GEROULD: CAUDINA. 161 
elongated oval shape, being 6-8 mm, in length and 3 mm. or less in 
diameter, according to the degree of contraction of the circula: 
muscle fibers in its wall. 
The wall (Plate 6, fig. 75) is composed of (1) an outer layer of 
flat, ciliated peritoneal cells, (2) a thick layer of connective tissue, 
composed, as in the circular canal, chiefly of fibers running in a 
longitudinal direction, (3) a layer of circular muscle fibers, about 
equal in thickness to that of connective tissue, and (4) an internal 
epithelium of thin, flat cells. Thus in its finer structure the Polian 
vesicle closely resembles the circular water-canal, the relative thick- 
ness of the muscle layer as compared with that of connective tissue 
is, however, much greater in the Polian vesicle than in the circular 
canal. 
d. Radial Canals, 
Each radial canal in the region near its opening into the circular 
canal has a much larger caliber than along the rest of its course 
(Plate 5, figs. 67-70, ag. .); this is shown by a series of cross 
sections through the aquapharyngeal bulb. A cross section through 
the aquapharyngeal bulb where the radial and interradial plates 
are united into a continuous ring (Fig. 70) shows the median-ventral 
radial vessel (ag. „d.) dividing into three parts. The two lateral 
branches (Fig. 71, ag. ta.) pass forward, diverging till they reach the 
anterior border of the calcareous ring, where each enters a tentacle, 
Anterior to this point (compare fig. 67) the radial vessel gives off 
— in this case on the /g/f side—a short branch (aq. ta!., fig. 
72), which is larger than the continuation of the radial canal 
itself, and opens into the third or most ventral tentacle; greatly re- 
duced in caliber, the radial canal then curves outward passing 
through the anterior notch in the radial plate, and runs backward to 
accompany the radial nerve and neural vessels to the posterior 
extremity of the body. The branches to the tentacles may, however, 
be given off at three different levels instead of at two, and the branch 
opening into the median-ventral tentacle may arise on the right side 
of the median-ventral radial vessel (Fig. 67), instead of on the left 
as in the example cited. I shall refer to this matter again in treating 
of the arrangement of tentacles in the interradii, 
The histology of the radial canals in Caudina (Plate 1, fig. 1; 
Plate 3, fig. 40; Plate 4, fig. 43; and Plate 6, fig. 79) is similar to 
