REPORT ON THE CRINOIDEA. 69 
protected by it, is a group of three tentacles arising from the radial water-vessel. These 
groups of tentacles alternate on the opposite sides of the ambulacrum along the whole 
length of the arm, and they accompany its lateral branches on to the pinnules. 
In the irregular genus Actinometra the mouth is not central or sub-central, but 
excentric or even marginal. It is sometimes radial in position, as in Actinometra strota 
and Actinometra jukesi (Pl. LV. figs. 1, 2), and sometimes interradial, as in Actino- 
metra magnifica (Pl. LVI. fig. 7); while the number of ambulacra diverging from the 
peristome may vary, especially in the multiradiate species, from four to ten or even more. 
In most cases, however, the large interpalmar area which contains the central or sub- 
central anal tube is bounded by two large aboral groove-trunks, which form a horse-shoe 
shaped curve, and give off the ambulacra of the lateral and posterior arms. If the mouth 
be radial these grooves are often tolerably equal and symmetrically arranged as in 
Actinometra strota and Actinometra jukesi (Pl. LY. figs. 1,2). But in types with an 
interradial mouth like Actinometra magnifica or Actinometra stelligera (Pl. LVI. 
figs. 7, 8),’ the right or western limb of the curve is much larger than its fellow. [See 
p. 88.] The latter supplies the arms of the left lateral ray only; while the larger right 
limb represents the posterior ambulacrum combined with part or the whole of the right 
lateral ambulacrum as well. 
In many forms of Actinometra more or fewer of the arms which arise from the 
posterior and postero-lateral portions of the disk are in the ungrooved and non-tentacu- 
late condition, as has been already described; and the disk ambulacra corresponding to 
them are less developed than those of the normal grooved arms. In fact they may be 
altogether absent in some of the large multiradiate species such as Actinometra magnifica 
and Actinometra nobilis, both of which are remarkable for the presence of ungrooved 
arms on each ray. In Actinometra magnifica for example, which has an interradial 
mouth (Pl. LVI. fig. 7), not only are all the seventeen army of the hinder ray entirely 
unprovided with food-grooves, but more or fewer of the closely crowded arms on each of 
the other rays are in the same condition, as they do not receive branches of the dividing 
groove-trunks which supply their fellows. 
Faint lines are sometimes visible, indicating the directions which these grooves should 
have taken, z.¢., the positions of the simple water-vessels which are distributed to the 
ungrooved arms, but give off no tentacular branches. This is particularly well seen in the 
case of the posterior ray and parts of the hinder divisions of the two lateral rays of Actino- 
metra magnifica (Pl. LVI. fig. 7). The primary groove-trunk, which leaves the right lower 
corner of the peristome to supply the right and posterior rays, divides into two principal 
branches before reaching the equator of the disk in which the anal tube is situated. 
' The specific formula of Actinometra jukesi is—a. R. ae [ “ ; indiyidas | ‘ 
a 
* The specific formula of Actinometra stelligera is—a.2.2. as 
