60 S. LOVEN, ON rOUKTALK.SIA, A GKNUS OF ECHINOIDEA. 



the petala, are in tliemselves uniutermptecl, »open» contiiuuitions of tlie ainbtihifral 

 series of plates, thiis in Cassiclulus, iii tlie Adetes, in Spatangus, Micraster, Maretia, 

 Echinocardium, Lovenia. When, in others, as in Agassizia, Meoma, Brissopsis, Kleinia, 

 Brissus, Eupatagus, Plagionotus, Breynia, they are »closed» inferiorly, this is an acci- 

 dental feature, the constriction being caused solely by the pressure exerted by the 

 peripetalous fasciola. ^) The petaline plates are longitudinally narrow, extended trans- 

 versely, and the peripodium, generally placed towards the external margin, incloses 

 with a narrow rim the always geminous, widely separated, transversely placed per- 

 forations, and unites them with a slender compressed ridge or with a furrow, a more 

 or less distinet suture marking the junction of the margins from either side. Upon 

 the whole a structure like this is found prevalent in the great majority of forms com- 

 ing into view from the first appearance of the Cassidulidffi. But among the Spatan- 

 gidsö their exists a gi-oup, of a feAv genera, differing widely enough in other respects, 

 but held together by a common character, the absence of petala. They are Apetalous; 

 whether Abranchian further researches will decide. Their ambulacra, instead of 

 being, in their dorsal portions, deepened and crowded Avith compressed plates, in order 

 to give room to numerous gill-leafiets, are all along even with the general surface, 

 and only gradually contracting up to the top, and their plates, as high as they are 

 broad, or nearly so, are regularly hexagonal, the pores being very minute, placed cen- 

 trally or subcentrally, and the pedicels small and simple. These genera are Pala30- 

 tropus, Argopatagus; Urechinus; Cystechinus; Genicopatagus; Aceste, Calymne, Aerope. 

 They are all abyssal. 



In the väst majority of Echinoidean forms, Arhseonomous and Neonomous, — 

 and presumably in the whole of them, — it is seen that the adoral and inner perforation 

 of the peripodium is prolonged into a short and narrow slit, cut through the wall, and 

 that this slit widens below into a separate but smaller perforation. This is very di- 

 stinctly seen in the Cidaridte, Echinida3, Echinoconidge, Echinoneida3, and, above 

 all, in the phyllodean peripodia of the Spatangidas. In those of the Cassidulidas, as 

 generally in the subanal and fronfcal, as well as petaline peripodia, it is less distinet, 

 but mostly to be recognised as a minute notch in the wall. This is the particular 

 little foramen that gives passage to the branch of the ambulacral nerve which is seen, 

 on the inside of the plate, to enter the pore along Avith the vessel, and, as easily, on 

 the outside, to emerge from it and distribute its branchlets, all through the connective 

 tissue, to the external organs. ^) 



Such is a sketch of the pedicels in the most prominent of Neonomous Echinoidea, 

 as they are variously specialised in order to meet requirements more varied than those 

 essential to the earlier types: developed into delicate organs of touch, or combining 

 Avith tactual function that of prehension, moditied for purposes still obscure, or evi- 

 dently subservient to respiration. I have dwelt at some length on these diversities, 

 with a vieAv not only to add a few facts more to those knoAvn already, but mainly to 



1) Étucles p. 62. -) Ib., p. 8, pl. II, fig. 29, 30, 31, 



