Behaviour of Medusoids 47 



posive in character, and could not have been 

 acquired by imitation or from having been 

 taught. That these motor effects are, however, 

 due to a common cause is shown by the fact 

 that in whatever part of the world Hydroids 

 exist they respond in like manner to stimuli, 

 and the response is repeated by succeeding 

 generations of these animals. 



We may now pass on to consider the be- 

 haviour of Medusoids. The origin of the 

 nerve cells of this order of animal is similar 

 to that of Hydra; both of them being derived 

 from, and remaining in close connection with 

 the living substance of the ectoderm, out of 

 which also the various sensory organs are 

 developed. 1 



With sensory organs and a nervo-muscular 

 system such as those we have described as 

 entering into the structures forming the body, 

 and tentacles of Medusoids, we can understand 

 how it comes to pass that, if we stimulate a 

 point situated on the under surface of the 

 margin of a jelly-fish's swimming-bell, its 



1 A Treatise on Zoology, Edited by E. Ray Lankester, Part II., 

 "The Anthozoa," by Prof. G. C. Bourne, p. 10. 



