46 HANDBOOK OF. INVERTEBRATE ZOOLOCV. 



3. Examine the hull) at the tip of one of the branches, 

 and notice that the endoderm i.s wanting here, while the 

 greiitly thickened ectoderm is packed with large nema- 

 tocysts. 



IV. Cut off a portion of the nianubrimn. and tea/ing it 

 out in a drop of glycerine, notice the large granular endo- 

 denn cells which line its cavity, the transparent ectoderm 

 cells which cover its outer surface, and the supporting l.-iyer 

 i >et ween the two. 



V. Examine the inner surface of a piece of the umbrella, 

 and notice : 



1. The scattered nuclei of the greatly-flattened ecto- 

 derm cells which cover it. 



2. Under these the layer of longitudinal mu-cular fibres 

 which encircles the sub-umbrella, and which, by its con- 

 traction, drives the water out of the cavity, through the 

 opening of the velum. 



3. Here and there a dark brown stellate ganglion cell, 

 which consists of a central body with a nucleus, and two 

 or three long, fine, radiating nerve-fibres. 



4. Along the lines of the radiating chymiferous tubes, 

 notice a second layer of muscles, perpendicular to the 

 circumference of the umbrella. 



VI. Examine a piece of the velum, and notice : 



1. An outer layer of cells, continuous with those upon 

 the outer surface of the umbrella. 



2. An inner layer, continuous with those on the sub- 

 umbrellar surface. 



3. A thin, transparent, supporting layer, separating these 

 two layers of cells. 



4. The muscular layer of the velum, between the sup- 

 porting layer and the inner layer of cells. 



VII. The nerve-ring. Examine a piece of the lower 



