158 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
axial side of a radial calcareous plate, sends off three tentacular 
canals, bends outward at the anterior extremity of the aquapharyn- 
geal bulb (Plate 5, fig. 67), and then runs backward between the 
hyponeural canal and the body cavity to the tip of the tail, where 
it terminates in three blindly ending branches. Of these branches 
one is median and two lateral, the former lying in an anal papilla, 
the latter embedded in the connective-tissue layer of the body-wall. 
After describing these parts I shall treat of (e) the tentacles. 
The supposed rudimentary ambulaera of Caudina and other Mol- 
padiidae described by previous authors having been discussed under 
«Integument,"—in connection with which certain somewhat prob- 
lematical structures in Caudina are described, — I shall confine my 
attention in this part of the paper to certain unquestionably rudi- 
mentary ambulaera, which I have discovered in connection with 
(^) the three posterior branches of the radial canal, 
a. Circular Canal. 
The circular canal (Plate 5, fig. 66) surrounds the pharynx 
immediately behind the calcareous ring, and is situated as far from. 
the wall of the pharynx as are the posterior extremities of the radial 
plates, to which its anterior wall is attached. Strands of connective 
tissue covered with epithelium pass from the axial side of its wall to 
that of the pharynx and thus form a further support. The diameter 
of its lumen measures nearly or quite 1 mm. Its wall (Plate 6, fig. 76), 
though relatively very thin (13-20 a), is composed of five layers. 
These from without inward are (1) a flat, ciliated endothelium, (2) 
a comparatively thick layer of connective tissue, (3) an exceedingly 
thin hyaline, structureless membrane, separating “3” from (4) a thin 
layer of muscle fibers, which are circular, . e., lie in planes perpen- 
dicular to the direction of the canal,! and finally (5) an internal 
epithelium composed of flat ciliated cells. 
The connective tissue is composed of fibers — for the most part 
running parallel to the direction of the canal —embedded in the 
usual transparent matrix, which here is especially abundant. Stellate 
connective-tissue cells are scantily present; spheruliferous corpus- 
cles in great numbers. The circular muscle fibers are continuous 
1] find that in Cucumaria frondosa the muscle fibers are not parallel to the direction 
of the canal, as Semper (68, p. 123) asserts, but perpendicular to it. 
