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OF THE REGULAR ECHINOIDEA. 367 
In the walls of the intestine there are numerous small plates having irregular or 
smooth margins (Plate XLVIII. figs. 1, 2, 3). 
In the mesentery they are usually large, and in some species have an acicular or tri- 
radiate character (Plate XLVIII. figs. 4, 5); a superficial layer of irregular, generally 
delieate, reticulate spicula is frequently found on the surface of the mesentery, from 
which it can be easily removed, when, owing to their interlacing, its character as a 
membrane is retained (Plate XLVIII. fig. 6). Where the mesentery is united to the 
intestine, and on the inner border where the loops of intestine are strengthened by a free 
membrane, the plates assume the form of spines projecting from the membrane, in which 
their bases are inserted ; they have generally a solid axis, with a dextrally spiral arrange- 
ment of the areolæ on their surface (Plate XLVII. figs. 6, 7, 8, 9). 
The membranous and ligamentous bands at the base of the lantern are generally 
abundantly supplied with spicula; in the one which extends inwards from the trans- 
verse muscle they are sometimes of large size, interlocking and overlapping by their 
lateral processes (Plate XLVIII. figs. 9, 10); this membrane has sometimes a tuft of 
spines on its inner border (Plate XLVII. fig. 10). 
The spicula of the ambulacral tubes are most numerous towards their free extremity, 
and lie transversely to their length, are curved to adapt them to their cylindrical form, 
smooth on the concave and with rings of short spines on their outer or convex border ; 
they are frequently branched or acicular in character, these more irregular forms being 
usually of less refractive power. The dorsal ambulacral tubes have the spicula reduced 
to a few irregular caleareous threads; and the rosette, &e., which in the oral ambulacral 
tubes is well developed, is here represented by a few delicate plates of indefinite form 
(Plate XLVIII. figs. 11, 12). 
In Goniocidaris the walls of the intestine are crowded with triradiate spicula, having 
their points generally rounded, although frequently one is produced as a sharp curved 
spine projecting at an angle of 45? to the plane of the others (Plate XLVIIL fig. 13); 
towards the edge of the intestine they assume the form of perforated plates, but show 
well-marked evidence of their triradiate origin (Plate XLVIII. fig. 14). As in Cidaris, 
the intestine is fringed by spines along at least one of its borders; they are, however, 
more slender than in that genus; and the basal plate from whieh they spring being 
narrow and much produced makes them resemble the letter T (Plate XLVIII. fig. 15). 
In the ovaries they are very abundant, and coarser than in other parts, but retain the 
same triradiate plan of growth. (Plate XLVIII. fig. 16, 2.) : 
The spieula of the ambulacral tubes differ but little from those of Cidaris; they are, 
however, much smaller, and somewhat more branched and spinous in proportion to their 
size, than is generally the case in that genus. (Plate XLVIII. fig. 16, a.) ; 
In Diadema the intestine has imbedded in its substance a very few small irregularly 
curved and sometimes branched spicula, having often an enlargement about the centre 
(Plate XLVIII. fig. 18, a); in a membrane obtained from near the anus I found similar 
spicula, but much larger and more numerous (Plate XLVIII. fig. 17, 5); in a small 
Diadema from China they were much branched, and often on a triradiate type (Plate 
XLVIII. fig. 18, b). "ia 
F 
