156 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
calcareous ring which forms the framework of the aquapharyngeal 
bulb. It is composed, as in all known Molpadiidae, excepting 
perhaps Trochostoma,! of ten pieces (Plate 5, figs. 64 and 65), five 
radial plates alternating with five smaller ones situated in the 
interradi. They are so closely bound together that they cannot be 
readily separated, except by the use of a solution of potassium or 
sodium hydrate. Each radial plate, as seen in looking upon its 
outer surface (Fig. 65, b), is of a somewhat hexagonal shape, 
tapering anteriorly and posteriorly from near the middle; its length 
(6-7 mm.) is twice its greatest breadth (3-3.5 mm.), and its thick- 
ness is about 1 mm. The plate is slightly notched in front to 
admit the radial water-tube at the point where the tube turns to 
run outwards to the body-wall; posteriorly the plate is forked. 
When viewed edgewise (Fig. 65, a), the plate is seen to be slightly 
concavo-convex. The external surface of the anterior part of each 
radial piece is slightly corrugated on one side for the attachment of 
the longitudinal muscles of the body-wall, and furrowed on the 
other side to receive a tentacular ampulla. The internal surface 
(Fig. 65, c) has a shallow furrow, running parallel to the long axis 
of the plate along its median line, which accommodates the radial 
canal as it runs forwards from the circular canal of the water-vascular 
system. 
Although the configuration of the external surface of the radialia 
in Caudina is not so irregular as in many holothurians, yet the struc- 
ture is to a considerable extent modified in adaptation to the over- 
lying tentacle on one side and the termination of the longitudinal 
muscle on the other. The position of tentacle and radial muscle in 
respect to each other results therefore in the symmetry of. radialia 
already described by Ludwig (8992, p. 87-88; "91%; 916). As in 
other Molpadiidae and in some Cucumariidae, the two right radial 
plates in Caudina arenata are congruous with each other, each having 
against the anterior part of the external surface a tentacle dorsad, 
a muscle ventrad ; likewise the two left radialia are congruous with 
each other, J. e., a tentacle dorsad, a muscle ventrad; consequently 
the radialia of the right side are symmetrical with those of the left 
both in position and form. The median, ventral radial plate in C. 
arenata may be congruous with the right radial plates, and therefore 
symmetrical with the left ones, as in the Hast-Asiatic Caudina 
Ludwig (89-92) states (p. 82-83) that T. arenicola has only the five radial plates; in 
T. oöliticum the plates are absent altogether. 
