28 HTDKOZOA. 



structure and mode of development are very dif- 

 ferent {g). 



In accordance with these several modifications, 

 the class Hydrozoa has been divided into seven 

 orders: Hydridce; Corynidce; Sertularidce; Caly- 

 cophoridce ; Physophoridce ; Medusidce ; and Lu- 

 cernaridce. 



The Hydrozoa vary exceedingly in size. When 

 a coenosarc is present, it indicates a tendency on 

 the part of the organism to increase by a process 

 of continuous gemmation, and such forms often 

 attain considerable dimensions, though the sepa- 

 rate polypites are often so small as to be almost 

 microscopic. This is the case with most of the 

 complex fixed Hydrozoa, furnished with a branched 

 horny coenosarc. In the oceanic species, the poly- 

 pites are somewhat larger, yet, when the hydro- 

 soma consists of only one of these, its size is 

 usually inconsiderable. But, should the hydrosoma 

 develop an umbrella, its subsequent increase may 

 be extremely rapid, and, in this manner, the most 

 gigantic members of the class appear to be pro- 

 duced. 



The animal fabric of the Hydrozoa is, perhaps, 

 best described as consisting of — 



a. Organs of nutrition, 



b. Prehensile apparatus, 



c. Tegumentary organs, 



d. Muscular system and organs of locomo- 



tion, 



e. Nervous system and organs of sense, and 

 /. Eeproductive organs. 



3. Organs of IVutrition — In every Hydro- 



