ECHINODERMATA 71 



trolled, both for opening and closing, as well as protruding 

 and retracting, by a somewhat complicated muscular 

 arrangement. This apparatus has been termed " Aristotle's 

 lantern." It is shown in the Figure just referred to. 



The internal anatomy consists of an alimentary canal, 

 extending from the base of the " Aristotle's lantern " to 



Fig. 28. Test and teeth of a Sea-urchin. SphcKrechinux 

 brevispinosus. 1 Natural size 



the anal opening at the apex of the test. Five large lobes 

 fill the greater part of the cavity. These are the repro- 

 ductive glands, and look as if made up of bunches of millet 

 seed. These may contain male or female elements 

 i.e. spermatozoa or ova but only the microscope can de- 

 cide which. The sexes, as we have said, are separate. 

 The products of these glands are expelled through little 



