46 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
that in other holothurians. The wall of the radial vessel is composed 
of connective tissue, lined with a layer of Hat, ciliated cells. Longi¬ 
tudinal muscle libers exist between these two layers on one side of 
the vessel only; the side which, throughout the most of its course, 
is turned toward the exterior and is consequently adjacent to the 
lacunar vessel of the connective-tissue layer. A few of these libers 
accompany the radial canal into the aquapharyngeal bulb, being 
there naturally on the inner, or axial side of the radial canal, between 
it and the haemal vessel. Enveloping the radial canal and lying just 
external to the muscle fibers is a thin, structureless, hyaline membrane. 
e. Tentacles. 
Arrangement in interradii .— The tentacular ampulla which passes 
over each of the paired radial plates lies dorsad to the adjacent radial 
muscle attached to the plate. This is coordinate with the fact that 
in the dorsal interradius there are four tentacles, while there are 
three each in the right-dorsal and in the left-dorsal interradii. In 
like manner each of the paired radial canals sends two tentacular 
branches dorsad, one ventrad; thus the three interradii of the bivium 
possess in all ten tentacles. 
These conditions, as already pointed out by Ludwig (’91 a , ’91 c ), 
hold good for several species of Haplodactyla, Ankyroderma mus- 
culus and the East-Asiatic Caudina, which Ludwig has described as 
probably identical with C. caudata. The arrangement of the five 
remaining tentacles belonging to the two interradii of the trivium 
varies, however, in C. arenata in the same way as in A. musculus. 
For there may be either two tentacles in the left-ventral interradius 
and three in the right-ventral or rice versa. In the former case 
the median-ventral radial canal sends two branches to the right, one 
to the left,— a condition which obtains generally, according to Lud¬ 
wig, in the East-Asiatic Caudina which he described; in the latter 
case two tentacles are of course sent to the left, one to the right. 
Ludwig’s criticism (’89-’92, p. 588) of Yon Marenzeller’s state¬ 
ment (’82) in regard to the calcareous ring of C. arenata is entirely 
just. The statement in brief is that, if the calcareous ring be rolled 
out fiat and viewed from the abaxial surface, the tentacle is found to 
lie immediately to the right of the median line of each interradial 
plate, and that then comes the attachment of a muscle, whereas to the 
left of its median line there are two tentacles. Ludwm has alreadv 
