13 



tube the cells are not so tiglitly packed, and eacli one 

 bears at its base a process containing a myopkan tkread 

 (PI. II., fig. 12). Tkese tkreads are so arranged as to 

 form a circular muscle band by tke constriction of which 

 the diameter of the tube may be diminished. 



It is a perfectly easy operation to scrape away on to a glass 

 slide a number of the endodermal myo-epithelial cells in 

 Alcyonium, and mount them for microscopic examination. 

 As a matter of fact they afford us the most readily 

 accessible example of this kind of cell which can be 

 procured. 



The Nervous System of Alcyonium can only be de- 

 monstrated in sections that have been specially prepared. 

 It consists of a few minute star-like cells situated in the 

 mesogloea close to the endoderm and ectoderm layers of 

 the polyps, connected together by very fine anastomosing 

 nerve fibrils. 



Mesogloea. — The substance of the mesogloea is, accord- 

 ing to Brown, chiefly composed of a Hyalogen. Previous 

 to the conversion of the Hyalogen into Hyalin it yields a 

 Mucin. It does not contain Gelatine, and consequently 

 to speak of it as a gelatinous structure is liable to mis- 

 interpretation. 



Sexual Organs. — In the month of April several 

 mesenteries bear close to their free borders little groups 

 of cells derived from the endoderm. These cells give rise 

 to the sexual cells, and at this stage the sexes cannot be 

 distinguished. Later in the year the groups of cells 

 become diiferentiated. In the females the protoplasm of 

 the cells composing a group fuses into a common mass, 

 the nuclei diminish in number, and at length there 

 remains only one large spherical cell with one nucleus 

 (See PI. III., fig. 19). This cell is a youug ovum. It 

 grows ver}- slowly in size, and spherical globules of some 



