LESSON XXI 



HYDRA. 



We have seen that with plants, both Fungi and Algge, the 

 next stage of morphological differentiation after the simple 

 unicellular or non-cellular organism is the linear aggregate . 

 Among animals there are no forms known to exist in this 

 stage, but coming immediately above the highest unicellular 

 animals, such as the ciliate Infusoria, we have true solid 

 aggregates. The characters of one of the simplest of these 

 and the fundamental way in which it differs from the plants 

 described in the two previous lessons will be made clear by 

 a study of one of the little organisms known as " fresh-water 

 polypes " and placed under the genus Hydra. 



Although far from uncommon in pond-water, Hydra is not 

 always easy to find, being rarely abundant and by no means 

 conspicuous. In looking for it the best plan is to fill either 

 a clear glass bottle or beaker or a white saucer with weeds 

 and water from a pond and to let it remain undisturbed for 

 a few minutes. If the gathering is successful there will be 

 seen adhering to the sides of the glass, the bottom of the 

 saucer, or the weeds, little white, tawny, or green bodies, 

 about as thick as fine sewing cotton, and 2 — 6 mm. in 

 length. They adhere pretty firmly by one end, and examin- 



