150 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
the tube. The last is covered with a hyaline structureless cuticula, 
except in the small intestine. 
(a) The peritoneal epithelium of the alimentary tube is ciliated 
throughout its extent. It consists of cubical cells, which, treated 
with silver nitrate, present somewhat regular polygonal outlines 
(Plate 5, fig. 56). The diameter of the cells (10-16 a) is about the 
same as that of the epithelial cells of the reproductive organs, to be 
described later. 
In Cucumaria (Plate 5, fig. 53) the external epithelium of the 
stomach is composed of spindle-shaped cells, the deeper portions of 
which run through a layer of connective tissue, on the inner surface 
of which they terminate. 
(6) The outer layer of connective tissue underlying this external 
epithelium in Caudina is so extremely thin as to be scarcely notice- 
able except in the wall of the stomach. 
(¢) The muscle layer consists in theanterior part of the pharynx 
of circular fibers only; at about the middle of the length of the 
pharynx longitudinal muscle fibers are inserted in the connective 
tissue beneath the layer of circular muscle fibers. These longitudinal 
fibers increase in number from in front backwards, and throughout 
the extent of the stomach and small intestine form a continuous 
sheet immediately beneath the layer of circular muscle fibers. 
Whereas the latter is well developed from mouth to anus, the 
longitudinal muscle fibers are scanty in the small and large intestines 
and entirely absent in the cloaca, They are functionally replaced 
here, however, by about twenty isolated longitudinal muscles of 
small size (Plate 5, fig. 61, mu. ly.) lying externally to the band 
of circular muscles of the cloaca and disposed at irregular intervals 
in the external layer of connective tissue. 
The numerous strands attaching the cloaca to the body-wall are 
composed of connective tissue in which are embedded radial muscle 
fibers. Kach strand has a covering of peritoneal epithelium. 
In Cucumaria frondosa I have found the arrangement of the two 
muscle layers in the wall of the alimentary tube to be the same 
as in Caudina, +. e, the longitudinal fibers lie within the circular 
muscle layer (Fig. 53). 
The same arrangement of muscle layers has been found in all the 
Molpadiidae and Cucumariidae thus far studied. In Holothuria, 
