94 THE VOYAGE OF H.M.S. CHALLENGER. 
by the. blood-vascular plexus (Pl. LVII. figs. 1, 4, U7), which has a water-vascular trunk 
on each side of it; and the water-tubes extend outwards in a radial direction as long as 
the water-vessels remain double. This is very evident in some horizontal sections 
through the upper part of the disk of Pentacrinus naresianus, which were made for 
Sir Wyville Thomson by Dr. Stirling. The double row of water-tubes may be seen 
extending along the sides of the ambulacra beneath the line of tentacles, to a distance of 
3°5 mm. from the peristome, and then it is not complete. 
The position of the water-tubes beneath the primary ambulacra is well shown im 
Pl. LVI. figs. 1, 38, 4. They are seen, as it were, coiling round the subambulacral 
plates to open below into the upper part of the circumvisceral coelom. When, however, 
the radial extensions of the labial plexus gradually thin out and the two lateral water- 
vessels unite into a single median trunk, the water-tubes become less numerous, and are 
only found in the first two or three sections beyond the point of union. They are thus 
really in the same position as in the Comatulz, if we consider the double water-vessels as 
expressing extensions of the angles of the water-vascular rmg in the direction of the 
rays. 
The water-pores which pierce the ventral perisome, whether it be plated or not, are in 
a close functional relation with the water-tubes. They are the openings of minute canals 
which are lined by columnar epithelium, and expand almost immediately into enlarge- 
ments where the epithelium is ciliated (Pl. LVIL. figs. 1, 3,4; Pl LIX. figs. 2, 4, 6—wwp). 
The inner end of the canal beyond the enlargement is lined by pavement epithelium, and 
opens into the body-cavity. According to Perrier’ the primary water-tubes of the early 
larval Comatule are directly continuous with the inner ends of the water-pores, without 
the intervention of the body-cavity. 
He has not yet figured this connection, however ; and after the careful observations of 
Ludwig upon the subject, with which my own are in complete accordance, I have 
considerable hesitation in accepting Perrier’s statements, especially as he admits that the 
water-pores of mature Comatulz do establish communication between the body-cavity and 
the exterior, just as described by Ludwig in the Cystid phase. This subject is discussed 
more fully elsewhere.” [See Appendix, Note D. | 
In the smaller and simpler types of Crinoids the water-pores correspond in number to 
the water-tubes. The young Antedon has one in each interradius ; and the same is the 
case in Rhizocrinus, the single water-pore piercing the oral plate. In Hyocrinus, 
however, the number of pores is larger. In both the specimens which I have examined 
there are two pores in the oral plate of the anal interradius, and there are no others in 
any of the anambulacral plates which lie between it and the edges of the radials. In the 
other interradii the disposition of the water-pores is as follows :— 
1 Sur le développement des Comatules, Comptes rendus, t. xeviii., 1884, pp. 444-446. 
2 On some points in the Anatomy of Larval Comatule, Quart. Journ. Micr. Set., N.S., vol. xxiv., 1884, p. 320. 
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