660 ON AN ABNORMAL ECHINUS. [June 16, 



broken plates would remain to demonstrate the occurrence of an 

 accident is not to be expected, for the researches of Prouho* 

 show that, in Boi'ocidaris papillata at least, broken plates are 

 rejected and are replaced by new ones. All therefore that one 

 could expect to find in place of the plates originally damaged 

 would be plates irregular in outline and in structure, and such 

 exist in our specimen at the truncated end of the ambulacrum 

 {vide p. 653). 



It is more diiScult with a particular-accident explanation to 

 account for the abnormalities which occurred on all sides of the 

 test simultaneously with the destruction of the ambulacrum. A 

 serious accident to any animal is followed by a general loss of 

 vitality, and such a loss is very likely to be reflected in those 

 parts which are in most delicate equilibrium, that is, in the 

 parts where growth is taking place. Of the thirty abnormal 

 plates which occur in the band coinciding with the truncated 

 ambulacrum, twenty -two are deficient in the number of primitive 

 plates of which they are composed, while of the remaining eight, 

 four are deficient as regards the possession of pore-pairs. These 

 deficiencies agree well with the idea of loss of vitatility, for they 

 seem to indicate that at that period, primitive plates were formed 

 with less than the usual rapidity, the result being that owing to 

 the decrease of pressure from newly-formed plates, only two 

 pore-plates, instead of three, were compressed to form a compound 

 ambulacral plate. 



An explanation of the abnormalities in the Shetland specimen 

 such as that given above is of necessity largely conjectural. 

 Whether it altogether meets the requirements of the case must 

 be left to the decision of experiments, in which the natural 

 conditions surmised are reproduced as nearly as possible by 

 artificial methods. 



SUMMABY. 



The imperfect development of this test of Echinus esculentus 

 is expressed in :— 



(a) General shape. — Flattened, with a bulge at one portion of 

 the ambitus, and above the bulge a depressed area of the corona. 



(b) Majo7' symmetries. — The left posterior ambulacrum does 

 not exist after its twenty-ninth plate (counting fi-om the 

 peristome). The corresponding ocular plate is present, but its 

 shape is unusual and its pore is a mere pin-hole. 



(c) Plate details.— On the twenty-eighth, twenty-ninth, or 

 thirtieth plate of each of the ten ambulacral rows a series of 

 abnormalities begins, the abnormalities consisting, for the 

 most part, of deficiencies in the number of primitive plates 

 forming a compound plate. 



Less marked divergencies from the normal can be detected in 

 the sizes of some of the plates and in the irregular courses of 

 certain of the sutures. 



* Prouho, H., I. c. p. 251. 



