256 



LECTURES ON BIOLOGY 



In sponges and jelly-fishes bud-formation can proceed from 

 any part of the body, but the higher animal-forms which are still 

 able to produce by budding, above all numerous tunicates and 

 the moss-animalcules, possess a special generative gland, the 

 Stolo prolifer, from which the formation of the young proceeds. 



A beautiful illustration of 

 the first method of budding is 

 offered by the green fresh-water 

 polyp, Hydra viridis. The 

 structure of this delicate animal 

 is still exceedingly simple and 

 corresponds in its essentials to 

 the organization of a gastrula. 

 The mouth aperture, situated 

 at the distal extremity of the 

 body, and surrounded by filiform 

 tentacles, leads directly into the 

 body-cavity, which is covered 

 on all sides by a double cell- 

 layer, endoderm and ectoderm. 

 Here the mouth has still a 

 double task to fulfil : to take 

 in the food - particles and to 

 eject the digested food. It 

 serves, therefore, at the same 

 time as mouth and anus. The 



proximal end of the Hydra is closed by an adhesive disc, whereby 

 the animal is commonly attached to some water-weed. 



If the polyps are kept in an aquarium and are liberally 

 supplied with their favourite food, minute crustaceans, which 

 they seize skilfully with their tentacles, one is soon able to 

 observe remarkable changes in most of the individuals. While 

 the body was at first slender and uniform, small prominences 

 now appear on the body-wall, increasing rapidly in size. The 

 microscope shows that both endoderrn and ectoderm jointly 

 participate in this new formation and that the maternal body- 

 cavity is continued into the bud. Gradually these prominences 

 assume the form of a polyp, with mouth and tentacles being 

 formed at its distal extremity. The hydra has become a polyp- 



FIG. 60. GREEN FRESH-WATER POLYP 



(Hydra viridis). 



