THE COASTS OF SICILY. 57 



Costa *, and Krohn f have demonstrated its existence 

 in the Echinoderms to which belong the star-fishes, 

 while MM. Ehrenberg, Grant, Milne Edwards and 

 Agassiz have described it in the Acalephas, while 

 we ourselves have discovered it in the Nemertes and 

 the Planarias, animals which are very nearly allied to 

 certain intestinal worms, although they live in water. 

 Fully half of the Radiata and all the Worms, there- 

 fore, possess nerves as distinct as the higher animals. 



A most interesting question is connected with 

 that of the existence or absence of the nervous 

 system. What relations exist between the external 

 world and these lowest representatives of the animal 

 creation ? Do Annelids, Star- fishes, and Medusa? 

 possess the senses of sight and hearing ? Lamarck, 

 influenced by theoretical ideas, denied them all 

 sensation, and designated the majority of the inferior 

 animals by the denomination of animaux apathiques, 

 and, without being equally explicit, Cuvier seems to 



* Dr. Costa, a physician and naturalist at Naples, has published a 

 great number of very interesting memoirs, which bear principally 

 upon the marine animals of his native country. He was one of the 

 first who studied with care the anatomy of the Amphioxus, a very 

 remarkable fish, to which I have referred in a subsequent part of the 

 present chapter. 



f The naturalist Krohn has also specially devoted his attention 

 to the marine animals. Amongst other works, we are indebted to 

 him for a very interesting memoir on the mode of reproduction of the 

 Salpae, in which he has confirmed, and at the same time more 

 fully developed, the facts announced by Chamisso, which had long 

 been treated as fabulous. He has shown that the mode of genera- 

 tion of these Molluscs, compels us to include them among those 

 animals in which Steenstrup's theory of alternate generations is 

 exhibited. See VoL I. p. 219. 



