1889-90-] On the Echinoidea or Sca-UrcJiins. 343 



as the Echinoidea or Sea-urchins — in many respects the most 

 highly developed members of the group. The name " urchin " 

 is often applied to that familiar animal the hedgehog, the 

 old Scottish form of the word being "hurcheoun," French 

 h4risson, from Mrisscr, to bristle, to stand up on end. The 

 sea-urchin is therefore known also as the sea-hedgehog, as 

 well as the sea-egg. In Prof. Edward Forbes's interesting book 

 on 'British Star- fishes and other Animals of the Class Echino- 

 dermata,' amongst the humorous drawings from his own pencil 

 in which the author so greatly deliglited, there is a most amus- 

 ing sketch of two boys at the sea-beach setting a hedgehog at 

 an echinus. The amazement of the two animals, strangers to 

 each other yet with a certain kinship in appearance, is evi- 

 dently mutual, as each sets its spines on end and prepares 

 cautiovisly for the fray. Beginning with the external structure 

 of the sea-urchin, the covering or box containing the animal 

 first claims attention. The problem has to be here solved of 

 a living creature, thus enclosed, constantly increasing in size, 

 and therefore requiring a house which shall continue to fit 

 its ever-altering conditions. The hermit-crab, as is well 

 known, under such circumstances simply " conveys " the 

 abode of another animal to suit its own needs, but the sea- 

 m-chin is far too much " attached " to its early home to permit 

 it to pursue such tactics. Its shell must therefore grow with 

 it, and remain with it during the whole period of its exist- 

 ence — never being cast or moulted like the shell of a crab, 

 for instance. This regular systematic growth is effected by 

 the shell being composed of numerous pentagonal plates, 

 accurately dovetailed together. The thin muscular coats which 

 line the interior and exterior of the shell cross and recross 

 between the joints or sutures of the plates, and secrete from 

 the sea-water the carbonate of lime which composes these 

 plates, and which is constantly being added in a regular man- 

 ner round the edges of each plate. New plates are also in- 

 serted at certain parts of the shell during the process of 

 growth. In one family of living urchins, however (the 

 Echinothuridee), and in some extinct forms, the plates are not 

 firmly attached by their edges, but are imbricated or over- 

 lapping, thus rendering the shell flexible. In the living 

 condition, the whole structure of the common egg-urchin'a 



