664 



ON THE SHAPES OF EGGS 



[CH. 



shell, that is to say a shell so strong that the influence of gravity 

 becomes negligible as a cause of deformation. The ordinary 

 species of Echinus begin to display a pronounced depression, and 

 this reaches its maximum in such soft-shelled flexible forms as 

 Phormosoma. On the general question I took the opportunity 

 of consulting Mr C, R. Darhng, who is an acknowledged expert 

 in drops, and he at once agreed with me that such forms as are 

 represented in Fig. 328 are no other than diagrammatic illustrations 

 of various kinds of drops, " most of which can easily be reproduced 



Fig. 328. Diagrammatic vertical outlines of various Sea-urchins : A, Palaeecliinus ; 

 B, Echinus acutus; C, Cidaris; D, D' Coelopleurus ; E, E' Genicopatagus; F, 

 Phormosoma luculenter; G, P. tenuis; H. Asthenosoma : I, Urechinus. 



in outUne by the aid of liquids of approximately equal density to 

 water, although some of them are fugitive." He found a difficulty 

 in the case of the outhne which represents Asthenosoma, but the 

 reason for the anomaly is obvious ; the flexible shell has flattened 

 down until it has come in contact with the hard skeleton of the 

 jaws, or "Aristotle's lantern," within, and the curvature of the 

 outhne is accordingly disturbed. The elevated, conical shells 

 such as those of Urechinus and Coelopleurus evidently call for 

 some further explanation ; for there is here some cause at work 



