284 PROCEEDINGS OF THE ACADEMY OP [1885. 



We have noticed (Rev. ii, p. 31) narrow grooves upon the inner 

 surface of the vault, which meet beneath the median part of the 

 oral plate, and follow the subtegminal galleries which enclose the 

 ambulacral tubes. The condition of these grooves can be studied 

 most profitably from natural casts, in which they appear as string- 

 like elevations along the ventral surface. They have been ob- 

 served most frequently among the Actinocrinites, where they 

 seem to be universally developed, while no traces of them are to 

 be seen in the twenty or more casts of Platycrinus which we 

 examined. That they do not represent the ambulacral tubes, is 

 proved by the fact that these are always located at a distance 

 from the inner floor, as beautifully shown in the casts (PI. 4, fig. 

 5, iiinl PL 5, fig. 9), and wherever we found the tubes intact, they 

 occupy the same position. That the strings are in no way con- 

 nected with the tubes, is further shown by the fact that they always 

 meet in the centre, while the tubes form a ring around the centre, 

 as also by the irregularity which they exhibit. It is shown by 

 our figures (PL 4, fig. 4, and PL 8, figs. 1 and 3), that there are 

 always two of them side by side, which at places connect, and 

 again at others depart from one another, with irregular knots at 

 each bifurcation. This structure could not be explained if the 

 strings represented the inner cavity of the ambulacral tubes, as 

 these are very regularly arranged. That the grooves are placed 

 along the solid walls of the test, has led us to suppose that they 

 were axial canals, and that these Crinoids possessed an orocentral 

 nervous system like all other Echinoderms, but contrary to the 

 Neocrinoidea, in which the nervous system, as now generally admit- 

 ted, is connected with the chambered organ within the basal cavity. 

 Our interpretation becomes more plausible when we consider that 

 in the Camarata the radials arc never pierced by canals, and it 

 would be difficult to understand how these ponderous arms could 

 have moved without axial cords, unless their movements were 

 altogether passive. That the canals have been observed only in 

 certain groups, may be explained by supposing that in many 

 cases they probably rested againsl the wall, without piercing the 

 floor. 



That the perisome, wherever found in place, extends all the 

 way from the top of the first interradials to the central piece, is 

 very interesting, and shows a complete resemblance between the 

 ventral perisome of a recent Crinoid, and the body beneath the 



