Prof. S. Loven on the Structure oftJie EcMnoidea. 287 



in both plates a small cavity, and in this a sphseridium, placed 

 as in the preceding. In the broad, perfectly smooth fm-row 

 which in Arachnoides occupies the middle of each ambula- 

 crum, nothing indicates the position of the sphajridia ; but we 

 find them concealed in the shell at the same distance from the 

 pores as in Clypeaster. 



Echinoneus has, near the tentacular pores on the first and 

 second plates, segregated globular sphasridia, Avliich are seated 

 uncovered in slight depressions. In this, as in much else, it 

 •resembles the regular Echinoidea. 



In most of the latter the splireridia are numerous and distri- 

 buted alternately on both rows of plates of the ambulacra. 

 Echinus Flemingi^ E. escidentus, Toxojmeustes drobachensis, 

 Loxechinus albus^ Tripneustes venfricosicSj Echinometra lu- 

 cunter^ and Amhhjpneustes ovum have their usually ellipsoidal 

 sphajridia arranged near the sutures, with the long axes nearly 

 parallel to the surface of the test. In Te'm7iopleurus^ 8al- 

 macis^ and Mes])ilia their form is spheroidal, and they stand 

 in the apertures of the deep cavities in the angles of the plates. 

 In all these Echinid^ the row of spha3ridia is separated on 

 both sides from the tentacular pores by the series of large 

 radiolar tubercles. In Diadema, on the contrary [Astropyga)^ 

 the sphffiridia are seated near the tentacular pores, and the 

 radiolar series of tubercles is situated between them and the 

 suture. Echinocidaris is quite different, as it has in each 

 ambulacrum only a single sphgeridium in a rounded notch in 

 •the suture quite close to the margin. In Cidaris sphaeridia 

 are not found. 



These organs, which arc so well and peculiarly protected in 

 many genera [Brissojysis lyrifera seems to protect its uncovered 

 sphseridia by binding together the neighbouring small radioli 

 over them), cannot be any thing but a sensorial apparatus, 

 probably destined for the perception of the changes which, 

 take place in the smTOunding water and in the substances 

 which this holds in solution or suspension, consequently an 

 organ of taste. Brissopsis lyrifera holds them quite still for 

 hours ; then follows a half-circular movement around the 

 point of attachment, which very soon ceases. The strong 

 nervous stem which runs internally along the suture in each 

 ambulacrum gives off alternate branches, one for each plate. 

 Every such branch enters with the tentacidar vessel into the pore 

 of the plate, and passes through this out to the outer side of the 

 test ; in this way it may furnish both tentacles and spha3ridia 

 with nerves, although I have not succeeded in demonstrating 

 this. It is easier, in Brissopsis lyrifera, especially in the 

 "bivium, to ascertain how the nerve, after issuing through the 



