200 



ZOOLOGY 



SECT. 



FIG. 153. Minyas. 

 /. float. (After 

 Andres.) 



position : some forms, such as Edwardsia (Fig. 150) and Cerianthus, 

 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 

 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 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 " (Alcyonium, Fig. 154), where 

 spicules of varying form are found distributed throughout the 

 mesogloea of the ccenosarc. In Tubipom (Fig. 149), the " Organ- 

 pipe Coral," the mesoglceal spicules become closely fitted 

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



FIG. 154. Alcyonium p alma turn, A, entire 

 colony; B, spicules. (After Cuvier.) 



