THE STRUCTURE OF XENIA HICKSONI. 197 



Muscles. — The retractor muscles are situated on the ventral faces of 

 the mesenteries and the protractor muscles on the dorsal faces, as in 

 Alcyonhim. These muscles are somewhat 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 Mesogloea of Mesenteries. — On examining a transverse section 

 through the mesenteries, there is seen to be a considerable quantity of 

 mesoglcea between the two endodermic lamellae covering the mesentery 

 (Fig. 20). In this mesoglcea 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 mesoglcea. In older polyps 

 there is a larger number of these cells in the mesoglcea of the 

 mesentery, though they are not equally numerous in all parts. In 

 the upper portion of the polyp, about the level of the stomodeeurn, 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 

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

 numerous, and in some cases completely fill up the mesoglcea, 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 mesoglcea 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 mesoglcea of the mesenteries at the base of the polyp and 

 in the upper portions of the stem. That this is the case is shown by 

 the following : 



(1) These cells are most numerous in those parts of the colony 

 where gonads occur in greatest numbers. 



