72 COELENTERATA [CH. 



polyps are developed, mesenteries and oesophagus making their 

 appearance. Something like these tubes does, in fact, occur 

 amongst Hydromedusae : a complete colony is found to consist of 

 a number of upright branches ending in polyps but connected at 

 their bases by tubes called stolons which creep along the sea floor: 

 the endodermal tubes of Alcyonium may be compared to these 

 stolons, the great difference being that in their case, owing to the 

 thickness of the jelly, the ectoderm is stretched uniformly over a 

 mass of tubes, instead of each tube having its own ectodermal 

 covering as in the Hydromedusae. 



Still examining a section of the polyp the next point we notice 

 is the structure of the mesenteries. These end in a free edge 

 below, which is much thickened and folded, and since it stands out 

 in contrast with the rest of the mesentery as if it were an inde- 

 pendent structure it has been called a mesenteric filament 

 (Figs. 30 and 32). The cells comprising six of these filaments are 

 very tall and secrete a juice which helps to digest the prey : the 

 remaining two filaments are composed of ciliated cells of moderate 

 height which maintain a constant outward current of water. But 

 there is reason to believe that both kinds of filaments are produced 

 by the downgrowth of strips of ectoderm along the edges of the 

 mesenteries starting from the lower edge of the stomodaeum. The 

 surface of the mesenteries is covered by cells which become very 

 much folded so as to produce a marked projection from the face of 

 the mesentery. The cells of the folded area are all produced into 

 vertical muscle-tails so that together they give rise to one of the 

 powerful longitudinal muscles (Figs. 30 and 31), by which the 

 sudden retraction of the polyp is brought about. The slow ex- 

 pansion is effected by the reaction of the elastic jelly or mesogloea 

 and perhaps also by the pressure exerted on the fluid contained in 

 the body by a layer of circular muscles developed as outgrowths 

 from the endoderm cells of the intermesenteric chambers. 



A second difference is found in the position of the eggs and 

 sperm cells. These are developed from the endoderm on the face 

 of the mesenteries, very low down in the base of the polyp and 

 nearer the free edge than the longitudinal muscle. The eggs when 

 ripe are cast out into the coelenteron and pass out by the mouth, 

 though in many species they come in contact with the male cells 

 whilst they are in the - coelenteron of the parent. 



The gullet has at one side a deep indentation or groove which 

 is lined by powerful cilia (2, Fig. 31). The groove is termed a 



