GEROULD: CAUDINA. 171 
Caudina the sexes are separate, Since it is now shown that Caudina 
is a dioecious form, it may be stated with confidence that in the 
family Molpadiidae, as far as known, the sexes are without exception 
separate, 
2. HISTOLOGY. 
'l'he walls of the genital tubules in both sexes consist of four layers 
of cells: (a) peritoneal epithelium, (5) circular muscle fibers, (c) con- 
neetive tissue, and (d) the internal, germinal epithelium. 
a. The peritoneal epithelium consists of flat, ciliated cells with 
polygonal outlines (Plate 6, figs. 81, 82). . The diameter of the cells 
is about 3 m, that of the nuclei about 1 a. 
In two forms of Cucumariidae which I have examined, Thyone 
briareus and Cueumaria frondosa, this external layer consists of 
peculiar club-shaped cells, such as Jourdan (’83) has described in 
connection with the testes of Cucumaria tergestina and the genital 
tubules of Phyllophorus, and likewise similar to those in Colochirus 
Lacazii as described by Hérouard (789). Jensen (283) has also 
described them in Cueumaria. It is probable that this club-shaped 
type of peritoneal epithelial cell is characteristic of the genital 
tubules of the Cucumariidae. The walls of the testes and ovaries in 
Thyone are, as I have found, ciliated. 
There are no muscle fibers differentiated from the peritoneal 
epithelial cells of the genital tubules in Caudina. In this respect 
Caudina differs from Holothuria tubulosa, where Hamann has 
described muscle processes running longitudinally. 
b. The only musculature of the genital tubules consists of long, 
slender, unbranched fibers (Plate 6, fig. 84), circular in cross section 
and tapering very gradually toward either extremity. They run 
around the tubule (Fig. 83) in planes perpendicular to its length; 
longitudinal muscle fibers are found. only in the genital duet. The 
nucleus is situated upon the side of the fiber about half way between 
its two extremities. The length of the fibers is so great that in the 
genital tubules of full-grown individuals they pass nearly or quite 
around the tubule, The one figured (Fig. 84) is of about the 
average size, 0.7 mm, long and 4 or 5 u in thickness. They are 
so abundant as to form a nearly continuous layer immediately beneath 
