THE STETTCTDRE OF XENIA HICKSON[. 263 



Muscles. — The retractor muscles are situated on the ventral 

 faces of the mesenteries and the protractor muscles on the 

 dorsal faces, as in Alcyonium. These muscles are some- 

 what feebly developed, as might be expected from the non- 

 retractile nature of the polyps. Shortening of the retractor 

 muscles produces a slight contraction of the oral disc and 

 consequent formation of the funnel-like depression leading to 

 the mouth, to which reference has already been made (p. 251). 



Cells in Mesoglcea of Mesenteries. — On examining a 

 transverse section through the mesenteries, there is seen to be 

 a considerable quantity of mesoglcBa between the two endo- 

 dermic lamellae covering the mesentery (fig. 20). In this 

 mesogloea there are cells which have the reticulate protoplasm 

 and general appearance of endoderm cells. These cells migrate 

 into the mesoglcea from the endoderm covering the surface of 

 the mesentery, and even in a young polyp '8 mm. long a few 

 cells have already taken up their position in the mesogloea. 

 In older polyps there is a larger number of these cells in the 

 mesogloea of the mesentery, though they are not equally 

 numerous in all parts. In the upper portion of the polyp, 

 about the level of the stomodseum, the mesogloeal cells are few 

 in number and small in size, but from this part downwards 

 their number and size gradually increase, until in the mesen- 

 teries in the upper portion of the stem they are large and 

 numerous, and in some cases completely fill up the mesogloea, 

 so that the mass of cells is in close contact on both sides with 

 the endoderm covering the two sides of the mesentery. Towards 

 the base of the stem the cells become fewer in number and 

 slightly smaller in size. These cells are found in the mesogloea 

 of all the mesenteries, but they are less numerous in the dorsal 

 mesenteries than in the remaining six. Many of the cells are 

 at first somewhat elongated or pear-shaped, with one or more 

 processes in connection with, or pointed towards, the endoderm ; 

 but later many of them become rounded and larger, and their 

 nuclei become much larger. 



The primitive genital cells are derived from these large 

 rounded cells in the mesogloea of the mesenteries at the base 



