BEHAVIOR OF CCELENTERATA 189 



special interest from the standpoint of behavior are the nerve cells. In 

 Hydra these consist of comparatively few, small cells with long, branched 

 processes, scattered among the ectoderm and entoderm cells. They 

 apparently serve to connect the other cells. In the sea anemones the 

 nerve cells are more numerous than in Hydra, but are likewise scattered 

 throughout the body, in both ectoderm and entoderm. They are some- 

 what more numerous in the neighborhood of the mouth than elsewhere. 

 In Medusae the nervous system is more concentrated. The cells and 

 fibres form two rings about the edge of the body : one lies just beneath 

 the ectoderm of the exumbrella, the other beneath that of the subum- 

 brella. These rings are interconnected by scattered fibres. A plexus 

 of nerve fibres covers the entire concave surface of the subumbrella 

 and manubrium, beneath the ectoderm. This plexus is compared by 

 Romanes as regards texture to a sheet of muslin. Nerve cells and fibres 

 are found also in the tentacles, but are not known on the convex surface 

 of the exumbrella. The two marginal nerve rings are often spoken of 

 as the "central nervous system" in medusae. 



i. ACTION SYSTEM. SPONTANEOUS ACTIVITIES 



In the ccelenterates we take up animals with action systems differing 

 much from those of the organisms we have hitherto studied. The chief 

 movements are due to contractions and extensions of parts of the body 

 and tentacles, produced by contractions of the muscle fibres. The body 

 is flexible, and being radially symmetrical may contract or bend with 

 equal ease in any direction. 



Under natural conditions, Hydra and the sea anemone are usually 

 attached and at rest, while the medusa may be in movement. Let us ex- 

 amine the behavior under such conditions, when no observable stimulus 

 is acting on them, aside from the usual conditions of existence. 



If we observe an undisturbed green Hydra attached to a water plant 

 or the side of a glass vessel, we find that it usually does not remain still, 

 but keeps up a sort of rhythmic activity. After remaining in a certain 

 position for a short time it contracts, then bends to a new position, and 

 reextends (Fig. 114). In this new position it remains for one or two 

 minutes, then it again contracts, changes its position, and again extends. 

 This continues, the changes of position occurring every one or two min- 

 utes. In this way the animal thoroughly explores the region about its 

 place of attachment and largely increases its chances of obtaining food. 

 This motion seems to take place more frequently in hungry individuals, 

 while in well-fed specimens it may not occur. 



Thus contractions take place without any present outward stimulus ; 



