REPORT ON THE CRINOIDEA. 03 



But if the food-groove on the ventral surface of the arm or pinnule remains undeveloped 

 (PI. LXI. fig. 3), not only are the amhulacral epithelium, nerve, and blood-vessel absent 

 altogether, but the water-vessels are simple tubes like the integumentary water-vessels of 

 the Molpadidse, and have no lateral extensions, as tentacles are absent (fig. 4 on p. 

 113,21;). 



This condition may occur in a majority of the arms and even on the disk of 

 Actinometra (PI. LVI. fig. 7) ; on more or fewer of the lower pinnules of Antedon acoela 

 and Antedon angusticahjx (PL LIV. figs. 1-3, 5) ; and on the proximal pinnules of 

 Antedon eschrichti and Antedon rosacea, which receive no branches from the brachial 

 ambulacra. 



In Metacrinus, on the other hand, the ambulacra, and ^\-ith them the water-vessels, 

 of the large basal pinnules may start dii-ectly from the primary ambulacra of the 

 disk, or even from the peristome itself (PI. XXXIX. fig. 2 ; PI. XLIII. fig. 3 ; PL L. 

 fig. 2). 



The radial water-vessels which underlie the disk-ambulacra of the Comatulte 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 Ijlood- 

 vascular plexus in this position (PL LVII. figs. 1, 3, 4, Ir); 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 w^ater- 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 RhLocvinus 

 lofotensis. They are multiple in Bathycrimis, though not abundant ; whUe 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 — ivt). 



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 w^ater-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 



