ECHINOIDEA. 227 



The structure of the spines affords important systematic 

 characters for the diagnosis of families and to a certain extent 

 of genera.* 



Sphaeridia are small, densely calcareous, glassy, spherical 

 bodies, composed of a stratified vitreous substance and placed 

 upon short stalks of which the calcareous tissue is more reticu- 

 lar ; the stalks are articulated to prominences on the test. They 

 are found on the ambulacral plates only, and particularly on the 

 ambulacral plates bordering the peristome. They are covered 

 by a ciliated epithelium and round their base is a muscular 

 sheath and a sub -epithelial circular nerve tract as in the case 

 of the large spines. They may project freely, or be placed in 



FIG. 168. Diagram of a Spatangoid showing the fascicles (after Gregory), i internal, I lateral, 

 m marginal, p peripetalous, sa subanal fascicle. 



depressions of the test, which are sometimes completely closed 

 (Clypeastroida, Cassidulidae). They are probably sensory 

 structures, and, from their position near the mouth, they have 

 been supposed to be olfactory or gustatory. On the other hand 

 it has been suggested that they are for orientation. They are 

 to be regarded as modified spines. They are present in all 

 Echinoids except Cidaris. 



The jaws and five teeth appear to be present in all Echinoids 

 except Spatangoids. In the typical sea-urchins they form the 

 structure known as the lantern of Aristotle. The jaws consist of 

 a complicated framework of calcareous plates and rods by means 



* E.Hesse, NeuesJahrb. f. Mineral. Geol. u. Palaeont. Beilageband, 13, 

 1889-1901, p. 185. 



