No. 5-] TH1-: HABITS OF PYCNOGONIDS. 2OI 



the specially arranged articulation between the first and second 

 joints. In order to prevent the specimens from swimming 

 while making these observations, and to force them to crawl, 

 a small collar of tinfoil was clasped around the body between 

 the second and third pairs of legs, care being taken that it was 

 small enough not to interfere with the movements. 



Reaction to Light. 



As has been stated, one of the most important factors con- 

 cerning the movements of the Pycnogonids when placed in the 

 water is the direction of the source of light. If the dish 

 containing them is placed near a window, the animals either 

 swim or crawl quickly to the light side of the dish. 1 This fact 

 has been noted by Loeb, 2 who says of Anoplodactylus : " Es ist 

 wie die meisten frei beweglichen Bewohner der Oberflache des 

 Meeres positiv heliotropisch," and he also states that when the 

 body was severed between the second and third pairs of legs, 

 the anterior portion still reacted in the same way, while the 

 posterior portion, which was comparatively inactive, moved 

 independently of the light. These results were easily verified, 

 and it was further ascertained that the oculiferous tubercle 

 (Figs. I and 4, oc.} is the photo-recipient organ, for when this 

 was cut off the animals failed entirely to show any response to 

 the light. Although there are, of course, individual variations, 

 it is surprising how quick this response usually is, especially 

 if the dish is covered above and on the sides away from the 

 window, so as to exclude all light from other directions. In 

 the diffuse light three or four feet from a northwest window, 

 lively specimens usually traveled an average of 12 cm. to 15 cm. 

 in thirty to forty seconds. The response seems to be more 

 pronounced when the light enters horizontally. 



In moving towards the light the animals may adopt any one 

 of the modes of locomotion previously described : (i) they may 



1 This is not the case if there are hydroid stems or similar objects which the 

 I'ycnogonid can grasp. The tendency to cling to anything of this character 

 seems to be stronger than the reaction to light. 



- Loeb, J., " Bemerkungen iiber Regeneration," Arch. Entwick-Mech. Bd. ii, 

 pp. 250-256. 1896-97. 



