104 



COELENTERATA. 



the adjacent chambers in communication (Fig. 84, c). Between 

 these mesenteries, which join the oesophageal tube and are called 

 primary mesenteries, there may be intercalated mesenteries which, 

 as a rule, do not reach the oesophagus and are called accessory 

 mesenteries. There are secondary, tertiary, etc., accessory mesenteries. 



All the mesenteries 

 decrease in breadth 

 towards the base of 

 the polyp (Fig. 89). 

 The mouth open- 

 ing is rarely round, 

 but usually has the 

 form of a slit, at the 

 two ends of which 

 (or sometimes at 

 one end) there is a 

 groove lined with 

 long cilia. These 

 grooves are the 

 oesophageal grooves 

 or yonidial grooves 

 (siphonoglypJies). 

 They remain open 

 when the walls of 

 the rest of the 

 oesophagus are ap- 

 plied to one another 

 (Figs. 88 and 89). 



The tentacles are 

 hollow, and may be 

 smooth (Zoantharia, 

 Fig. 86) or pinnate 

 (Alcyonaria, Fig. 

 85), and are placed 

 in one or in several 



rows. Pores may be present at the tips of the hollow tentacles, 

 and on the side walls of the body, in which case they are called 

 cinclides (Fig. 84). In the genus Cerianthus and its allies there 

 is a large aboral pore. The mesenterial thickenings or filaments 

 are specially characteristic of the Anthozoa. They consist of thick- 

 enings of the endoderm containing gland -cells and thread -cells. 



FIG. 87. Longitudinal section through a Hexactinian (Phellia 

 limicola), from Chun, after Andres, a acontia ; c, c' septal 

 ostia ; g gonad ; m.f mesenterial filament; m.l longitudinal 

 muscles ; o mouth ; o' internal opening of oesophageal tube ; 

 oes oesophageal tube : s primary mesentery ; s' secondary 

 mesentery; s" tertiary mesentery; t tentacle. 



