Prof. S. Lovdn on the Structure of the Echinoidea. 429 



of series I. 5~V. «, both primary plates of which have coalesced, 

 has become depressed to a great extent ; and its sphasridimn, 

 which was the first to appear and stood on the margin of its 

 primary plate 1, has disappeared. Sphaeridium 2 still remains, 

 but, during the progress of the depression, has approached the 

 margin ; 3 stands, as before, on the binary large plate of series 

 I. ^-V. a, and 4 on the ternary plate of series I. a-V. h ] and a 

 new sphaeridium has made its appearance on the primary plate 

 1 of large plate 2 of series I. h-\. a. These spheeridia, 2, 3, 

 4, 5, are still present after the large plates 1 and 2 of both 

 series have coalesced into secondary large plates, and the animal 

 has become 11 millims. in diameter (fig. 6). But in a some- 

 what larger animal of 15 millims. diameter (fig. 7), which has 

 five sphajridia, these are not 2, 3, 4, 5, 6, but 3, 4, 5, 6, 7 ; for 

 the sphseridium 2, gradually approaching the margin, has 

 disappeared, a sphgeridium, 6, has been added apparently on the 

 primary plate 1 of large plate 2 in series I. a-V. Z>, and yet 

 another, 7, apparently on the primary plate 1 of large plate 2 

 in series L h-Y. a. Finally, at the size of 52 millims. (fig. 8), 

 when the ambulacral peristomial plates are large plates of the 

 third order, consisting of 1, 2, and 3 united, their six sphferidia 

 have the ordinal numbers 3, 4, 5, 6, 7, 8, the last of which 

 apparently belongs to primary plate 1 of large plate 3. Thus 

 by the resorption which takes place in the margin of the peri- 

 stome, two sphseridia have been lost, and one radiole with its 

 tubercle. 



Thus the asymmetry in the Echinoidean skeleton ^with relation 

 to its antero-poster tor axis ^\% expressed within each ambulacrum, 

 in its two subordinate rows of plates, most strikingly in the 

 arrangement, size, form, changes, and movements, during growth, 

 of the peristomial j^lates and those immediately following them, 

 in the number and position of their pores, in the order of the 

 appearance and disappearance of the sphasridia; and it will 

 probably not fail, upon closer investigation, in the relations of 

 the radioles and pedicellariaj. In eftect it abrogates tl: e radiate 

 plan of structure and displays the homologies between the 

 fepatangidaj and Echinidas, that even under the latter appa- 

 rently radiate form we find in the bivium, which symmetrically 

 encloses an unpaired interradium, a bilateral arrangement on 

 the two sides of an antero-posterior axis, which is the same in 

 the mature animal in all the different groups in the class. 



If in the peristome of Toxopneustes drobachensis (fig. 1, 

 Plate XIV.), or of some other Echinid, we unite by straight 

 lines the five plates I. a, II. a, III. J, IV. a, V. h^ and the five, 

 1. 5, II. J, III. a, IV. Z», V. a, two pentagons of the same size 

 and form are inscribed in the circle, each of which has three 



