CHAP. XIII 



ANTHOZOA = ACTINOZOA 



327 



body, attached at one end (the aboral end) to the common colonial 

 matrix or to some foreign object. At the opposite or free ex- 

 tremity it is provided with a mouth surrounded by a crown of 

 tentacles. In these respects, however, they resemble in a general 

 way some of the Hydrozoa. It is only when the internal 

 anatomy is examined that we find the characters which are 

 absolutely diagnostic of the group. 



In the Hydrozoa the mouth leads directly into the coelenteric 

 cavity ; in the Anthozoa, however, the mouth leads into a short 

 tube or throat, called the " stomodaeum," which opens into the 

 coelenteric cavity. Moreover, this tube is connected with the 

 body-wall, and is supported by 

 a series of fleshy vertical 

 bands called the mesenteries 

 (Fig. 146). The mesenteries 

 not only support the stomo- 

 daeum, but extend some dis- 

 tance below it. Where the 

 mesenteries are free from the 

 stomodaeum their edges are 

 thickened to form the im- 

 portant digestive organs 

 known as the mesenteric 

 filaments (rnf). It is in the 

 possession of a stomodaeum, 

 mesenteries, and mesenteric 

 filaments that the Anthozoa 

 differ from all the other Coelen- 

 terata. There is one character that the Anthozoa share with 

 the Scyphozoa, and that is, that the gonads or sexual cells (G) 

 are derived from the endoderm. They are discharged first into 

 the coelenteric cavity, and then by way of the mouth to the 

 exterior. In the Anthozoa the gonads are situated on the 

 mesenteries. 



Nearly all the Anthozoa are sedentary in habit. They begin 

 life as ciliated free-swimming larvae, and then, in a few hours 

 or days, they become attached to some rock or shell at the 

 bottom and immediately (if colonial) start the process of budding, 

 which gives rise to the colonies of the adult stage. Many of 

 the Sea-anemones, however, move considerable distances by gliding 



Fig. 146. Diagram of a vertical section 

 through an Anthozoan zooid. B, Body- 

 wall ; G, gonads ; M, mesentery ; mf, 

 mesenteric filament ; St, stomodaeum ; 

 T, tentacle. 



