248 HYDROID POLYPES LESS. 



short buds arise in a circle : these are the rudiments of the 

 tentacles : the narrow portion beyond their origin becomes 

 the hypostome (L). Soon the cuticle covering the distal end 

 is ruptured so as to set free the growing tentacles (M) : an 

 aperture, the mouth, is formed at the end of the hypostome, 

 and the young hydroid has very much the appearance of a 

 Hydra with a broad disc of attachment, and with a cuticle 

 covering the greater part of the body. 



Extensive budding next takes place, the result being the 

 formation of the ordinary hydroid colony. 



Thus from the oosperm or impregnated egg-cell of the 

 medusa the hydroid colony arises, while the medusa is 

 produced by budding from the hydroid colony. We have 

 what is called an alternation of generations, the asexual genera- 

 tion or agamobium (hydroid colony) giving rise by budding 

 to the sexual generation QI gamobium (medusa), which in its 

 turn produces the agamobium by a sexual process, i.e. by 

 the conjugation of ovum and sperm. 



Two other Hydroids must be briefly referred to in con- 

 cluding the present lesson. 



Floating on the surface of the ocean in many parts of the 

 world is found a beautiful transparent organism called 

 Diphyes. It consists of a long, slender stem (Fig. 58, A, a\ 

 at one end of which are attached two structures called 

 swimming-bells (m, m) in form something like the bowl of a 

 German pipe, while all along the stem spring at intervals 

 groups of structures (e\ one of which is shown on an 

 enlarged scale at B. 



Each group contains, first, a tubular structure (B, n) with 

 an expanded, trumpet-like mouth, through which food is 

 taken : this is clearly a hydranth. From the base of the 

 hydranth proceeds a single, long, branched tentacle or 



