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 arc longer and stronger than the in- 

 ternal. The two former are of considerable thickness, and are 

 generally as long, sometimes more than twice as long, as the 

 cell. 



The polype has from fourteen to sixteen ciliated tentacula, of a 

 light orange-colom*, rather more than three-fourths of the length of 

 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 (b), 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 oeso- 

 phagus [d) and stomach, and sometimes in greater number to 

 the former than the latter. The inner surface of the pharynx 

 (/), the oesophagus, 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 a) . 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. 



