THE ORIGIN OF THE NERVOUS SYSTEM 139 



from the sense-cells, numerous multipolar ganglion-cells whose processes 

 add to the fibrillar material already mentioned. A careful study of this 

 fibrillar material has recently been made with the result that a true 

 nervous network has been demonstrated in hydroids (Wolff, 1904; 

 Hadzi, 1909), siphonophores (Schaeppi, 1904) and sea-anemones 

 (Wolff, 1904; Groselj, 1909). In the sea-anemones in particular this 

 network appears to be a perfectly continuous and diffuse one, notwith- 

 standing Havet's previous declaration (1901) to the contrary. The 

 third layer is composed of parallel muscle-fibers that rest against the 

 supporting lamella on one side and are in contact with the nervous net- 

 work on the other side. The muscle-cells of this layer are much elon- 

 gated, spindle-shaped cells. These three layers, the epithelial layer, the 

 nervous layer and the muscular layer, constitute the structural elements 

 in the ectodermic neuromuscular mechanism of a sea-anemone. 



The nervous type of ectoderm just described covers practically the 

 whole surface of a sea-anemone and has been designated as a diffuse 

 nervous system in contrast to a centralized one. The fact that the 

 nervous layer is more fully developed on the oral disk than elsewhere 

 has given anatomical grounds for the assumption that this portion is a 

 central nervous organ, but, as will be shown later, the physiological 

 evidence in favor of this opinion is so slight that the designation of the 

 nervous system as a diffuse one is more consistent with facts. 



From the standpoint of our original analysis, it is quite plain that 

 in the sea- anemones we are dealing with at least two elements of the 

 typical neuromuscular mechanism, namely, receptors as represented by 

 the sense-cells, and effectors as seen in the muscle-fibers. Whether the 

 fibrillar material that intervenes between these two structures represents 

 an adjuster or central apparatus will be discussed after the action of 

 this nervous mechanism has been more fully described. 



The feeding habits of the sea-anemones throw considerable light on 

 the physiology of their nervous structures. If particles of meat are 

 dropped on the tentacles of an expanded Metridium, they become en- 

 tangled in the mucus on these organs and are quickly delivered to the 

 mouth, where they are swallowed. If fragments of clean filter-paper 

 soaked in sea water are similarly dropped on the tentacles, they are 

 usually discharged from the edge of the oral disk without having been 

 brought to the mouth. Thus the animal appears to discriminate be- 

 tween what is good for food and what is not. If, however, pieces of 

 filter-paper soaked with meat juice are put on the tentacles, they are 

 usually swallowed as though the sea-anemone had been deceived. On 

 the basis of these simple experiments a still more striking combination 

 can be devised. If a sea-anemone is provided alternately with pieces 

 of meat and pieces of filter-paper soaked in meat juice it will in the 

 beginning swallow in sequence both materials, but after ten or a dozen 



