fi72 SPILERIDIA. 



A covering of this kind, which is tlic exception among the Spatangoids, is 

 the rule in the Cassidulidae and Clypeastridae. Khynchopygus has, on every 

 plate of the first live pairs in each ambulacrum, a sphseridium which is grad- 

 ually overgrown by the outer layer of the test, and which leaves finally only 

 a fine fissure open. (See PI. XV. f. ■: ; PI AT"./. >), 



The Clypeastridae exhibit two types. Echinarachnius, Lobophora, Mellita. 

 Encope, Rotula, Laganum, and Echinocyamus have in each radius only a 

 single sphanidiuni in common for both of its peristomal plates, and most 

 frequently, even in very young individuals, concealed in a crypt in the mass 

 of the shell. In Clypeaster and Araehnoides we have two sphseridia in 

 each ambulacrum, one in each of its two peristomal plates. Echinoneus bus. 

 near the tentacular pore on the first and second plates, segregated globular 

 sphseridia, which are seated in uncovered slight depressions. In this, as in 

 much else, they resemble the regular Echini. 



In the regular Echini the sphseridia are numerous, and distributed alter- 

 nately on both rows of plates of the ambulacra. In the Echinidae proper 

 they stand near the sutures of the plates: in the Diadematidae. near the 

 tentacular pores. In the Arbaciadae each ambulacrum has only a single 

 sphseridium in a round notch in the suture "lose to the margin. 



These organs, which are so well ami peculiarly protected in many genera. 

 cannot be anything lint a sensorial apparatus, probably destined for the per- 

 ception of the changes which take place in the surrounding water, and in 

 the substances which they hold in solution or suspension ; consequently an 

 organ of taste. The sphseridia make their appearance seemingly later than 

 the spines and pcdiccllarise. 



I have to thank Professor Loven for preparations showing the sphseridia 

 of several genera. 



