132 ZOOLOGY. 



whales. Some of the commonest forms are melon-shaped, 

 but not bigger than a boy's marble. The cilia are arranged 

 in eight bands, or ctenophores, extending from the upper to 

 the lower extremity of the body. They are also furnished 

 with a pair of long-fringed tentacles, which can be com- 

 pletely retracted within sacs. These animals are destitute 

 of any skeleton or hard parts. They contain a little 

 mass of matter, believed to be nervous, from which a few 

 filaments proceed. This is the only trace of a nervous 

 system found in the Ccelenterata. Some species of 

 Ctenophora are luminous at night. 



CLASS 2. Hydrozoa. 



188. The Hydrozoa (Gr. hudra, a water dragon; zoon, 

 an animal) are ccelenterate animals, in which the digestive 

 sac is not separated from the body cavity by any interven- 

 ing space as in the Actinozoa, the alimentary canal and the 

 somatic cavity forming one continuous tube. The repro- 

 ductive organs are external) being attached to the outer 

 surface of the body wall. 



189. This class includes the fresh-water hydra, Sertu- 

 laridse or sea-firs, the Corynidss or club-hydras, the 

 LucernaridSB or lantern-hydras, and the Medusae or 

 jelly-fishes. Some are simple, others form colonies by 

 budding or self-division. They are cellular in structure, 

 and the body consists of two layers, ectoderm and endo- 

 derm. The mouth is surrounded by tentacles, by means 

 of which the animals seize their prey. They possess 

 thread-cells, which are found in the tentacles, as well as 

 in other parts of the body. The mouth leads into a 

 digestive sac, " the outer wall of which is in direct con- 

 tact with the water in which the animal lives." This sac 

 leads by a wide aperture to the general cavity of the body, 

 which is merely a continuation of the digestive tube. 

 Thus, if a vertical section be made of a hydrozoon, only 

 a single tube will t> gut through j whereas^ if a simitar 



