CffiLENTERATA. 



53 



structures found in the integument of almost all the members of 

 this sub-kingdom, and sometimes in internal parts as well. They 

 are very beautiful objects of microscopical examination, and diflfer 

 very considerably in the details of their structure. Each thread-cell 

 (fig. 27) is essentially a double- walled sac or capsule filled with fluid, 

 and containing internally a longer or shorter spirally-coiled thread 

 or filament. The thread is pointed at its free extremity, and often 

 barbed at its bsise or serrated along the edges. When the external 

 capsule is touched it ruptures ; and the coiled-up 

 thread is thrown out, like a lasso, to a distance 

 often equal to many times the length of the en- 

 closing capsule. The pointed end of the thread 

 pierces the object which it strikes — supposing the 

 latter to be of a sufficiently soft nature — and ap- 

 parently envenoms the puncture thus produced 

 with the fluid contained in the sac, this seeming to 

 have some poisonous or iiritating character. 



In accordance with the above-mentioned differ- 

 ences in the arrangement of the digestive system, 

 the Coelenterata are divided into two great classes, 

 termed respectively the Hydrozoa and the Aetin- 

 ozoa. In the Hydrozoa, there is no body-cavity 

 distinct from the digestive cavity — or, in other 

 words, the body-cavity is the digestive cavity, the 

 mouth opening directly into the body-cavity (fig. 

 28). In the Actinozoa, on the other hand, there is 

 a distinct digestive cavity, but this opens directly 

 into the general body-cavity, so that the two form distinct but freely 

 communicating divisions of the same chamber. 



Fig. 28.— Diagram- 

 matic section of 

 Hydra; the darlc 

 line is the ecto- 

 derm, thefiiie line 

 and clear space 

 fidjat-ent indicate 

 the endoderm. 



Class I. Hydrozoa. 



The Hydrozoa are defined as Coelenterata in which there is no digestive 

 sac separate from the general cavity of the body. The reprodiKtive organs 

 are external, in the form of outward processes of the body-wall (fig. 30). 



The Hydrozoa are all aquatic in their habits, and, with few ex- 

 ceptions, all are inhabitants of salt water. The class includes both 

 simple and composite organisms, of which the most familiar are the 

 Sea-firs and their allies (Hydroid Zoophytes), the Fresh- water Polype 

 or Hydra, the Sea-jellies (Medusce), and the Portuguese Man-of-war 

 (Physalia). Owing to the extremely •complicated nature of many of 

 the Hydrozoa, it appears advisable to preface their description by 

 an explanation of some of the more important terms which are 

 employed in connection with various members of the class. 



