26 



BEGINNERS' ZOOLOGY 



scarce. Asexual generation (by budding) is common with 

 the hydra when food supply is abundant. After the bud 



grows to a cer- 

 tain size, the 

 outer layer of 

 cells at the base 

 of the bud con- 

 stricts and the 

 young hydra is 

 detached. 



Compare the 

 sponge and the 

 hydra in the fol- 

 lowing respects : 

 — many celled, 

 or one celled ; 

 obtaining food ; 

 breathing; tubes 

 and cavities ; 

 openings ; re- 

 production ; loco- 

 motion. Which 

 ranks higher 



Fig. 39.- 



■ Longitudinal section of hydra (microscopic 

 and diagrammatic). 



among the metazoa ? The metazoa, or many celled ani- 

 mals, include all animals except which branch ? 



Figure 39 is a microscopic view of a vertical section of a hydra to 

 show the structure of the body wall. There is an outer layer called the 

 ectoderm, and an inner layer called the e?tdoderm. There is also a thin 

 supporting layer (black in the figure) called the mesoglca. The mesoglea 

 is the thinnest layer. Are the cells larger in the endoderm or the ectoderm ? 

 Do both layers of cells assist in forming the reproductive bud ? The ecto- 

 derm cells end on the inside in contractile tails which form a thin line and 

 have the effect of muscle fibres. They serve the hydra for its remarkable 

 changes of shape. When the hydra is cut in pieces, each piece makes a 

 complete hydra, provided it contains both endoderm and ectoderm. 



