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EVOLUTION OF ANIMALS 



Part V 



Fig. 24.1. Ctenophores. A, Pleurobrachia pileus, the "sea gooseberry," named 

 because its size, streaks and translucence suggest a gooseberry. Common on the 

 northern Pacific and Atlantic coasts. B, Hormiphora plumosa, barely an inch long. 

 Tortugas, Florida. (Courtesy, Mayer: Ctenophores of the Atlantic Coast of North 

 America. Washington, Carnegie Institution, 1912.) 



Ecology. Ctenophores are carried about by currents but they also swim 

 feebly by means of their combs. Venus's girdle swims by undulations of the 

 body similar to those of a leech or an eel. 



Ctenophores are carnivores that feed voraciously upon any animals that 

 they can swallow. Swarms of billions of Uttle Pleurobrachia can sweep the 

 surface water clean with their tentacles that trail for several inches behind 

 them. In his study of the food relationships of animals in the Gulf of Maine, 

 H. B. Bigelow writes that "of all the members of the plankton [surface or- 

 ganisms], the most destructive to smaller or weaker animals are the several 

 coelenterates, and especially the ctenophores, genus Pleurobrachia, a pirate to 

 which no living creature small enough for it to capture and swallow comes 

 amiss." All ctenophores have a unique means of catching their prey, the glue 

 cells presently described. 



Structure. Each comb is composed of long cilia fused together as if the teeth 

 of a comb were united nearly to their tips (Fig. 24.2). A ctenophore swims 



