658 MESSRS. RITCHIE AND McINTOSH ON [June 16, 



has become somewhat distorted. As m our case also, the 

 complement of apical plates is perfect, while the ocular pore 

 (Bateson says merely " ocular ") corresponding to the imperfect 

 area is absent. There is therefore a remarkable correspondence 

 between the two examples. 



Possible Origins of the Abnormalities. 



In the majority of such cases as have been described, authors 

 have made no reference to the probable origin of the abnormality. 

 ■Gauthier dismisses the case oi He7niaster hatnensis, above mentioned, 

 with the rather depreciatory remark, " il ne presente qu'une 

 .simple atrophie." Bateson in his remarks prefacing the summary 

 of the Echinoderm variations, says that " it cannot be dou.bted that 

 the variation[s] seen in Echini . . . are truly congenital. Similarly, 

 though in Asterias, &c., reduction in the number of arms might 

 otherwise be thought to be due to mutilation, it cannot be so in 

 Echini." * But while the majority of the abnormalities appear 

 to be congenital, so sweeping a statement must be avoided, since 

 it would preclude our even considering the possibility of reaction 

 to immediate external influences. For, although in the meantime 

 we cannot definitely point to any member of a major symmetry 

 which has demonstrably suffered alteration thi'ough external 

 factors, the occurrence of such alteration is not at all improbable, 

 considering the extraordinary sensitiveness of Sea-Urchins to 

 unusual conditions of environment t. 



Hamann, realising the difficulty of confining attention to only 

 one of several possible causes, says, in his resume of the form- 

 abnormalities in Echinoids, that, should the aberrancies not be 

 due to discontinuous congenital variation, their origin might 

 be set down to loss and subsequent regeneration or to fusion +. 

 Renamed in accordance with this conception of the potential 

 influence of external factors, Class (2) of Bateson becomes, 

 according to Hamann, that of incomplete regeneration (" unvoll- 

 standige Regeneration "). But even this conception confines 

 the possibilities within far too narrow limits. " Loss " implies 

 the previous existence of some part which disappears, and it 



* Bateson, 1894, I. c. p. 433. 



t Lo Bianco states that on the coast-line, where, previous to the 1906 eruptions 

 of Vesuvius, thousands of Echini had been scattered on the rocks, not a single live 

 specimen could he found subsequent to the ash showers. None of the other marine 

 invertebrate groups mentioned by Lo Bianco suffered to the same extent as the Sea 

 Urchins. Lo Bianco also demonstrates that in the case of the artificial introduction 

 of ashy material into a vessel containing Echini, the Echini had alreadj' begun to 

 putrefy on the morning of the third day after the experiment began, while two 

 days later the organs were completely macerated and the spines had fallen ofl'. 

 The rapidity with which the Sea Urchins succumbed shows sensitiveness to derange- 

 ment of function. Lo Bianco, Salvatore, "Azione della pioggia di cenere, caduta 

 durante I'eruzione del Vesuvio dell' Aprile 1906, sugli animali marini " : in 

 Mittheil. Zool. Stat. Neapel, Bd. xviii. Heft i. 1906, pp. 91 et seq. 



X "Wenn die Abnormitaten nicht sprungweise congenitale Varietaten sind so 

 tame fiir ihren Ursprung Verlust und nachtragliche regenerative Processe .... 

 -oder Verschmeltzung in Betracht." Hamann, 0., I. c, p. 1294. 



