504 ECHINODERMATA ECHINOIDEA chap. 



oral surface of the arms of Starfish, and hence in Urchins the 

 aboral surfaces of the arms have, so to speak, been absorbed into 

 the disc, so that the oral surfaces have become bent in the form 

 of a semicircle. The radii are separated from one another by 

 meridional bands called " interradii," which correspond to the 

 interradial angles of the disc of a Starfish and to the sides of 

 its arms. The small area enclosed between the upper termina- 

 tions of the radii is called the " periproct," and this corresponds 

 to the entire dorsal surface of the Starfish, including that of 

 the arms. 



One of the commonest species of British Sea-urchin is Ech Inus 

 esculentus. In sheltered inlets, such as the Clyde, it is often left 

 exposed by the receding tide, whilst everywhere on the coast in 

 suitable localities it may be obtained by dredging at moderate 

 depths on suitable ground. In the Clyde it is easy to observe 

 the habits of the animal through the clear still water. It is 

 then seen to frequent chiefly rocky ground, and to exhibit a 

 liking for hiding itself in crevices. Often specimens will be seen 

 clinging to the rock by some of their tube-feet, and, as it were, 

 pawing the under surface of the water with the others. In the 

 Clyde it feeds chiefly on the brown fronds of Laviinaria, with 

 which the rocks are covered. In more exposed situations, such 

 as Plymouth Sound, it does not occur in shallower water than 

 18 to 20 fathoms. At this depth it occurs on a rocky ridge; 

 but in 1899, after a south-west gale, all the specimens had 

 disappeared from this ridge, showing at what a depth wave 

 disturbance is felt. 



A full-grown specimen is as large as a very large orange ; its 

 under surface is flattened, and it tapers somewhat towards the 

 aboral pole. The outline is that of a pentagon with rounded 

 angles. The spines in Echinus esculentus are short in com- 

 parison to the diameter of the body, and this is one of the 

 characteristics of the species. 



The animal is provided with a well-developed skeleton, con- 

 sisting of a mail of plates fitting closely edge to edge, and 

 carrying the spines. This cuirass bears the name " corona " 

 (Fig. 227). It has two openings, an upper and a lower, 

 which are both covered with flexible skin. The upper area 

 is known as the "periproct" (Fig. 227, 2) ; it has in it 

 small isolated plates, and the anus, situated at the end of a 



