MUSEUM OF GOMl'AK.V'J'IVE 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 entoderm 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 centi'al 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 iu 

 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, Elders ('90, pp. 64, 65) has described two sys- 

 tems for Ascopodaria which I have not seen in Urnatella, namely, lateral 



