CCELENTERATA. 



CHAPTER I. 



THE SUB-KINGDOM CCELENTERATA. 



I. General characters, — 2, Classes. 



I. CJeneral characters. — The animal forms 

 included under the sub-kingdom Coelenterata pre- 

 sent modifications of a type of structure better 

 marked than that which is characteristic of the 

 Frotozoa. All are furnished with an alimentary 

 canal, freely communicating with the general, or 

 somatic, cavity. The substance of the body consists 

 essentially of two separate layers, an outer, or 

 ^ ectoderm,' and an inner, or ^ endoderm.' These 

 two membranes, but especially the former, are in 

 general provided with cilia. 



Another distinctive characteristic of the Coslen- 

 terata is found in the presence of the peculiar 

 urticating organs, or Hhread-cells,' which are met 

 with so constantly in the integument of these 

 organisms {fig. i). 



Thread-cells, for which the term ^ cnidse ' has 

 been proposed, usually occur as colourless, trans- 

 parent, elastic, double-walled sacs, rounded or 



B 2 



