BRANCH CCELENTERATA 



This branch comprises our fresh-water Hydra, and a few 

 alHes, and the marine forms, jelly-fishes, corals, and sea-ane- 

 mones. This branch finds representatives from the shore line 

 and the surface to the profound depths of the ocean. 



The body, which is usually radially symmetric, consists es- 

 sentially of a two-layered sac, which is open at one end and 

 closed at the other, and in which there is a simple or branched 

 gastric cavity. The outer layer is called the ectoderm; the 

 inner layer, the endoderm, and a gelatinous non-cellular layer 

 between them, the mesoglea. Some coelenterates are soft- 

 bodied, others secrete a calcareous or limy substance called 

 coral. Around the free open end of the sac-like body are a 

 varying number of tentacles. 



Nettle Cells. — Stinging or nettle cells are characteristic of 

 this branch, except in Cetenoph'ora, where they are replaced 

 by adhesive cells. These stinging cells, which are especially 

 abundant on the tentacles, contain a fluid, and a spirally wound 

 thread provided with barbs, which, when the animal is disturbed, 

 are discharged into the body of the intruder, paralyzing it. It 

 is then seized by the tentacles and drawn into the mouth. 



Size. — Coelenterates vary in size from the little fresh-water 

 hydra, a fraction of an inch in length and of the diameter of a 

 pin, to the giant jelly-fishes, as the Cya'nea, which sometimes 

 reach 7 or 8 feet in diameter and have tentacles more than 100 

 feet long. 



Locomotion. — Some members of this branch are free, as the 

 jelly-fishes; some are permanently fixed; as the Corals, while 

 some, as the Hydras, are temporarily fixed, moving from one 

 position only to adhere to another, and thus making slow pro- 

 gression. 



Multiplication is both sexual (by eggs) and asexual (by 

 budding) . 



Origin. — They are of ancient origin, being abundant in the 

 Cambrian Period. 



