PHYSIOLOGY OF THE INVERTEBRATA. 349 



The Porifera or Spongida. 



In these animals the sense-organs are not further diifereu- 

 tiated than those of the Protozoa. 



The Ccelenterata. 



The sense of touch in these animals is believed to be chiefly 

 located in the tentacula, which surround the m.outh, but in 

 Hydra, as well as in other forms, every cell is sensitive to 

 tguch. 



■The small pits in connection with nerves, and provided 

 with an epithelial lining of hair-bearing sense cells, in the 

 Medusw, are regarded as the simplest olfactory organ. They 

 are situated round the margin of the bell ; in fact in all the 

 MediiscB the seAse-organs are marginal. The small pigmented 

 spots are undoubtedly eyes ; and according to some writers, 

 otolithic sacs or simple auditory organs are also situated on 

 the edge of the bell. 



The rudimentary eyes of the Medusae are much better 

 developed than those which are supposed to exist in the 

 Protozoa ; for in certain species, nerves penetrate manifestly 

 into the capsule (Gegenbaur). But the exact function of the 

 ocular spots in these animals was not understood until 

 Dr. G. J. Romanes, P.R.S.,* investigated their nature from 

 a physiological standpoint. His mode of investigating this 

 subject was to put two or three hundred Sarsice into a large 

 bell-jar, and then to completely shut out the daylight from 

 the room in which the jar was placed. By means of a dark 

 lantern and a concentrating lens, he cast a beam of light 

 through the water in which the Sarsice were swimming. 

 " From all parts of the bell-jar they crowded into the path of 

 the beam, and were most numerous at that side of the jar 

 which was nearest to the light. Indeed, close against the 

 glass they formed an almost solid mass, which followed the 

 light wherever it was moved. The individuals composing 

 * JPhilosophical Transactions, 1875, p. 295 ; ibid., 1879, p. 189. 



