SPHiERIBIA. 



In addition to their well-known outer appendages, the recent Echini, with 

 the exception of Cidaris, possess external organs which have only lately 

 been detected b}' Loven ; he calls them Sphaeridia* 



They are small button-like bodies, — spheroidal, ellipsoidal, or somewhat 

 irregular balls, furnished with a short stalk, movable upon slightly projecting 

 tubercle. The sphaeridia are hyaline, shining, hard, solid, clothed with con- 

 nective tissue rich in pigment, with epithelium and a ciliated cuticle. Their 

 pedicel has the reticulated texture typical of the Echini, 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 great many of the balls have on their surface small ele- 

 vations, tubercles, or spines, and many also depressions, which are sometimes 

 shallow, but frequently sink deeply in towards the axis in a conical form. 

 But the greater mass of the ball is formed of very numerous and very thin 

 concentric layers, and there are some which do not present anything but 

 these. Their solid contents are dissolved by a weak acid. 



The sphaeridia belong exclusively to the ambulacra, and in all the genera 

 where they occur they are found in the peristomal plates. They always 

 occupy a definite position. In the Spatangidae they stand generally uncov- 

 ered ; one, two, or more in a little group by the base of the tentacular cirri 

 of the buccal area, near the side turned toward the median suture of the 

 ambulacra, decreasing thence the farther from the mouth, especially on the 

 bivium, — not unfrequently four, three, or two upon each of the first plates, 

 only one on each of the ordinary plates immediately following ; more nu- 

 merous on the trivium, in depressions or like rows of beads in narrow, elon- 

 gate, well-defined furrows. In Lovenia the segregated sphaeridia are con- 

 cealed under domes, which have a small, narrow, transverse opening at their 

 apex. 



* The following abbreviated account is tnken from the translation by Dallas, in Ann. and Mag., Oct., 

 1872, of Loven's article in the Oefversigt of Kongl. Vet. Akad. Forhandl., 1871. 



