No. 5.] THE HABITS Ol-' PYCNOGONIDS. 197 



species, being found among both light and dark colored hydroids 

 and algae. Pallene impresses one as the smallest form of the 

 three owing to its extreme slenderness, though it is really 

 almost twice the extent of Tanystylum, measuring some 12 mm. 

 to i 3 mm. across. 



Swimming and Crawling Movements. 



/v 



The activities of the three species of Pycnogonids under 

 consideration are in a way directly correlated with their struc- 

 tures. Tanystylum, a short-legged and compact form, is very 

 sluggish and inert ; if placed at the surface of a dish of water, 

 it kicks hardly at all, but sinks immediately to the bottom, 1 

 where it does not attempt to crawl but usually draws its legs 

 together over its back and remains quiet. Pallene, on the 

 other hand, under the same conditions does not sink to the 

 bottom, but by vigorous kicking movements of its long slender 

 legs remains suspended in the water, for a considerable time at 

 least, its further movements being determined by the condi- 

 tions, one of the most important of which, as will be shown 

 later, is light. In the actions of Anoplodactylus there is great 

 individual variation, but in general it may be said that they 

 are intermediate between those of Tanystylum and Pallene. 

 Some specimens sink almost at once to the bottom, where they 

 rest in whatever position they may strike ; others may crawl 

 along upon the sand, or partly swim, touching the sand with 

 only the tips of certain of the legs ; or still others may swim 

 entirely free from the bottom. As with Pallene, just what the 

 animal does appears to depend largely upon the conditions. 

 Most of my observations were made upon Anoplodactylus, for 

 the reason that it was easiest to obtain and of convenient size 

 for observing the movements in detail. 



Before going further it may be well to give a brief explana- 

 tion of the terminology which I shall use. Various authors 

 have used different names for the seven pairs of appendages 



1 In these experiments the bottom of the dish was covered by a layer of fine 

 sand. The depth of water was usually about 5"cm. to 7 cm., though deeper water 

 was tried with no difference in the results. 



