CCELENTEEATA. 



CHAPTEK I. 



THE SUB-KINGDOM CCELE NTERATA. 

 I. General characters. — a. Classes. 



I. General characters. — The animal forms 

 included under the sub-kingdom Goelenterata pre- 

 sent modifications of a type of structure better 

 marked than that which is characteristic of the 

 Protozoa. 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 Ccelen- 

 terata is found in the presence of the peculiar 

 urticating organs, or < thread-cells,' which are met 



so constantly in the integument of these 

 organisms (fig. i). 



Thread-cells, for which the term « cnidee ' has 

 been proposed, usually occur as- colourless, trans- 

 parent, elastic, double-walled sacs, rounded or 



B 2 



with 



