30 HYDRA. 



2. The supporting lamella is seen in transverse sections as a 



very thin transparent layer between the ectoderm and 

 the endoderm. 



3. The endoderm cells differ greatly in shape in different 



parts of the body and at different times. During 

 life some are amoeboid, and their processes may 

 extend so far as to almost obliterate the cavity of the 

 animal. Others contain glandular secretions which 

 are used for the digestion of food in the ccelenteron. 

 The nucleus is usually nearer the outer or basal end, 

 and is flattened in shape. The flagella can very 

 seldom be seen in sections. 



In the foot the endoderm cells are much smaller 

 than in the body, and form a layer of short columnar 

 or almost cubical cells containing granular concre- 

 tions. 



OBELIA. 



Obelia is a delicate white organism consisting of a branch- 

 ing stem rising an inch or so from a root-like stolon. The 

 stem is bent in a zigzag fashion, each angle supporting a 

 polyp or hydranth comparable to a Hydra. If the buds of 

 Hydra remained in contact we should have an organism 

 not unlike Obelia. 



Obelia is, however, marine. It lives fixed to the fronds 

 of seaweed, the shells of lobsters, and the surface of wood- 

 work immersed in the sea. Its food consists of the minute 

 animals and plants that accumulate as microscopic debris 

 along the coast. 



A. Structure. 



The structure of an Obelia-hydroid is somewhat 

 similar to that of Hydra. Each polyp consists of a 

 digestive cavity and a capacious mouth surrounded 

 by a circle of tentacles that rise at the base of the 

 lips. Adjacent polyps are united by the central 

 stem. The outer ectoderm of the mouth, digestive 

 cavity, and tentacles is lineal by an inner endoderm 



