PEROPHORA ANNECTENS. 65 



to indicate that they have no function to perform different 

 from that performed by the remaining portions of the 

 epitheHum. I am also unable to determine that the cells 

 are essentially different in them. 



Histologically the digestive tract likewise presents some 

 conditions of structure that are worthy of notice. The 

 wall of the oesophagus is composed, as is apparently uni- 

 versal in Tunicates, of very regular, columnar, ciliated 

 epithelium, with a thin basement membrane on its ex- 

 ternal surface. In almost all cases the inner third of the 

 cells stains much more deeply than do the remaining por- 

 tions. 



As we pass to the walls of the stomach, a pronounced 

 change in structure is seen. The cells become much 

 higher and the cilia are absent. Fig. 27, pi. iii, represents 

 the typical structure in this region. As here seen, the 

 wall appears to be made up of uniform, high, cylindrical 

 cells, that are not in contact with ojie another. These 

 cells stain readily in hematoxylin, carmine or cochineal,' 

 and each contains a distinct, round, clear nucleus, in the 

 center of w^hich is seen a single large nucleolus. The 

 nuclei are situated, on an average, about one-fourth of 

 the length of the cells from their outer ends, and are 

 usually so great in diameter as to make them appear to 

 extend entirely across the cell. I have said that the epi- 

 thelium appears to be made up of these cells, which are 

 not in contact. What the actual structure is, becomes 

 obvious when a tangential section is examined, particu- 

 larly where such a section is at the niveau of the nuclei of 

 the cells described. A drawing of such a section, highly 

 magnified, is shown in fig. 28, pi. iii. We here see the 

 same large, clear nuclei, c. d., with a thin layer of stained 

 protoplasm around them, and among these cells numer- 

 ous much smaller, unstained, clear, circular areas, c. c. 



2d Ser., Vol. IV. ( 5 j October 26, 1893. 



