LIGHT RECEPTIVE FUNCTION OF GONIONEMUS. 3 



much pigment to answer this requirement. In regard to this 

 point Beer ' ('oi) said : " Mitdar viel zu alt gewordenen Ansicht, 

 dass Pigment der wesentlichste und urspriiglichste Bestandtheil 

 eines jeden Sehorganes sei muss endgultig gebrochen werden." 

 And in a considerable paper he makes good this statement. 



In regard to the structure of the marginal papillae it may be 

 said that most Trachomedusge and some Leptomedusae possess a 

 welt or ring of tissue made up of ectoderm, enlarged especially 

 at, or near, the bases of tentacles by the rapid growth of cells for 

 the production of nettling organs 2 which are to migrate out on 

 the tentacles. In Goto's description of Gonionemus depressum* 

 it is stated that " these bulbs . . . contain hollow prolonga- 

 tions of the entoderm of the circular canal, and the ectoderm is 

 clogged with developing nettle cells." Perkins shows this struc- 

 ture in two figures. 4 The main point is that there is here no 

 definite arrangement of either cells or pigment such as is present 

 in so-called primitive eyes or eyespots. So that these papillae 

 would scarcely be expected to function as special organs for light 

 stimulation. This does not preclude their being tactile ; they 

 are ciliated. 



From experiments Yerkes finds 5 that when the oral surface is 

 up, Gonionemus is much more sensitive to light than when the 

 aboral surface of the animal is turned toward the light. This is 

 supported by his observations that in a weak light the bell 

 margin is turned upward and in strong light it is turned 

 downward. The tentacles are cut off and, testing the animal, he 

 finds that while the tentacles are sensitive to light they are not 

 the cause of the difference in sensitiveness between the oral and 

 aboral surfaces of the medusa. After removing the whole mar- 

 gin of the bell Yerkes finds that the remainder of the bell is not 

 sensitive to light, or at least does not provoke response. Then 



1 Beer, Theodore ('oi), " Ueber primitive Sehorgane," 1901. 



2 Murbach, L. ('94), " Beitrage zur Anatomie und Entwicklung der Nesselor- 

 gane d. Hydroiden," Archiv fur Naturgeschichte, 1894. 



3 Loc . cit. 



4 Perkins, Henry F. ('02), "The Development of Gonionema murbachii," 

 Proceedings of the Acad, of Nat. Sciences of Philadelphia, November, 1902 (March, 



1903). 



Loc. cit., page 300. 



