3U 



SCIENCE 



[N. S. Vol. XXX. No. 757 



by the writer in the Philosophical Magazine 

 for May, 1904. James Baenes 



Bbyn Mawe College 



some comments on the reactions op 

 perich^ta 

 In a discussion of the method of trial in 

 Science, Vol. XXVI., 662, Professor H. B. 

 Torrey referred to the writer's description of 

 the light reactions of Perichoeta. He inter- 

 preted the behavior of Perichmta in weak light 

 as displaying " Untersehiedsempfindlichkeit " 

 and not the tropic reaction, with the resulting 

 conclusion that there would be no orientation 

 in weak light. The writer had stated that 

 Perichmta responds to weak light chiefly when 

 the anterior end is extended, presumably be- 

 cause of the greater exposure of photoreceptor 

 cells in the integument. Torrey called these 

 " Unterschiedsempfindlich " reactions because 

 apparently due to an increase in the intensity 

 of the light on the cells. Loeb first introduced 

 the distinction between the tropism as a con- 

 stant stimulus efiect and the reaction to 

 change of intensity in the case of Serpula, 

 which bends towards the light and also with- 

 draws suddenly into its tube from the stim- 

 ulus of a shadow cast upon the oral end. 

 Except for the opposite sign of the helio- 

 tropism the reactions of Perichmta and this 

 other annelid bear a family resemblance. I 

 need only refer to the familiar facts that 

 Perichmta or Lumhricus turn away from all 

 but the weakest light and retract into their 

 burrows on sudden illumination. In the open, 

 the worm gives the " Unterschiedsempfind- 

 lich " reaction of retracting its head on sud- 

 den illumination with strong enough light 

 and after a period of backward creeping 

 follows this up by a tropic, turning response 

 effects of sudden illumination are conspicu- 

 ously wanting in weak light, as ordinarily 

 only turning movements appear. 



A form of response to illumination of the 

 anterior end which is between these extremes 

 consists of creeping backward after a distinct 

 pause, which is often prolonged, and without 

 any sudden movements which would naturally 

 be related to the change produced by the stim- 



ulus. These weaker responses might naturally 

 be regarded as constant stimulus effects. Ee- 

 actions attributed to change of intensity 

 ought to give manifest evidence of the shock 

 in resulting movements or inhibitions. 



The objection is raised that the transitori- 

 ness of the stimuli in weak light, received 

 during extension movements, would preclude 

 their giving rise to orientation. The tropism 

 is ascribed to a differential tonus produced on 

 the muscles of the two sides. There is no 

 apparent reason why even transitory light 

 stimuli of any intensity might not produce 

 some appreciable tonic effects. It appears, 

 however, that a considerable change of inten- 

 sity is required to temporarily inhibit forward 

 movement, as is the case in retraction of the 

 head. If the sudden manifestations of shrink- 

 ing are absent in weak light is it not apparent 

 that the threshold for " Unterschiedsempfind- 

 lich " effects is higher than for purely tonic, 

 i. 6., tropic effects in the earthworm? As for 

 the application of the trial hypothesis to the 

 behavior in weak light, that is only giving a 

 name to the somewhat gradual process of 

 orientation, interrupted by movements con- 

 trariward which are less influenced by the 

 light. 



E. H. Harper 



NOETHWESTERN UNIVERSITY 



ENTOMOLOGICAL CONFERENCE ON TEE 

 PACIFIC COAST 

 The department of entomology of the Univer- 

 sity of California lias for several years past held 

 four conferences during the school year at stated 

 intervals, the place alternating v.'ith Berkeley. 

 Thus during the last school year four such con- 

 ferences were held, two at Berkeley, one at Wat- 

 sonville and another at Davis. The last of these 

 meetings, held in Berkeley, was planned to be 

 more inclusive, inasmuch as entomologists from 

 the entire Pacific coast were invited to attend and 

 present papers. The hope was also expressed that 

 a special organization of western entomologists 

 might be effected, inasmuch as the insect problems 

 of the Pacific slope are so difi'erent from those 

 on the other side of the Rocky Mountains. 



At this meeting, held April 20 to 23, the fol- 

 lowing general program was carried out: 



