Physiology of some Zoophytes. 387 
four, and these increasing in length reach its outer margin 
(fig. 3 a). These bars are hollow, are lined internally by a fine 
membrane, and almost entirely disappear when the polypidom is 
immersed in dilute muriatic acid. Neither these bars nor the 
three appendices to the cells above described, present themselves 
until the body of the cell and its containing polype have been 
fully formed. The spines attached to the cell are almost always 
four in number,—two to each superior angle of the cell,—are 
hollow, and the external two are longer and stronger than the in- 
ternal. The two former are of considerable thickness, and are 
oa, as long, sometimes more than twice as long, as the 
cell. 
The polype has from fourteen to sixteen ciliated tentacula, of a 
hght orange-colour, rather more than three-fourths of the lengthof _ 
the cell. The animal when retracted within its cell is folded up as 
in Flustra foliacea. Fig. 5 is a representation of the polype when 
expanded, and fig. 4 represents its appearance as seen from the 
posterior surface when it withdraws and folds itself within the 
cell. In this polype the part marked a in the figures had more 
of the appearances of an appendix of the stomach (6), or of a se- 
parate organ, than in some of the other ascidian polypes*. Its 
inner surface is so thickly covered with reddish brown granules, 
or more properly speaking, minute cells, that it is quite opake. 
Similar granules also adhered to the inner surface of the ceso- 
phagus (d) and stomach, and sometimes in greater number to 
the former than the latter. The inner surface of the pharynx 
(f), the cesophagus, the stomach, and a portion of the intestine 
(c) next the stomach are covered with cilia. A mass of dark- 
coloured egesta, apparently principally composed of the cells and 
granules thrown off from the inner surface of the digestive tube, 
is frequently observed about or above the middle of the intestine, 
and this part of the intestinal tube presents a dilatation fre- 
quently considerably larger than what is necessary to contain 
the inclosed mass. The polype in protruding itself first pushes 
out a short flexible tube attached to the inner margin of the aper- 
ture through which the tentacula pass. The muscles by which 
it withdraws itself within its cell are two in number,—one pro- 
ceeding from the lower and outer part of the cell, and dividing 
into two bundles as it passes upwards, which are attached to the 
sides of the lower part of the pharynx; the other arising from 
the lower part of the cell and attached to the lower end of the 
appendix of the stomach (fig.5@). The muscular bundles by which 
it protrudes itself cannot be distinctly traced from their proximity 
* From the contractility of these parts the form is not uniform, and in 
some individuals we find the stomach less and the appendix larger than they 
are here represented. 
