5-2 ZOOLOQ r. 



part formed by the digestive tract. The Ccelenterata pre- 

 sent striking "examples of self-division, gemmation, and 

 alternate generations, and very great extremes iu 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 {Hijdroids and Acaleplu.) 



General Characters of Hydrozoa. — An excellent idea of 

 ■the general structure of the Hydrozoa may be obtained from 

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

 the otiier forms being but a modification and elaboration of 

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

 "based on Avhat is found to constitute the structure of 

 Hydra. 



Order 1. Hydroidea. — The animal next higher iu struc- 

 ture than the sponge is the curious Protohydra 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 Coeleuterate yet discovered. As 

 "the form of the fresh-vrater Hydra is familiar, Frotoliydra 

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

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

 layeis (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 

 ])lants. There are two varieties of Hydra vulgaris 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 seives both as 

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

 ".".lUritive fluid, and is called the gastro-vascular cavity. 

 T'le mouth is surrounded with from five to cidit tentacles, 



