Prof. iS. Loveii on the Structure of tJte EcMnoidea. 285 



would prevent tlie application of this tlieorj to the great lakes 

 of North America. It is a singular fact, however, that whilst 

 both these Crustaceans have been discovered in Lake ^Superior 

 and also in Lake Michigan, only one of them has been found 

 in Lake Ontario, the Mysis seeming to be wholly wanting. 



XLIII. — On the Structure of the EcMnoidea. By S. Loven*. 



[Plate XIV.] 



Besides the well-known external organs, ocelli, spines, pedi- 

 cellariaj, the clavulfe of the fasciole, tentacles, and branchia3, the 

 recent Echinoidea possess another Icind of organs which have 

 hitherto been overlooked, although they occur so generally 

 that we seek them in vain only in Cldaris. These are very 

 small, button-like bodies, spheroidal, ellipsoidal, or somewhat 

 irregular balls, 0'll-0*375 millim. in their greatest diameter, 

 furnished with a short stalk, which is movably attached to a 

 small, slightly projecting tubercle. They may not unsuitably 

 be named splicvricUa. They are hyaline, shining, hard, solid, 

 and clothed with connective tissue rich in pigment, with epi- 

 thelium and a ciliated cuticle. Their pedicel has the reticu- 

 lated texture typical of the Echinoidea, which spreads more or 

 less distinctly and continuously around its starting-point. In 

 the direction of the axis of the ball we not unfrequently see 

 a tube which opens in its upper pole, and is either simple or 

 branched in a more or less regular manner. A gi;eat many of 

 the balls have on their surface small elevations, tubercles, or 

 spines — and many also depressions, which are sometimes shal- 

 low, but sometimes sink deeply in, towards the axis, in a 

 conical form. But the greater part of the mass of the ball is 

 formed of very numerous and very thin concentric layers ; and 

 there are some which do not preseait any thing but these. 

 Their solid contents are dissolved by a weak acid, so that only 

 the epithelium remains. 



The sphgeridia belong exclusively to the ambulacra (radii) ; 

 and in all the genera which possess them they are never 

 wanting on the peristomial plates, but differ in number and 

 distribution in a direction from the mouth. They always 

 occupy a definite position. In the Spatangida3 they stand, 

 generally uncovered, one, two, or more in a little group, by 

 the base of the tentacular cirri of the buccal area, near the 



* Translated by W. S. Dallas, F.L.S., from a separate copy communi- 

 cated by the author, from the ' CEfversigt af Kongl. Vetensliaps-Akad. 

 Forhandlingar,' 1871, no, 7. 



