100 



INVERTEBRATE ZOOLOGY 



Subclass Alcyonauia 



Eight feathered tentacles and eight single mesenteries (Fig. 57) 

 characterize members of this subclass which are also frequently 

 referred to under the name Octocoralla. Colonial forms, 

 frequently with polymorphism of the indi- 

 viduals, are common. Budding to form a 

 colony is usually stoloniferous. 



As in the preceding subclass, here also we 

 find diversity in skeletal structure. Horny 

 or solid calcereous rods give form and 

 support to the colonies of sea fans (Gorgo- 

 nians) and precious coral, while only clus- 

 tered spicules occur within the tissues of the 

 sea pens and sea pansies. 



Fig. 57. — Trans- 

 verse section of an 

 Alcyonarian. {From 

 Hertwig's Alaniial by 

 Kingsley, courtesy of 

 Henry Holt and Co.). 



B. PHYLUM CTENOPHORA 



The Ctenophora have very commonly 

 been considered as a class of the Coelenterata. The absence of 

 marginal tentacles, lack of netting cells, centralization of the 

 nervous mechanism, and transformation of the mesoglea into a 

 mesoderm mark the chief arguments in favor of considering the 



A B 



Fig. 58. — A ctenophore, Pleurohrachia rhododaclyla Agassiz. .-1, lateral view 

 B, aboral view. {After L. Agassiz). 



ctenophores as an independent phylum. The most striking 

 external feature is the presence on the surface of the body of 

 eight meridional bands of swimming combs or plates (Fig. 58) 

 each of which is composed of a linear series of short rows of cilia. 



