MUSEUM OF COMPARATIVE ZOOLOGY. 13 
the atrium above and the aboral wall of the cloaca below. The cells 
surrounding the anal opening are slightly larger than their neighbors. 
Lying apparently in the basement membrane is the anal sphincter 
already mentioned. 
Under the term body cavity I include the space lying between the 
alimentary tract and the outer body wall and atrium. This space, 
which is much reduced in the retracted condition of the polypide, con- 
tains indifferent mesenchymatous tissue, tubular cells, muscles, the 
excretory and sexual organs, and the nervous system. 
The body cavity is bounded by no other epithelium than the ecto- 
derm of the body wall and atrium, and the entodérm of the alimentary 
tract; that is to say, there is no mesoderm. 
The indifferent mesenchymatous cells have been seen to best advan- 
tage in the end of a regenerating stalk shown in Figure 3. Here the 
cells, which were studied while living, could be seen migrating on the 
inner surface of the ectoderm, and extending through the central region. 
Such wandering cells were filled with highly refractive granules. 
The tubular cells of the body cavity lie chiefly at the base of the 
calyx, near its attachment to the stalk. In this region also I have seen 
in the living animal a flickering ciliate movement, and in the sectioned 
animal flame cells. 
I have been able to distinguish only two systems of muscles in 
the body cavity of Urnatella, — the sphincter of the atrium, and the 
muscles of the tentacles. 
The sphincter of the atrial opening (Plate III. Fig. 18, Plate IV. 
Fig. 26, spht. atr.) is composed of circular fibres lying in the lip of the 
atrium. Taken together, the fibres form a folded sheet, U-shaped on 
cross section, the convexity of the U being directed upwards. The 
function of this muscle is, of course, to constrict the atrial opening, and 
thus to protect the tentacles and parts below. 
The tentacular muscles consist of the pair to each tentacle already 
mentioned. These run from the apex of the tentacle to the base, where 
they diverge to the right and to the left, and, after breaking up into 
many branches, pass through the ectoderm to become inserted upon the 
cuticula of the body wall. Leidy (84, p. 10) saw these tentacular 
muscles. 
Differentiated muscles do not seem to be abundant in the calyx of 
any of the Endoprocta. Tentacular muscles are unknown in other spe- 
cies. On the other hand, Ehlers (90, pp. 64, 65) has described two sys- 
tems for Ascopodaria which I have not seen in Urnatella, namely, lateral 
