KONGL. SV. VET. AKADEMIENS HANDL. BAND. 19. N.M) 7. 39 



the ten being uniporous, and much alike in the disposition of their pores, and the peri- 

 stomal formula is sustained, in the greater number of genera at least, solely by the 

 superior size of the first plates of I a, II a, III b, IV a, V b, or, more distinctly, of 

 the corresponding second plates, one genus, however, Ai"achnoides, being exceptional 

 even on that point. The relations between arnbulacrals and interradials are also va- 

 rious, the latter preserving their series uninterrupted in Echinocyamus and Laganum^), 

 while in Encope and Clypeaster, Echinarachnius and Arachnoides Zelandiae ^), the first 

 plate of all is alone adinitted into the peristome, as is also in Mellita and Rotula ^) the 

 tirst plate of 1, 2, 3, 4, Avhile the interradium 5 remains entire, whereas in the adult 

 Arachnoides placenta*) all are excluded, the 5, 1 alone appearing in the vicinity of 

 the peristome. 



I have given the insignificant name of Sphseridia"), Spherids, to certain sense or- 

 gans, of function as yet obscure, present in all forms of the Echinoidea, the Cidaridse, 

 and perhaps Echinothuriadaj, excepted. They are very minute, calcareous, generally glo- 

 bular or pyriform, pedunculated bodies, articulated to little knobs of the test, and com- 

 posed, like the spines, of an axial reticular tissue, and external compact layers of 

 transparent glossy substance, and covered with a thin connective tissue, an epithel and 

 a ciliated cuticule. They belong exclusively to the ambulaera, and to the adoral por- 

 tions of these. In the greater number of the Echinidas they are uncovered and mostly 

 numerous, being found singie, one only in each ambulacrum, in the genus Echinoci- 

 daris") alone, which, together with the allied form, Coelopleurus, deviates from the 

 Echinidan type in other respects also, in the sub-petaloid ambulaera bearing bran- 

 chial pedicels, in the quadri-partition of the central piece of the calycinal system, 

 in the anomalous forms of the spines. The Clypeasti-idse offer two types: one, repre- 

 sented by the plurality of genera^), in which there is only a singie spherid for each ambu- 

 lacrum, more or less concealed in a cavity; and another, to which belong Clypeaster 

 and Arachnoides*), having in each ambulacrum two spherids contained in crypts hollow- 

 ed out in the substance of the test. In a like manner the multiple spherids of Cassi- 

 dulus"), at first free and uncovered, successively become overgrown by the superficial 

 layer of the calcareous substance of the test, until the site of each is marked merely 

 by a very minute opening. 



In all these groups, in which the bilateral disposition of the skeletal systems is 

 often but slightly, though unfailingly expressed, the spherids are distributed by equal 

 numbers, or nearly so, between the five ambulaera, which are also nearly of the 

 same structure. In the Spatangidte^"), on the contrary, where the bilateral differ- 

 entiation of the ambulaera in a trivium and a bivium, apparent already in the Ho- 

 lastridee of the Older Cretaceous period, is gradually carried out in a decided manner, 



1) Études, p. 32, pl, XLIV, XLV. '^) Ib. p. 32, pl. XLVI, XLVII, L, LIl. ■') Ib. pl. XLVIII, XLVI. 

 *) Ib. pl. LI. '') Ib. p. 1—11, 36, pl. I— V, -VII— X, XVII. «) Ib. p. 7, pl. X, tia:. 91, 92. ') Ib. p. 6, 

 pl. VIII, fig. 68—73. 8) Ib. p. 6, pl. VIII, fi<?. 74—78. ") Ib. p. 6, 36, pl. VII, fisr. 61—66. ">) Ib. 

 p. 6, 36, pl. III, tig. 32—40, IV, 42—45; V, 48. 



