Ill] ACTINOZOA 69 



is comparatively easy to paralyse the members of the colony or 

 polyps by adding cocaine, or some similar reagent, to the water in 

 which the colony is living (Fig. 29). If then an expanded polyp 

 be cut off and examined with a lens, we shall be able to make out 

 most of its structure. We notice to begin with that there is a 

 single circle of eight tentacles, each of which has a double row of 

 short branches, so that it looks like a miniature feather ; within the 

 circle of tentacles there is, however, no trace of an oral cone ; there 

 is instead a flat disc, slightly sunken in the centre, where we find 

 the slit-like mouth. If we look in at the lower, cut end of the 



-,.4 



FIG. 29. Part of a colony of Alcyonium digitatum x 8, showing thirteen polyps 

 in various stages of retraction and expansion. 



1. Mouth. 3. Mesenteries with reproductive cells. 



2. Oesophagus. 4. Feathered tentacles. 



polyp we shall see that the internal cavity or coelenteron, instead of 

 being a simple cylindrical space like that of Hydra, is partially 

 divided into compartments by folds stretching in towards the centre, 

 but not meeting. These folds are called mesenteries, and there are 

 eight of them, corresponding in number (but not in position) with the 

 tentacles (Fig. 30). We shall further see that the mouth does not, 

 as in Hydra, open directly into the coelenteron, but leads into a 

 flattened .tube which projects into the interior of the body. This 

 tube, the so-called oesophagus or gullet, is really lined by the 



