112 ECHINOIDEA. II. 



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

 developed ; the ocnlar piates as well as the abactinal jDlates of the paired ambulacra are rather indis- 

 tinct, bnt the conrse 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 Abatiis 

 cavernosiis by Loven (On Pourtalesia. jd. 20 — 22, PL XIV) and by Agassiz (Panamic Deep-Sea Echini. 

 p. 211 — 13. PI. 99). The piates of the anterior ambulacrum are comparatively large and elongate, with 

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

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

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

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

 s^'stem is quite embryonal, round (PL XIII. F'ig. 4), the labrum not at all prominent. The sternum is 

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

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

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

 Echini. p. 78) to be the case in j-oung Schizasters. 



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

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

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

 piates develop between the two large genital piates and the anal system (PL XIII. Fig. i). The fasciole 

 now presents a very imjjortant 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). — Piates are now con- 

 tiiiually developing in the odd posterior interambulacrum, the new ones appearing at the jjosterior 

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

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

 ambulacral piates 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, 

 PL XIII, representing side views of the test in specimens of 3 and 4'5'"'" length. The latero-anal fa.sciole, 

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

 area, viz. passing just behiud it. In specimens of ca. 6""" 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 erilarges considerably, the 

 piates becoraing 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 



I Agassiz (loc. cit.) sa3's of the quite similar stenuun in the jouiig Abatus cavcrnostts that it is < ahnost a true me- 

 ridosternum«. As I have pointed out above (p. 84), it is not at all nieridosternous biU typically aniphisternous. 



