200 



ZOOLOGY 



SECT. 



position : some forms, such as Edward 'sia (Fig. 150) and Cerinnthus, 

 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, live an actually free life, habitu- 

 ally lying on the sea-bottom with the longitudinal 

 axis horizontal like that of a worm : a few, such 

 as Minyas (Fig. 153), have the aboral end dilated 

 into a sac containing air and serving as a float ; 

 by its means these animals can swim at the 



J , n ,1 i Fia. 153. Minyas. 



surtace of the sea, and are thus, alone among the /. float. (After 

 Actinozoa, pelagic. 



Dimorphism. With the exception of one genus of Stony Corals, 

 the Zoantharia are all homomorphic, i.e. there is no differentiation 

 of the zooids of a colony. But in the Alcyonaria dimorphism is 

 common : the ordinary zooids or polypes are accompanied by 



smaller individuals, called 

 siphonozooids (Fig. 148, 

 s.), having no tentacles, 

 longitudinal muscles, or 

 gonads. 



None of the Actiniaria 

 have a true skeleton : 

 in some, however, there 

 is a thick cuticle, and 

 several kinds enclose 

 themselves in a more or 

 less complete tube (Fig. 

 150), which may be 

 largely formed of dis- 

 charged nematocysts. 

 The simplest form of 

 skeleton is found in the 

 solitary Alcyonarian 

 genus Hartea (Fig. 145), 

 already referred to, in 

 which minute irregular 

 deposits of calcium car- 

 bonate, called spicules 

 (sp.}, are deposited in 

 the mesoglcea. A similar 

 spicular skeleton occurs 

 in the 'Dead-men's finger " (Aleyonium, Fig. 154), where 

 spicules of varying form are found distributed throughout the 

 mesogloea of the ccenosarc. In Tnbipora (Fig. 149), the 'Organ- 

 pipe Coral," the mesogloeal spicules become closely fitted 

 together, and form a continuous tube for each polype, the 





l.">4. Alcyonium palmatum, A, entire 

 colony; B, spicules. (After Cuvier.) 



