48 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
lumen of the canal. The ordinary epithelium which everywhere 
lines the vessels of the system covers both sides of the valve. The 
turning of the concavity of the valve towards the tentacle and the 
presence of numerous blood corpuscles lying against this concave 
side in sections of the valve in action show that the valve serves to 
prevent the flow of fluids from the tentacle. 
The tentacular ampullae (Plate 5 , figs. 66, 67) hang free in the 
body cavity, as in other Molpadiidae and the Cucumariidae, being 
attached to the anterior part of the calcareous ring for a short 
distance only. They are slender tubular organs about 7 mm. in 
length, which taper to a point and end blindly at about the level of 
the ring canal or a little behind it. 
The anterior attached portion of an ampulla presents a different 
histological condition from that of the posterior blind sac. The 
latter has the simpler structure, being composed of (1) ordinary 
peritoneal epithelium, (2) a thin middle layer of connective tissue 
containing stellate connective-tissue cells and numerous amoebocytes, 
and (8) a Hat internal epithelium. In the anterior or attached 
portion of the ampulla, but only in that part of its wall which is 
nearest the body-wall, muscle fibers make their appearance between 
the inner epithelium and the connective tissue. These have a 
longitudinal direction, and run forward as a thick sheet, which is 
continuous with the muscular layer of the tentacle. 
f. Posterior Branches of the Radial Canal. 
As has been already stated, each radial canal ends blindly in an 
inconspicuous anal papilla, homologous to the terminal tentacles in 
the Asteroidea, Ophiuroidea, and Echinoidea. A pair of rudimen¬ 
tary ambulacra (Plate 6, figs. 79, 80) communicate by narrow 
openings with the radial canal near its end, where it opens into the 
terminal papilla. These lateral rudiments of ambulacra are situated 
within the connective-tissue layer and run outward nearly to the 
epithelium (Fig. 79), but there are no corresponding elevations of 
the surface of the body. 
The walls of the papilla (Plate 4 , fig. 50, pa. an.) consist from 
without inward of an outer epithelium of cubical or flattened cells, a 
thin layer of fibrous connective tissue without spicules, in which 
blood sinuses similar to those found in the genital tubules arise, a 
hyaline homogeneous sheath enveloping a layer of longitudinal 
