io6 A Guide to the Zoological Collections in the 



muscles which move the jaws. Next to this preparation is 

 dried and denuded test of Cidaris^ also opened to display 

 the "auricles", which however in Cidaris are upstanding 

 processes of the interradial plates. The ^- auricles " which, 

 like the "pyramids", jaws, and teeth, are five in number, 

 are known collectively as the " perignathic girdle ". 



The division of the shell into radii and interradii corre- 

 sponds with certain constant arrangements of the internal 

 organs : a radial water-vessel, accompanied by a nerve and 

 a blood-vessel, traverses each radius, while a generative 

 gland lies in each interradius. 



The details of the digestive, water-vascular, and repro- 

 ductive systems are shown in the spirit preparations of 

 Salmacis (Case 29), which also display well the mouth-parts 

 in situ, and the muscles by which they are moved. 



The above remarks apply only to the Regular Sea- 

 urchins— the Cidaroida and Diadernatoida — that have 

 spheroidal tests, at the poles of which the alimentary canal 

 opens. 



There are, however, two large groups of Irregular Sea- 

 urchins, in which the test is no longer spherical or even cir- 

 cular, and in which the mouth and anus are often excentric. 



The first of these groups is the Clypeastroida, of which 

 we may take Echinodiscus Ixvis as a type. 



In Echinodiscus Isevis (Cases 32-33) we find the body 

 to have the form of a flat shield-shaped cake or irregular 

 disk. On the flat under (ventral) surface we find, in a 

 central position, the mouth, and nearly midway between 

 the mouth and the edge of the disk the anus. Radiating 

 from the mouth are the ambulacral grooves, which bifurcate 

 and branch again and again, and have pores so minute as 

 to be visible only with a pocket-lens. 



On the upper (dorsal) surface we notice, in the centre, 

 the basal (genital) plates all fused together to form a single 

 large pentagonal madreporite, in only four of the angles of 

 which are genital pores found. Radiating from the apical 

 madreporite is a regular five-rayed " rosette", each petal 

 of which is formed by series of ambulacral pores. 



