XV] 



AND OF SEA-UKCHINS 



947 



tunity of consulting Mr C. R. Darling, who is an acknowledged 

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

 represented in Fig. 454 are no other than diagrammatic illustrations 

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

 in outline 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 outline which represents Asthenosoma, but the 

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



Fig. 454. Diagrammatic vertical outlines of various sea-urchins: A, Palaeechinus; 

 B, Echinus acutits; C, Cidaris; D, D', Coelopleurus; E, E', Genicopatagus ; 

 F, Phormosoma liiculenter ; G, P. tenuis; H, Asthenosoma; I, Urechinus. 



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

 or " Aristotlci's lantern," within, and the curvature of the outline 

 is accordingly disturbed. The elevated, conical shells such as those 

 of Urechinus and Coelopleurus evidently call for some further ex- 

 planation; for there is here some cause at work to elevate, rather 

 than to depress the shell. Mr Darling tells me that these forms 

 "are nearly identical in shape with globules I have frequently 

 obtained, in which, on standing, bubbles of gas rose to the summit 

 and pressed the skin upwards, without being able to escape." The 

 same condition may be at work in the sea-urchin; but a similar 



