REPORT ON THE CRINOIDEA. 93 
But if the food-groove on the ventral surface of the arm or pinnule remains undeveloped 
(Pl. LXI. fig. 3), not only are the ambulacral epithelium, nerve, and blood-vessel absent 
altogether, but the water-vessels are simple tubes like the integumentary water-vessels of 
the Molpadide, and have no lateral extensions, as tentacles are absent (fig. 4 on p. 
113, w). 
This condition may occur in a majority of the arms and even on the disk of 
Actinometra (Pl. LVI. fig. 7); on more or fewer of the lower pinnules of Antedon acoela 
and Antedon angusticalyx (Pl. LIV. figs. 1-3, 5); and on the proximal pinnules of 
Antedon eschrichti and Antedon rosacea, which receive no branches from the brachial 
ambulaecra. 
In Metaerinus, on the other hand, the ambulacra, and with them the water-vessels, 
of the large basal pinnules may start directly from the primary. ambulacra of the 
disk, or even from the peristome itself (Pl. XXXIX. fig. 2; Pl. XLIII. fig. 3; Pl. L. 
fig. 2). 
The radial water-vessels which underlie the disk-ambulacra of the Comatule pass off 
from the angles of the somewhat pentagonal water-vascular ring as single trunks, situated 
beneath the median lines of the ambulacra. But in Pentacrinus, at any rate in Pentacrinus 
decorus and Pentacrinus wyville-thomsoni, there is a radial extension of the labial blood- 
vascular plexus in this position (Pl. LVII. figs. 1, 3, 4, lv); and the two trunks which 
ultimately unite into the single water-vessel of the ambulacrum are thus kept separate 
from one another to a considerable distance, 1°5 mm. or more, from the edge of the 
peristome; that is to say, the angles of the water-vascular ring are produced in the 
direction of the rays, so that its outline is that of a short-armed Asterias rather than the 
more regularly pentagonal figure of a Goniaster. 
The ciliated water-tubes (‘ Steincaniile,” Ludwig) by which the water-vascular system 
communicates with the body-cavity, and thence with the exterior, vary very greatly in 
their development. The early Pentacrinoid has but one, situated in the same interradius 
as the fore-gut. In the later stages of Pentacrinoid life and in the young Comatula just 
free there are five, one in each interradius; and the same is the case in Rhizocrinus 
lofotensis. They are multiple in Bathycrinus, though not abundant ; while in the adult 
Antedon rosacea there are about thirty in each interradius; and in Antedon eschrichti 
and in Pentacrinus the number becomes still larger (Pl. LVII. figs. 1,3, 4; Pl. LIX. 
fig. 5—wt). 
The radial water-vessels of Comatula commence as single trunks arising from the 
water-vascular ring at the edge of the peristome ; and in a large Comatula like Antedon 
eschrichti the water-tubes may be found depending from the bases of the radial vessels 
beneath the middle line of the ambulacrum in the first two or three sections beyond the 
edge of the peristome. 
In Pentacrinus, however, the middle line of the commencing ambulacrum is occupied 
ho 
