20 HYDROZOA. 



CHAPTER IT. 



THE CLASS HYDROZOA. 



Section I. 



MORPHOLOGY AND PHYSIOLOGY OF HYDROZOA. 



I. Type of the Class : Hydra. — 2. General Morphology. — 3. Organs 

 of Nutrition. — 4. Prehensile apparatus. — 5. Tegumental y Or- 

 gans. — 6. Muscular System and Organs of Locomotion. — 7. Ner- 

 vous System and Organs of Sense. — 8. Reproductive Organs. 



I. Type of the class : Hydra. — The Hydra, 

 or fresh-water polype, is the t3^e of the class 

 Hydrozoa {fig. 2, e, and fig. 3). ^ 



The Hydra possesses a gelatinous, sub-cylin- 

 drical body, liable, from its contractility, to undergo 

 various mutations of form, having one end ex- 

 panded into an adherent disc, or foot, a mouth 

 being situated at its opposite extremity. The 

 mouth leads into a capacious cavity, excavated 

 throughout the entire length of the animal. From 

 the margin of the oral aperture, or rather, at a 

 little distance below it, arises a circlet of prehen- 

 sile tentacles, varying in number from five to 

 twelve, or even more. These are exceedingly 

 contractile, at one moment assuming the appear- 

 ance of delicate filaments, the next, shrunk up 

 into little wart-like knobs. Numerous thread-cells 

 are embedded in their substance, and project 

 freely from their surface. These are of two kinds, 

 one being much larger than the other. The 



