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 jioint of 

 view, highly interesting construction of the apical area has been described and figured for ^l ha fits 

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

 p. 211 — 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 fasciole. 

 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 fasciole, which consists only of a broad band encircling both 

 apical and anal system, as is also the case in Abahis cavcriiosjis 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,' though the plate 5. a. 2 is longer than b. 2. The test is almost oval in circum- 

 ference, with a \-er\- sliglit siuuation at tlic 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 jjlates 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. F'ig. i). The fasciole 

 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 (PI. XIII. Fig. i). — Plates are now con- 

 tinually develophig 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 liackwards, till 

 it comes on tlie 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 ajncal system form the prominent abactinal keel; 

 the shape of the test is thereby \-cr)- much altered, as seen b\- a comjiarison of the Figs, g and 7, 

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

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

 area, viz. pa.s.sing just behind it. In specimens of ca. 6""" length its anal j^art cannot l)e seen from 

 above any longer. 



We may now follow the development of the abactinal aml)ulacra. The odd anterior amliulacrum, 

 which is at first not much l)roader than the paired lateral ambulacra, .soon enlarges consideralily, the 

 plates becoming much broader and coinjjaratively lower. The sinuatiou in the front edge becomes 

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

 adjoining antero-lateral interamlmlacra. At alxjut 4""" length the ]5ores become dnul)lc, the outer ]-)ore 



> Agassiz (loc. cit.) says of tlic- quite similar steinuiii in the young Abnlus cavcriwsus tliat it is •almost a truL- me- 

 ridosteroum*. As I have pointed out alii.v<- ip, S.|>, it is not at all nieridosteruous but typically amphisternous. 



