80 PANAMIC DEEP SEA ECHINI. 



The anal system of Bolhriocidaris globulus Eichw. a.s figured by Jaeckel l 

 shows, as in the very .young Cidaridte, five anal plates in the angles of 

 the ocular plates, with secondary smaller and intercalcated plates. The 

 mode of growth of the ambulacra] zones is well shown in his figures, from 

 the position and size of the abactinal plates, and does not differ in any way 

 from that of recent Echinidae. It is natural that there should be no 

 difference from the mode of growth, in the pluteus of Echini and following 

 postembryonic stages, of succeeding pairs of tentacles immediately at the 

 base of the odd terminal tentacle. The difference in the number of inter- 

 ambulacral plates in the several rows is not more marked than in the 

 Cidaridse, where they vary from one to two plates, according to their 

 position with reference to the youngest interambulacral abactinal plate. 



It is by no means certain that the plate indicated as madreporic by 

 Schmidt and Jaeckel is to be considered as such. It is true that in Echini 

 the madreporic body is not always in a definite plate. It is not so placed 

 in Clypeastroids, and in many Spatangoids and in Echinothurias we sec it 

 encroaching upon the ocular plates. 



The existence of an interambulacral zone composed of a single row of 

 plates does not give us any clue to the mode of formation of the Palcechinid 

 type of tnterambulacra with its manifold rows of plates. 



An examination of specimens of Oligoporus missoitriensis Jackson, from 

 Webb City, Mo., and of Lepidechimis imbiicatus Hall, from Burlington, Iowa, 

 shows that young plates are added in the interambulacral area in the abactinal 

 region adjoining the ocular plates, forcing their way down on each side of 

 the genital plates and pushing the older plates towards the centre of the 

 interambulacral area. 2 But the exact mode of formation of new vertical 

 rows cannot be determined until very young specimens are available. 

 That they are formed at the aprcal system in the oldest known Echinids 

 is clearly shown by Jaeckel and Schmidt's figures. 



In Bothriocidaris the interambulacral zone stops short of the actino- 



1 Jaeckel, Otto, Uber die alteste Echiniden-Gattung Bothriocidaris, Sitzungs-Ber. d.Ges. f. naturf. 

 Freunde zu Berlin, Dec. is, isfii fig. l, p . 045. 



2 The abactinal system of Melonites figured by Keyes (Iowa Acad, of Sc, PI. XX, fig. lb) shows 

 plainly that new abactinal interambulacral plates are formed adjoining the genital plates. They 

 form a loop of six plates, the oldest in the centre. Immediately below there is a band of only four 

 or five plates; there are eight rows in the sixth licit, ami the same number in the equatorial region 

 and beyond the ambitus towards the actinal system. Unfortunately there are no figures of the 

 abactinal part of the ambulacra! areas, or specimens available to enable us to obtain au idea of the 

 mode of formation of the polyporous plates of the ambulacral system. 



