



a 



h 



a 



6 



a 



b 



No of plates.. 



24 



24 



22 



22 



22 



22 



Table showing the number of interambulacral plates in the respective series. 

 The heavy line indicates the relative position of the area of disturbance. 



In those areas which bound the incompletely developed ambu- 

 lacrum, all the series have suffered reduction of plates. Roughly, 

 they contain two plates shoi-t of the number normal for the 

 remainder of the test. Amongst the interambulacral plates there 

 is little abnormality other than the increase in length and breadth 

 in areas 4 and 5 already recorded, except in the two plates which 

 between them include the termination of the truncated ambu- 

 lacral area. These, instead of being rudely rectangular, are 

 pentagonal, the extra face abutting against the terminal plate of 

 the ambulacrum ; and not only is the aboral half of each longer 



