24 NERVOUS SYSTEMS AND SENSE ORGANS 



showed a direct connection between ectoderm and entoderm, a con- 

 clusion which Parker, 1917, and Parker and Titus, 1916, have 

 shown on both anatomical and physiological evidence. 



Von Heider, 1877, was of the opinion that the mesenteries of 

 some actinians might contain nervous elements. Wolff, 1904, and 

 Kassianow, 1908, were of the opposite opinion but a number of 

 investigators seem to have shown that Von Heider's opinion is the 

 right one, among them Hickson, 1895, Ashworth, 1899, Kiikenthal 

 and Proch, 1911, and Liedermeyer, 1914. 



In recent years Parker has given this group considerable atten- 

 tion and some of his conclusions will be employed in the following 

 discussion. There is also a paper on the histology of actinians by 

 Sanchez, 1918, but in this the nervous system is not considered very 

 extensively. 



The effector systems of sea-anemone are mucous glands, ciliated 

 epithelium and muscles. Although nematocysts are considered by 

 some to be under control of the nervous system, there is good evi- 

 dence that they are independent of it. The only system under the 

 control of the nervous system is the muscular. By means of experi- 

 ments it was learned that the bases of the anemones were especially 

 sensitive, but nervous transmission may be accomplished from 

 almost any portion of the ectoderm to its longitudinal mesenteric 

 muscles. By several experiments it was proved that the trans- 

 mission might be by means of almost any narrow bridge of tissue, 

 proving quite conclusively that the transmission is by a nerve-net. 



Many muscles responded at some distance from the point stimu- 

 lated and in some cases muscles were capable of responding directly 

 to a stimulus ; whether these muscles were also under the control of 

 the nervous system at other times was not clearly established in 

 every case. In the acontia, however, there seemed to be no inter- 

 mediation of nerve impulses in the response to stimuli. Connections 

 from ectoderm to entoderm was proved in many cases. In con- 

 necting the ectodermic and entodermic system the lips and oesopha- 

 gus seemed not as important organs as other parts of the body. 



Although the system of the actinians is diffuse there is some 

 degree of specialization. If the tentacles are stimulated by a nu- 

 trient fluid the oesophagus gapes by contraction of the transverse 

 mesenteric muscles, while weak acid causes a retraction of the oral 

 disc by means of a contraction of the longitudinal mesenteric 

 muscles. The two kinds of response suggest independent receptors 

 and relatively independent transmission tracts. 



In the tentacles the ectodermal surface is more receptive than 

 the entodermal ; if there is a nervous structure in the latter it is 

 probably very simple. The tentacles are complete neuro-muscular 

 organs and may react quite independently of the polyp, as shown 

 when severed from the body. 



