22 ZOOLOGY. 



SUB-KINGDOM VI. (telenterata, 



20. The Coelenterata (Gr. Jcoilos, hollow ; enteron, an in- 

 testine) form a very distinct division of the animal 

 kingdom. The red and white corals, many of the coral- 

 lines, sea jellies, &c., belong to it. These were placed by 

 Cuvier, along with some other groups, in the division 

 named by him Eadiata. From their resemblance to 

 plants many of them have also been called Zoophytes. 

 (Gr. zoon, an animal ; phuton, a plant). 



The body wall always consists of two layers, an outer 

 and an inner. The external covering is called the 

 ectoderm (Gr. ectos, outside ; derma, skin) ; the inner 

 lining, the endoderm (Gr. endos, within). These layers 

 have a cellular structure, and are generally furnished 

 with cilia (Lat. cilium, an eyelash), which constantly 

 vibrate. 



There is a circle of tubular arms round the mouth. 

 These are called tentacles (Lat. tento, I touch), and are 

 used by the animals in seizing their prey. 



The most distinctive feature in this group is the 

 digestive apparatus. In all the divisions hitherto de- 

 scribed it was seen that the digestive canal is completely 

 shut off from the body cavity, and communicates with 

 the exterior by an anus or vent. In the Cffilenterata, 

 however, the alimentary tube opens at its inner end into 

 the general cavity of the body, and communicates freely 

 with it. As there is no anal opening, the indigestible 

 portions of the food are expelled from the mouth. 



Another peculiar feature of the group is the possession 

 of stinging organs in the form of "thread cells." These 

 may be examined by the microscope in the common fresh- 

 water hydra. " These cells are oval elastic sacs, contain- 

 ing a long coiled filament, barbed at its base, and serrated 

 along the edges. When fully developed the sacs are filled 

 with fluid, and the slightest touch is sufficient to cause 

 the retro version of the filament, which then projects be- 



