io8 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



by a special development of longitudinally running muscles (Fig. 47, m) : 

 these are largest on the pair of mesenteries next the ciliated groove and 

 are smaller and less conspicuous on those furthest removed from it. 



The mesenteries are prolonged downwards beyond the truncated 

 end of the stomodaeum and are then attached only by one — the outer 

 — edge to the body wall, the opposite margin projecting freely into the 



coel:' 



■mes. 



Fig. 46. 



Alcyonium. Vertical section througii part of colony, coel, Coeienteron ; end^ endodermal tube 

 connecting coelentera of two neighbouring polyps ; m.f, mesenterial filament ; m.j*, dorsal, uncoiled, 

 mesenterial filament ; wes, mesogloea ; sp, spicule ; st, stomodaeum. 



coeienteron (Fig. 47, B). The free edge is thickened to form a con- 

 spicuous thread — the mesenterial filament {M.f). 



The coelenteric cavity of the projecting portion of the polyp is pro- 

 longed downwards into the substance of the colony, the mesenteries 

 being also prolonged downwards. In the case of the two mesenteries 

 furthest from the ciliated groove ("dorsal" mesenteries) the mesenterial 

 filament is straight, is continued right down into the basal part of the 

 coeienteron, and is covered with cells bearing powerful cilia which beat 



