FIRST LESSONS IN ZOOLOGY 



the bottoms of which have been gradually lifted above the 

 water's surface. Similarly the rock called Tripoli, found 

 in Sicily, and the Barbados earth from the island of Bar- 

 bados, are composed of the shells of ancient Protozoa. 



Hydra. — One of the most interesting of the simple 

 animals found in fresh-water ponds is Hydra (fig. 85). 



Though very small com- 



pared with most animals we 

 know, it is much larger than 

 any of the Protozoa, being 

 when expanded nearly one- 

 fourth of an inch long. 

 It is also not composed of 

 a single cell but of hundreds 

 of cells. It is one of the 

 simplest of the many-celled 

 animals, i.e., Metazoa. Hy- 

 dra may be found attached 

 to bits of sticks, stones, 

 and leaves in pools not too 

 stagnant. There are two 

 common kinds, one brown 

 and one green. Specimens 

 should be brought into the 

 schoolroom alive, and kept 



Fig. 85.— Hydra: note two tentacles j^^ ^ ^Jj^^ ^f water in thc 

 catching an insect larva; note the 

 building young Hydra. (Natural light. To observe the 



si^e, one-sixth inch; from life.) \^^\y\ts of Hj'dra, examine a 



live specimen, attached to a bit of leaf or stick, in a 

 watch-glass, under the low power of a compound micro- 

 scope, or with a good magnifier. 



Note the cylindrical body, attached at its base, and 

 with a series of tentacles projecting from its free end. 

 How many tentacles are there ,' They arise in a circle 

 about the mouth. Have some small water-fleas in the 



