52 ZOOLOG Y. 



part formed by the digestive tract. The Coelenterata pre- 

 sent striking examples of self-division, gemmation, and 

 alternate generations, and very great extremes in degree of 

 complexity of structure. 



The different groups have a high geological antiquity; 

 the species of Hydroid and coral-polyps serving as time- 

 marks to measure off geological periods. 



CLASS I. HYDROZOA (Hydroids and Acalephs.) 



General Characters of Hydrozoa. An excellent idea of 

 the general structure of the Hydrozoa may be obtained from 

 a study of Hydra, the type or example of the whole class, all 

 the other forms being but a modification and elaboration of 

 this simple type. The characters of the class as a whole are 

 based on what is found to constitute the structure of 

 Hydra. 



Order 1. Hydroidea. The animal next higher in struc- 

 ture than the sponge is the curious Protoliydra discovered 

 by Greef among diatoms and sea- weeds at Ostend. It is re- 

 garded by Greef as the marine ancestral form of the Ccelen- 

 terates. It is the simplest Ccelenterate yet discovered. As 

 the form of the fresh-water Hydra is familiar, Proluhydra 

 mav be best described as being similar to that, except that 

 it is entirely wanting in tentacles. It is made up of two 

 layers (an ectoderm and endoderm, no mesoderm having yet 

 been discovered), with a mouth and stomach (gastro-vascular 

 cavity). 



A more complicated form is the fresh-water Hydra, which 

 is commonly found on the under side of the leaves of aquatic 

 plants. There are two varieties of Hydra vulgar is appar- 

 ently common to the fresh waters of the old and new world ; 

 they are Hydra viridis and fusca. The somewhat club- 

 shaped body consists of two layers, the inner (endoderm) 

 lining the general cavity of the body, which serves both as 

 mouth and stomach, as well as for the circulation of the 

 nutritive fluid, and is called the gastro-vascular cavity. 

 The mouth is surrounded with from fi\v to eight tentacles, 



