Prof. S. Lov^n on the Structure of the Echinoidea. 437 



unpaired interradium (the episternum) and its plates 4 and 5, 

 and over plates 6, 7, 8, and 9 of the inner rows of the bivium. 

 The fasciola consequently grows nearly equally with the plates 

 of the test — but not perfectly ; for it shifts a little on the plate 

 within the limits of which it remains. On the inside of the 

 test no indication of it is seen ; it does not occupy an interspace 

 between the plates of the test, but is entirely external, and 

 belongs to a stratum of the test which lies without the radioli. 

 For we see sometimes, e. g. in Agasstzia, how, perfectly un- 

 affected and entire, it covers groups of radiolar tubercles which 

 are perfectly recognizable, as to form and arrangement, as through 

 a piece of gauze, and in size are but little inferior to those of 

 the same group which bear radioles immediately beyond its 

 margin. Or we see, for example in Plagionotus^ a crack in it ; 

 and through this the subjacent layer with its radiolar tubercles 

 sticks forth. It has fractures which run transversely ; during 

 growth joints arise, when its close rows of tubercles change 

 their direction ,* and sometimes such a joint coincides with the 

 suture between subjacent plates, sometimes not, when the 

 fasciola passes over it unaltered. Both the peripetalous and 

 infraanal fascioles contain special forms of tentacles which do 

 not overstep their boundaries. In Brissopsis the peripetalous 

 fasciola contains, in the unpaired ambulacrum, the powerful 

 tentacles with annular calcareous disks, and in the paired ones 

 branchige ; the infraanal constitutes the limit between the large 

 tentacles wreatlied with cirri, belonging to the inner rows of 

 the bivium, which are produced so that their pores fall within 

 its circle, and the simple, finger-shaj)ed ones, which issue from 

 the arabulacral plates of the sides. When we observe the 

 entirely external position of the fasciola, how it glides over 

 ready developed radiolar tubercles, how the most powerful ex- 

 ternal organs stand forth only within its circle, how in certain 

 genera (such as Plagiotiotus, Eu^yatagus^ and Breynia) the 

 tubercles of the test, which on one side of the boundary indi- 

 cated by it are small and but slightly developed, suddenly make 

 their appearance of large size and strongly marked on the other 

 side, and if we, moreover, note the opposition between the 

 dissimilarity of the regions thus distinguished in the Spatan- 

 gidge, and the thorough uniformity in the Cidaridge, Cassidulidas, 

 and Echmoneus, which have no fasciola, we are induced to ask 

 whether a membrane, perhaps following the largest circumfe- 

 rence of the test, may not cover the sides of the dorsal surface, 

 and then, in some forms, check the development of the radioli, 

 but beyond its border, which is the fasciola, leave two free fields 

 for the outer organs and the hard structures of the test, one 

 around the vertex and one infraanal. But this is of little con- 



