112 ECHINOIDEA. II. 



posterior ambulacra end off the posterior edge of the anal area. The posterior genital plates are not 

 developed; the ocular plates as well as the abactinal plates of the paired ambulacra are rather indis- 

 tinct, but the course of the ambulacra is sufficiently distinct. The same, from a phylogenetic point of 

 view, highly interesting construction of the apical area has been described and figured for Abatus 

 cavernosus by L,oven (On Pourtalesia. p. 20 22, PI. XIV) and by Agassiz (Panamic Deep-Sea Echini, 

 p. 2ii 13. PI. 99). The plates of the anterior ambulacrum are comparatively large and elongate, with 

 single pores, and only two tube-feet in each series of plates have as yet appeared within the fascicle. 

 They are rather large as shown in Fig. 3. PI. XIII, but can by no means be said to be of very promi- 

 nent size. Especially interesting is the fascicle, which consists only of a broad band encircling both 

 apical and anal system, as is also the case in Abatus cavernosus of a corresponding size. The actinal 

 system is quite embryonal, round (PI. XIII. Fig. 4), the labrum not at all prominent. The sternum is 

 typically amphisternous, 1 though the plate 5. a. 2 is longer than b. 2. The test is almost oval in circum- 

 ference, with a very slight sinuation at the front, but the frontal ambulacrum is not deepened. The 

 shape of the test is rather flat, not at all globular, as is maintained by Professor Agassiz ( Blake >- 

 Echini, p. 78) to be the case in young Schizasters. 



In the course of the further development the following changes take place. The postero-lateral 

 ambulacra and the two series of plates of the odd posterior interambulacrum grow forwards along 

 each side of the anal system, which is by and by pushed backwards, and a pair of interambulacral 

 plates develop between the two large genital plates and the anal system (PI. XIII. Fig. i). The fascicle 

 now presents a very important change: from the primary fasciole has developed a transverse branch, 

 passing over the postero-lateral interambulacra and between the apical and anal system. This trans- 

 verse band, together with the anterior part of the primary fasciole develops into the peripetalous 

 fasciole, whereas the part of the primary fasciole posterior to the transverse band becomes the latero- 

 anal fasciole. - This stage is found at a size of 3""" length (PL XIII. Fig. i). - - Plates are now con- 

 tinually developing in the odd posterior interambulacrum, the new ones appearing at the posterior 

 end of the two large genital plates. Thereby the. anal area is pushed more and more backwards, till 

 it comes on the posterior edge of the test and is at last not at all seen from above. These inter- 

 ambulacral plates between the anal area and the apical system form the prominent abactinal keel; 

 the shape of the test is thereby very much altered, as seen by a comparison of the Figs. 9 and 7, 

 PI. XIII, representing side views of the test in specimens of 3 and 4-5""" length. The latero-anal fasciole, 

 of course, is gradually pushed more backwards, as it must retain its original relation to the anal 

 area, viz. passing just behind it. In specimens of ca. 6 mm length its anal part cannot be seen from 

 above any longer. 



We may now follow the development of the abactinal ambulacra. The odd anterior ambulacrum, 

 which is at first not much broader than the paired lateral ambulacra, soon enlarges considerably, the 

 plates becoming much broader and comparatively lower. The sinuation in the front edge becomes 

 gradually deeper, and at the same time the ambulacrum deepens, forming a groove, bordered by the 

 adjoining antero-lateral interambulacra. At about 4""" length the pores become double, the outer pore 



1 Agassiz (loc. cit.) says of the quite similar sternum in the young Abatus cavernosus that it is 'almost a true me- 

 riilosternum. As I have pointed out above (p. 84), it is not at all meridosternous but typically amphisternous. 



