THE CtELENTERATES 33 



34. Hydrozoa, or Hydra-like animals. Attention has al- 

 ready been directed to the fact that the structure of Hydra 

 is the simplest of the coelenterates ; nevertheless, the thou- 

 sand or more species belonging to this class which present 

 a much more complicated appearance (Fig. 17) possess 

 many fundamental Hydra-like characters. It is owing to 

 this fact that this assemblage of forms has been placed in 

 the class of the Hydrozoa, or Hydra-like animals. 



With but very few exceptions the members of this class 

 are marine, usually living near the shore-line, where at 

 times their plant-like bodies occur in the greatest profusion 

 attached to rocks, seaweeds, or the bodies of other animals, 

 particularly snails and crabs. Fig. 17 (upper colony) gives 

 a good idea of one of the more complex forms, whose tree- 

 like body attains in some cases the relatively giant height of 

 from 15 to 25 c.m. (six to ten inches). In early life it bears 

 a close resemblance to a Hydra. Buds form in much the 

 same way, but they retain permanently their connection with 

 the parent, and in turn bear other buds, until finally the form 

 shown in the figure is attained. In the meantime root-like 

 processes have been forming which afford firm attachment 

 to the object upon which the body rests. Also during this 

 process the cells of the outer layer form a horny external 

 skeleton ensheathing the entire organism except the ter- 

 minal portions (the hydranths, Fig. 18, B) bearing the ten- 

 tacles. The gastric cavities of all communicate, and the 

 food captured by one ministers in part to its own needs 

 and, swept through the tubular stalks and roots, is also 

 shared by all other members. 



35. Jelly-fishes and the part they play. During the pro- 

 cess of growth a number of stubby branches arise which 

 differ from the ordinary type in shape, and also in many 

 cases as regards color/ . These club-like, fleshy portions de- 

 velop close-set buds (Fig. 18, c) which early assume a bell- 

 like shape, the point of attachment corresponding to the 

 handle, while the clapper is represented by a short, slender 



