OCELLATE HYDRO-MEDUSA. 47 



edge of the umbrella, and on its outer surface, just above 

 the insertion of the velum, notice a dark-colored band 

 (Fig. 25, m), which encircles the body parallel to, but 

 just outside of and below, the circular chymiferous tube. 

 In favorable specimens this band may be seen to consist 

 of:- 



1. A surface-layer of thickened ectoderm cells, with 

 cilia upon their outer surface. 



2. An inner layer of nerve-fibres, with a few scattered 

 ganglion cells like those of the sub-umbrella. 



VIII. Examining pieces from various parts of the body, 

 trace out the general relations of the various layers which 

 have been noticed, and observe: 



1. The ectoderm (Fig. 25, 1). This covers the outer 

 surface of the umbrella, the radial tentacles, the outer sur- 

 face of the velum, the inner surface of the velum, the 

 sub-umbrella, the outer surface of the manubrium, and 

 the outer surfaces of the mouth tentacles. 



a. On the outer surface of the umbrella the ectoderm 

 cells are very much flattened, and as they are easily 

 detached, they may not be present in a preserved speci- 

 men. In a specimen which has been recently hardened in 

 osmic acid, their nuclei may be seen in a surface-view of 

 the umbrella. 



I). At the lower edge of the umbrella the ectoderm sud- 

 denly becomes thickened to form the ciliated epithelium 

 of the nerve ridge. 



c. On the radial tentacles the ectoderm forms a thin 

 layer with nematocysts. 



d. The outer and inner layers of epithelium of the velum 

 are continuous with each other at the free edge, and are 

 formed of thickened cells. 



e. The ectoderm of the sub-umbrella is very thin, and 

 only the scattered nuclei can be recognized. 



