tNVEfcTEBRA-TA.. 435 



manubrium. In many cases the medusae are never set 

 free, but remain attached to the hydroid gemmarium and 

 show various stages of degeneration. There are also two 

 orders in which the medusa develops directly from the egg 

 or with a rudimentary hydroid stage. Another order is 

 that of the Siphonophora, which are free swimming pelagic 

 gemmaria, whose various zooids are differentiated for 

 various functions, so that the gemmarium is polymorphic. 



CLASS : ANTHOZOA. 



Coelenterates of cylindrical shape, attached at one end, 

 with mouth and tentacles at the other : the mouth leading 

 into a tubular stomodaeum, lined by ectoderm and opening 

 into the coelenteron below : the coelenteron around the 

 stomodaeum divided into compartments by radial parti- 

 tions called mesenteries : the free edges of the mesenteries 

 below the stomodaeum elongated and thickened, forming 

 the mesenterial filaments. The gonads are endodermic, 

 situated on the sides of the mesenteries, and the reproduc- 

 tive cells are discharged by the mouth. The majority of 

 the species form compound gemmaria, the corals. 



This class includes the sea-anemones and corals. There 

 are two great sub-classes : the Alcyonaria, represented by the 

 Dead Men's Fingers of our coasts and by the red coral of 

 commerce, and the Zoantharia, represented by the common 

 sea-anemones and the corals of the tropical coral reefs and 

 coral islands. In the Alcyonaria the number of mesenteries 

 and tentacles is always eight, in the Zoantharia the number 

 is usually six, twelve, or some higher multiple of six, but 

 in certain species the circles of tentacles are in multiples of 

 five. The mouth is always elongated, not circular, and 

 many of the internal structures are paired; an anterior or 

 dorsal can be distinguished from a posterior or ventral 

 side, so that the structure is always really bilaterally 

 symmetrical. 



PHYLUM PLATYHELMIA. 



The Platyhelmia or flat worms form a phylum of 

 which the different divisions show marked differences 

 from one another, so that it is somewhat difficult to state 



