32 BRANCH CCELENTEEATA 



tractile, and these cilia accomplish locomotion. They are 

 free and single, there being no polyp stage. They are found 

 from the tropical to the arctic seas. They are small — from 

 5 to 20 mm. in diameter — and their shape varies from that of a 

 pear to a sac-like or ribbon-like form. They have but two 

 tentacles. They are hermaphroditic, multiplying by eggs. 



The central nervous system is represented by a ciliated area 

 on the aboral pole, and is connected with a single sensory 

 organ. 



Economic Value. — The animals of this branch are of great 

 use to man, indirectly, by furnishing food for other animals, and, 

 directly, by the formation of great beds of limestone and of 

 coral reefs and islands, also by forming an article of commerce 

 of no small value. ^ 



" The red coral of commerce is obtained in the Me(hterranean 

 Sea off the coast of Africa and the west coast of Italy. The price 

 varies according to the color. The finest rose pink in large 

 pieces is valued at $400 or more an ounce. The common 

 article brings from $1 to $1.50 an ounce. "^ 



Geologic Distribution. — The hydrozoa are believed to be 

 represented by the Graptolites, which appeared in the Cambrian 

 Period, were numerous in the Ordovician, greatly diminished in 

 the Silurian, and almost extinct in the Devonian. Large 

 numbers of casts of jelly-fishes are found in the Cambrian rocks.' 

 Hydroids and true corals were important. Marine life and reefs 

 were formed in the Silurian Period. Corals vastly increased 

 in size and number in the Devonian Period, and were abundant 

 in the Carboniferous, contributing largely to the limestone. 

 Hydractinia were found in the Cretaceous Period. 



Important Biologic Facts. — In the Ctenophora is found for the 

 first time a true middle layer of mesoderm cells. '' 



In the hydroid colony is found the division of labor among 

 the different sets of individual zooids and a differentiation of 

 structure according to their function. 



' "The fishing for the red coral (CoraUiiim nihniiii) at Naples amounts 

 yearly to half a million dollars." — Kingsley. 

 * Adam's "Commercial Geography." 

 3 Scott's "Geology," p. 371. 

 ^Parker and Haswells "Zoology," vol, i, p. 207. 



