IV 



PHYLUM CCELENTERATA 



189 



communicates separately with the exterior by an aperture near 

 the anus, through which the genital products escape. 



Fixed and Free Forms. A large proportion of Actinozoa are 

 permanently fixed, such, for instance, as most of the Stony Corals, 



FIG. 13S. Fenja mirabilis. A, the<entire animal ; B, with the body-wall divided longitudinally 



a. anus. (After Daniel.) 



the Sea-fans, Black Corals, &c. Most Sea-anemones are temporarily 

 attached by the base, but are able slowly to change their position : 

 some forms, such as Edwardsia (Fig. 136) and Ceriantlms, usually 

 live partly buried in sand enclosed in a tube formed of discharged 

 stinging-capsules, the oral end with its crown of tentacles alone 

 being exposed : others, such as Peachia and Fenja, live an actually 

 free life, habitually lying on the sea-bottom with the longitudinal 

 axis horizontal like that of a worm : a few, such as Minyas (Fig. 139), 

 have the aboral end dilated into a sac containing air and serving 

 as a float ; by its means these animals 

 can swim at the surface of the sea, and 

 are thus, alone among the Actinozoa, 

 pelagic. 



Dimorphism. With the exception of 

 one genus of Stony Corals, the Zoantharia 

 are all homomorphic, i.e. there is no dif- 

 ferentiation of the zooids of a colony. But 

 in the Alcyonaria dimorphism is common : 

 the ordinary zooids or polypes are ac- 

 companied by smaller individuals, called 



sipli07iozooids (Fig. 134, s.), having no tentacles, longitudinal 

 muscles, or gonads. 



None of the Actiniaria have a true skeleton : in some, how- 

 ever, there is a thick cuticle, and several kinds enclose themselves 

 in a more or less complete tube (Fig. 136), which may be largely 



FIG. 130. Minyas. /. float. 

 (After Andre.s.) 



