130 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
the same histological conditions obtain as in the genital duct, save 
that the longitudinal muscle fibers of the latter are there: replaced 
by a layer of circular muscle fibers, which are external to the 
connective-tissue layer and continuous with the layer of circular 
muscle fibers surrounding the rest of the genital tubules. 
Circular muscle fibers forming a sphincter are found in the 
connective tissue of the terminal part of the duct which passes 
through the genital papilla. This portion of the duct is separated 
from the body wall by diverticula of the body cavity, and the con- 
nective tissue at the tip of the papilla, where the wall of the duet is 
continuous with the body-wall, forms an exceedingly thin layer. 
The peritoneal epithelium covering the genital duet presents no 
peculiar conditions; it is like that of the genital tubules. 
According to Hamann (84) muscle fibers are lacking in the geni- 
tal duct of Synapta digitata and (83) Cucumaria bucumis. In 
C. frondosa, I find, however, numerous longitudinal muscle fibers. 
The conditions in Trochostoma Thomsonii, as described by Daniels- 
sen and Koren (82), appear to be somewhat similar to those in 
Oaudina, since these observers describe a muscle layer intervening 
between two layers of connective tissue, — the direction of the fibers 
is, however, not stated. 
The oval nucleus of each of the colummar or spindle-shaped 
epithelial cells lining the duct is situated near the free end of the 
cell (Figs. 104 and 105). The cells gradually diminish in diameter 
from the region of the nucleus to their bases, which terminate in the 
connective-tissue wall of the duct; at the other or inner end the cell 
body tapers more rapidly from the region of the nucleus, and ter- 
minates in along flagellum; the tapering free end of the cell body 
and the base of the flagellum are enclosed within a xollar-shaped 
prolongation, which arises immediately beyond the nucleus, on the 
side next the lumen of the duct, (Fig. 108). 
12. PHYLOGENY. 
A result of the study which I have made of the structure of 
Caudina arenata and other holothurians has been to strengthen 
in my mind the conviction that the Molpadiidae are more closely 
related to the Cucumariidae than to any other family. The prin- 
