CHAPTER VI 

 THE CCELENTERATA 



A Simple Ccelenterate, Hydra -A Compound Coelenterate, Obelia. 



FROM the single-celled Protozoa we now ascend the first 

 step in the scale of animal life and turn to the Metazoa or multi- 

 cellular forms. These are animals whose many cells are arranged 

 in at least two layers, which are differentiated in structure and 

 function. The first phylum of the Metazoa with which we are now 

 concerned is the Phylum Ccelenterata and Hydra, a fairly widely 

 spread genus, furnishes a good example of a simple, little specialised 

 type. 



A Simple Ccelenterate Hydra. 



The various members of the genus Hydra are all inhabitants 

 of fresh water, hence their name of " fresh water polyps," and are 

 to be found adhering to aquatic plants and other submerged objects 

 in our ponds, ditches and streams. Three species, not differing m 

 essentials but only in details of structure, are commonly to be met 

 .with in this country : H. viridis is of a bright green colour ; H. vul- 

 garis (or H. grisea) is of a pale greyish colour ; and H. oligactis (or 

 H.fusca) is dark yellow or brown. An examination with a hand lens 

 is sufficient to show that the colouring matter in the first and last 

 is in the inner parts, and that the outer layer of the body wall is 

 transparent and practically colourless. The animals vary greatly 

 in appearance, according to the extent they are stretched out. In 

 a fully expanded condition they appear as long, slender, cylindrical 

 threads, from 6 to 7 mm. in length, attached by one end and bearing at 

 the other a circlet of much finer long filiform processes, the tentacles, 

 varying from six to ten in number, and these may extend as far 

 again beyond the body. The Hydra adheres by a flattened plate- 

 shaped foot or basal disc, and at its distal end, within the whorl of 

 tentacles, is a low conical projection, the " oral cone " or hypostome, 

 in whose centre is a circular opening, the mouth. The whole is 

 exceedingly contractile and extensile and very sensitive, the 

 slightest shock causing it to contract both its body and tentacles 



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