COELENTERATA 23 



tention from qute early times; Milne-Edwards in his natural history 

 of corals in 1857 wrote : 



"They enjoy a highly developed sensibility, not only do they con- 

 tract forcibly on the slightest touch, but they are also not insensible 

 to the influence of light. But no nervous system or organs of 

 sense are to be discovered in them." In these early times there 

 were, however, some vague suggestions of ganglia and nerve chords 

 in Actinia, but no confidence was placed in them. Huxley, in his 

 elements of comparative anatomy of 1864 says: "The nervous sys- 

 tem has at present not been determined in them." Alexander 

 Agassiz, in his seaside studies of 1871 says: "Only a few pigment 

 cells found at the tentacles are sense organs." 



Schneider and Ritteken, 1871, state that the chromatophores 

 are organs of sense, compound eyes. 



J. D. Dana in his Corals and Coral Islands, states that "they 

 sometimes possess rudimentary eyes." 



Duncan, 1874, describes in some detail the structure of the 

 "eyes" of actinians. He also recognizes a plexus or network of 

 nerve fibers and cells under the epidermis, and remarks that the 

 diffuse nature of the nervous system is what might have been 

 anticipated. 



The first rather complete account of the nervous system was 

 by the Hertwig brothers in 1879-80. They recognized sensory cells 

 in the epithelial layers and under the epithelium and next to the 

 muscular layers of both ectoderm and entoderm a laj^er of nerve 

 fibers and cells. The sensory cells when stimulated carry impulses 

 to the nerve cell layer and this in turn to the muscles beneath them. 

 Nerve impulses from the ectoderm to the entodermal muscles were 

 supposed by them to pass over the exterior to the oesophagus and 

 from its inner end to the entodermal musculature. They considered 

 the body of the sea-anemone to be rather uniformly supplied with 

 nervous tissue except at the oral disc where in the ectoderm the 

 cells were concentrated in a sort of center. Wolff", 1904, and Gros- 

 ley, 1909, in the main accepted Hertwigs' suggestions but they 

 placed the concentration of the nerve fibers in the wall of the 

 oesophagus and not in the oral disc. 



Kassianow, 1908, in Alcyonaria, believed the disc to be the 

 center of an individual member of the colony and Liedermeyer, 

 1914, although his observations were of sections alone, was of a 

 similar opinion from his study of one of the Pennatulacea. 



Havert, 1901, on a sea-anemone by means of the Golgi method, 

 maintained a difli'use nervous system for actinians. This author also 

 believed that the ganglion cells, so-called by the Hertwigs, were 

 really motor cells which receive impulses from sensory cells and 

 then transmit them to muscles, a condition more like that of the 

 central nervous system of forms with a reflex arc. This author also 



