KONGL. SV. VET. AKADKMIENS HANDUNGAR. BAND 19. N:0 7. 23 



maintains in the other species. vVcoording to Al. Agassiz it is indistinct in Spatago- 

 cystis, and absent in Echinocrepis. ') Upon the nature of the fasciola no fresh light is 

 thrown from its mode of existence in the Pourtalesia; it remains as obscure as 

 ever. The young of Abatus cavernosus, of 2,:^ mm., PL XIV, fig. 164, already pos- 

 sesses the peripetalous fasciola, minute knobs rather irregularly scattered conformably 

 to the elliptical outline of the test, and ti'aversing the interradia and the trivium, but 

 not the bivium. In the young of Echinocardium flavescens, of 1,7 mm., PI. XV, fig. 

 173, the star-like heads of the rods of the spinules of both fasciolse are very apparent, 

 sparse and easily counted, the peripetalous crossing the calycinal system, but not 

 touching the bivium, the ends of which it will traverse in the adult, causing, as 

 it seems, branchial leaflets to be replaced by simple tubular pedicels. Thus the fasciola 

 begins, along the greater part of its future course, with the development of solitary 

 spinules, whicli however are soon to become densely crowded and equal, like the nap 

 of a velvet. In the course of its growth'its position relatively to the underlying parts is 

 altered. In both the young specimens it does not cross the bivium, and in that 

 of Echinocardium flavescens it traverses even the calycinal system. I have elsewhere 

 shown, after a cai-eful comparison between a young, of 4,0 mm., and an adult Bris- 

 sopsis lyrifera, ") that in both the fasciohe traverse corresponding plates, and that the 

 movement becomes manifest solely from differences in the tracks of the former rela- 

 tively to the flgure and dimension of the plate. Upon the Avhole therefore the 

 fasciola may be said to become, at an early age, all but stationary, but not absolutely 

 so, as there seems to remain a small amount of reciprocal mobility. I once ventured to 

 show that the fasciola is a structural element independent of the skeletal systems, be- 

 longing, neither to the ambulacra nor to the interradial areas, but on the contrary, 

 so to speak, dominating them in some manner. No trace of it is to be seen on the 

 inner surface of the plates, and nowhere does it occupy their interstices. It is entirely 

 external to them, forming by itself a layer outside that of the spines, growing amidst 

 these, amidst pedicellariaj and pedicels, removing as much of them as lies in its v/slj, 

 and depositing in its stead its own band, incrusted with minute tubercles bearing the 

 densely packed club-shaped spinules. Sometimes it allows the markings it thus covers 

 to remain discernible through its substance, as in a specimen of Agassizia scrobicu- 

 lata, ^) in which the fasciola, intact and entire, like some gauzy tissue, lets clearly per- 

 ceive the underlying tubercles, which are perfectly recognisable as to place and form 

 and relation to the free tubercles contiguous to its margin, and parts of Avhich it even 

 covers. It is as though the fasciola, having caused the spine to drop, had grown over 

 its tubercle. Sometimes also, as in specimens of Plagionotus pectoralis and Brissus 

 Scillaj, ^) the fasciola has split, and the underlying tubercles stand forth in the cre- 

 vice, as though their spines had succeeded in resisting its subversive agression, and in 

 keeping the crevice open, in the one case aided no doubt by the presence of a pe- 

 dicel, which from its great muscular power was still more competent to check the ad- 

 vance of the spinules. 



') Rep. Chall. p. 141. -) Etudes, p. 62, pi. XXXVIl, fig. 213, 218. ») lb., p. G2, pi. XIII, fig. 121. 

 ••) lb., pi. XIII, fig. 122, 123. 



