ii COELENTERATA 35 



Each fertilised egg cell develops into a little 

 Development e l n & ate d mass of cells, which becomes covered 

 with cilia. In this stage it escapes from the medusa, 

 and swims slowly through the water. Finally it settles down 

 in an upright position on some rock or shell, loses its cilia, 

 and grows into a hydriform body, which soon buds freely and 

 forms a new hydroid colony of Bougainvillea. 



The formation of medusae in this way, by 

 Meml buddin ff from a hydroid colony, is only 

 found in the class Hydrozoa ; and the medusae, 

 if they develop fully, are always of the type described above, 

 having a velum, radially placed ovaries and spermaries, and 

 a continuous marginal nerve ring. In many of the Hydrozoa, 

 however, the reproductive individuals do not ever become 

 free from the colony, but remain attached to it in a rudi- 

 mentary condition, sometimes being nothing more than a 

 sac-like body enclosing eggs or sperms, as is the case in the 

 fresh-water form, Cordylophora, which occurs attached to the 

 underside of floating wood in fresh or brackish water. 



Hydra itself is included amongst the Hydrozoa, but here, 

 as has been described in Chapter L, the eggs and sperms are 

 borne direct on the body of the ordinary nutritive polyp. 



PRACTICAL WORK ON HYDROZOA 



Obtain some Hydra viridis from a pond, or, if this is im- 

 possible, from a dealer. 1 



Mount, in a drop of water on a slide, a small piece of weed 

 with a Hydra attached to it ; if the Hydra is detached it can be 

 taken up with a dipping tube and placed on the slide ; a small 

 piece of stick or weed should be put in the water before the cover- 

 slip is put on, otherwise this may crush the body of the Hydra 

 undesirably. Examine under the low power of the microscope, 

 watching the body expand ; when it is fully extended verify the 

 different facts stated in the text. Look for specimens that are 

 budding or reproducing sexually. Sketch in different stages. 



Prepared sections of the body should be obtained and examined 

 under the microscope if possible, so that the different tissues may 

 be studied in detail. 



Put several Hydrae into a small glass vessel partly filled with 



1 Supplied by T. Bolton, 25 Balsall Heath Road, Birmingham, in Is. tubes. 



