ECHINOTHURID.E. 81 



stome ; in the Palaechinidae it is probable that the vertical rows stop in a 

 similar manner at certain points of the test, being excluded from the lower 

 part by the crowding of the older plates. But the lowest plates of each 

 row are certainly the oldest and not the youngest, if we are to judge by 

 analogy from the whole Echini group. In the young of Goniocidaris canali- 

 culata the interambulacral plates are also excluded from the buccal mem- 

 brane, as well as in Salenia and Strongylocentrotus. 



The disappearance of ocular plates in Pourtalesise, and that of some of 

 the genital plates and their scattered position in such groups as the Holas- 

 teridae, Ananchytidae, Pourtalesiae, and other Spatangoids as well as in other 

 groups of Echini shows how great may be the variation in the position 

 of the plates composing the apical system. 



The zones of interambulacral plates in recent Echini may in reality 

 be said to form in their earliest stages, single, irregular, zigzag, vertical lines 

 of five plates. 1 Although the formation of these plates has not been deter- 

 mined in the youngest postembryonic stages, _yet in the earliest stages 

 the interambulacral plates are not as 3'et united, but form calcified areas 

 arranged in a quincunx manner, the oldest of which are alternately on the 

 right or left, and finally appear to form two distinct vertical rows from the 

 disappearance of the primordial actinal plate, and the lateral crowding 

 of the older plates with their increase in number. 



That we have more than one primordial interambulacral plate followed 

 by the older interambulacral plates is evident from the earlier stages of 

 Echini figured by Midler, 2 and by Theel 3 on the development of Echino- 

 cyamus, where there seem to be five interambulacral plates formed at 

 the same time. 



In the Echinothurise we have some data, as in the genus Sperosoma, 4 

 showing the manner in which four vertical rows of ambulacral plates may 

 be developed on the actinal side from such simple primordial rows as still 

 exist in the Cidaridae and many of the Clypeastroids and Spatangoids. The 

 passage is quite abrupt from four plates to two on the abactinal side, so 

 that no clear idea can be formed of the mode of origin of the four plates 

 on the actinal side. Dr. Mortensen describes a young Sperosoma of 27 mm. 



1 Bury, Q. J. Mic. Soi., Vol. .'38 (1895-96), Pis. 7, fig 34 : 8, 6g. 36. 



2 Larven u. Met. d Oph. u. Seeigel. Erste Abhandl., PI. VII and Tte Abhandhl., PI. VIII, fig. 11. 

 8 Nova Acta Reg. Soc. Sc. Upsala, 1892, p. 50. 



4 Koehler, R., PI. Ill, figs. 3, 4, Fascicule XII, Echinides et Ophiures. Resultats des Campagnes 



scientifiques du Priuce de Monaco. 1898. 



6 



