LESSON XXII 



HYDRA 



WE have seen that with plants, both Fungi and Algae, the 

 next stage of morphological differentiation after the simple 

 cell 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 generic name 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- 



