ACTINOZOA. 163 



Gorgonidce are permanently fixed, as are also 

 many of the higher coralligenous Actinozoa, es- 

 pecially those which multiply by continuous gem- 

 mation. Others, however, and these chiefly the 

 simpler forms, are free, but, like the unattached 

 Pennatulidce, not truly locomotive. Yet in the 

 greater number of the Actinozoa each polype, 

 though fixed, is contractile to some extent, shrink- 

 ing down under irritation, and again unfolding 

 itself at pleasure, while, among the Alcyonaria, 

 with a few exceptions, it is also retractile into the 

 fleshy substance of the coenosarc. Even this, too, 

 has its own share of contractility, most evident in 

 those species which possess an elastic sclerobasis. 

 Thus, on the South American coast, Mr. Darwin 

 observed a Sea-pen which, on being touched, 

 forcibly drew back into the sand some inches of 

 its compound, polype-covered, mass. 



All the Gteno'pliora are free-swimming animals, 

 but doubt yet hangs over the nature of certain 

 exceptional Zoantharia, reputed to be of similar 

 habit. The apical extremity of the genus Minyas 

 and its allies is represented by Lesueur and Lesson 

 as dilated into a large air-sac, excavated beneath 

 the floor of the somatic cavity, and furnished be- 

 low with an opening into the surrounding medium. 

 By means of this sac the creature is said to float 

 without effort, its oral disc being turned down- 

 wards; but further observations on its structure 

 are much wanting. Again, the Arachnactis alhida 

 of Sars, possesses, according to Professor E. Forbes, 

 not merely the power of swimming like a Medusid, 

 but " it can convert its posterior extremity into a 

 suctorial disc, and fix itself to bodies in the man- 

 ner of an Actinia.^'* But the aspect of the tentacles 



; M 2 



