1908,] AN ABNORMAL ECHrNUS. 659 



is almost impossible to conceive that, in a Sea-Urchin where the 

 morphological systems are welded together into a united whole, 

 a definite area could be lost in pai-t, in the way that the arm of 

 a Starfish could be lopped ofi". It is possible to conceive, however^ 

 that damage to the growing point of an area might check, 

 temporarily or permanently, the growth of that area. More- 

 over, it is not necessary to suppose that the damage be followed 

 by regeneration, although test-regulation must be an almost 

 inevitable consequent. We assume that, since the whole question 

 is one of major symmetries, the regeneration referred to by 

 Hamann is regenei-ation in a major symmetry as a whole (i. e., the 

 equivalent of the regrowth of the arm of a Starfish) and not the 

 insignificant substitution of new plates for broken ones, a form 

 of regeneration the comparative triviality of which is better 

 indicated by the term rejilace'ment (the reparation of Prouho). 



On account of these difficulties the phrase " incomplete re- 

 generation " cannot be taken to comprehend the connotation of 

 Bateson's Class (2), for neither loss nor regeneration is an essential 

 agent in producing such results as are included in that class. As 

 a wider designation and one which seems to include most of the 

 possibilities, we suggest arrested develop'ment. 



Congenital variation — facile phi-ase — might well account for 

 the original abnormalities, and test-adaption for the subsequent 

 and consequent distortions. But as an alternative to congenital 

 variation, reaction to immediate external influences appears to 

 ofier an explanation as probable and more simple. Although lack 

 of experimentation renders conjecture somewhat hazardous, it is 

 possible that some voracious enemy * or some wave-borne rock 

 fragment might break not only the newly-formed and extremely 

 delicate plates at the apical end of an ambulacral area, but along 

 with them might damage, either indirectly, by destroying the 

 controlling nerve, or directly, that portion of mesenchyme in 

 which the ambulacral plates are built up. The check to further 

 development received by the ambulacrum would give an oppor- 

 tunity to the interambulacral plates on each side to push outwards 

 and usurp the position hitherto occupied by the ambulacrum ; 

 and even if new generative mesenchyme were thereafter to be 

 regenerated in the old ambulaci-al position, it is conceivable that 

 by an adaptation of function these new plate-forming cells might, 

 instead of forming new and independent plates, reinforce the 

 interambulacral-forming cells, by depositing their calcareous 

 material along the edges of the interambulacral plates. Such 

 reinforcing power would account for the greater depth which 

 characterises the interambulacral plates between the abnormally 

 truncated ambulacrum and the apical disc. That in svich a case 



* Prouho has observed young Mullets not only snatching off the spines but even 

 raising the epiderm on the surface of Dorocidaris papillata. Such a wound is 

 sufficient to cause the underlying plates to be thrown off and replaced. Prouho, H.,, 

 "Recherches sur le Dorocidaris papillata et quelques autres Echinides de la 

 Mediterranee " : in Ai-ch. Zool. Exper. ser. ii, vol. 5, 1887, p. 250. 



