202 



ZOOLOGY 



SECT. 



transverse mesenteries. In such a form as Schizoimtlies (Fig. 151 , B) 

 there is thus recognisable an arrangement of the parts which might 



FlG. 151.— Antipatharia. A, oral face of zooid of Parantipathes. Ii, oral face of zoojd of 

 Scltiznpathe*. (After Delage et Herouard.) 



be interpreted as a dimorphism of the zooids, one set— the parts 

 containing the mouth and gullet — being regarded as gastrozooids, 

 and the others containing the gonads as gonuzooids. 



Fixed and Free Forms. — A large proportion of Actinozoa are 

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

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

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

 position: some forms, such as Edwardsia (Fiff. 149) and Cervmthus, 

 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 Pcachia, 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. 152), have the aboral end dilated into a sac containing 

 air and serving as a float ; by it? 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 



siphonozooids (Fig. 147, 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. 149). which may be largely 

 formed of discharged nematocysts. The simplest form of skeleton 

 is found in the solitary Alcyonarian genus Hartea (Fig. 144), already 



Fig. 152. —Minyas. /. float. 

 (After Andre*.)" 



