COELENTERATES IN GENERAL 



147 



are often very bright ly colored. Many jellyfishes are perfectly transparent 

 and the mesoglea reflects rays of light with crystal-like clearness. Some, 

 however, are beautifully tinted, and a few are strongly colored. Though 

 all colors may be found, the prevailing colors are blue, various shades of 

 rose or pink, yellow, or brown. Jellyfishes are more or less luminescent 

 at night. 



Fig. 63. — A, sea pen from Puget Sound, Ptilosarcus quadrangularis Moroff. X M- 

 The polyps are on the edges of the leaflike folds; the stalk is imbedded in mud or sand at 

 the bottom of the sea when the animal is under natural conditions, but does not anchor the 

 animal to a particular location. B, dried skeleton of a sea fan, Gorgonia sp. X /e- This 

 colony is anchored to a mass of coral rock. C, portion of a sea fan colony, showing the 

 polyps. X 8. All from preserved specimens. 



175. Polymorphism. — Polymorphism is a phenomenon which involves 

 the appearance of the same species of animal in different forms. It 

 is very generally exhibited by coelenterates. For instance, the colonies 

 of many of the marine hydroids have polyps of two types, one nutritive, 

 the other reproductive. In addition to these two forms there may also 

 be the medusa, which represents a third form of the same species (Fig. 

 66). In the case of certain colonial hydroids there are several distinct 

 types of polyps, accompanied by a division of labor between the indi- 



