THE ANATOMY OF PHORONIS AUSTRALIS. 135 
easy to perceive the use of a sense organ in this region of the 
body, close to the anus and nephridiopores. It is the only 
structure, which I have been able to find, which in any way 
suggests a sense organ. 
The Alimentary Canal.—The digestive system consists 
of a U-shaped tube divided into four regions; there are no 
accessory glandular outgrowths. The mouth leads into a 
spacious oesophagus, which gradually enlarges at the proximal 
extremity of the animal to form a stomach, beyond which the 
tube is constricted, and bends upon itself; a second smaller 
dilatation may be called the intestine, from which the rectum 
passes as a straight tube to the anus (figs. 23, 24). The 
mouth, as already mentioned, is placed between the outer and 
inner series of tentacles, and is continuous with the lopho- 
phoral groove of each side; it is overhung dorsally to a slight 
extent by the semicircular fold, spoken of as epistome. 
The esophagus is elongated, in section, from side to side ; 
that is, it is depressed dorso-ventrally. Its walls are thick and 
have the same structure as the oral surface of the epistome, 
being lined by a ciliated epithelium; a few circular and longi- 
tudinal muscle-fibres are present. 
The cesophagus is suspended to the body wall by three 
mesenteries ; a short median ‘ cesophageal mesentery” (a.) ; 
and two longer “ lateral mesenteries,” (4. 5.), placed on that side 
of the cesophagus which faces the intestine (fig. 18). So that 
the celom is here divided into three cavities—a “ rectal 
chamber” lying between the right and left lateral mesen- 
teries; and a right and left cesophageal cavity between the 
median mesentery and lateral mesentery of each side. Other 
fibres, probably muscular, cross the ceelom from the cesophagus 
to the body wall. 
The stomach, which lies at the aboral end of the body, is 
almost hidden, in a dissected specimen, by a whitish mass, 
which consists of the gonads. The wall of the stomach is no 
thicker than that of the cesophagus, but the cells are arranged 
in a peculiar way. The epithelium is seen (fig. 33) to be 
thrown into folds, so as to leave shallow crypts at more or less 
