202 COLE. [VOL. II. 



swim entirely free ; (2) they may partially swim, kicking along 

 on bottom with those legs that are down; or (3) they may 

 crawl with all the legs on the bottom. The second method 

 is the most common and the one in which the greatest speed 

 is made. But the striking thing to be noticed is that in the 

 first and second methods, those in which they swim or par- 

 tially swim, the movement is backwards, or nearly back- 

 wards, while in the third method of locomotion, when they 

 crawl on the bottom, they invariably go straight ahead, that is 

 to say, with the anterior end directed towards the source of 

 light. It thus appears that in moving towards the light they 

 orient themselves differently, according to whether they swim 

 or crawl ; and, as I have shown, whether they swim or crawl 

 depends directly upon the vigor or rate of the treacling move- 

 ment and not upon any difference in the direction of the stroke. 

 The question naturally presents itself, Why should there be 

 this difference in orientation in the two cases ? In order to 

 determine this, let us first consider specimens which are forced 

 to crawl by being weighted with tinfoil. If an animal so 

 weighted is placed in the water with its anterior end towards 

 the light, it crawls directly ahead without turning ; if its head 

 is pointed in any other direction, it gains this same orientation 

 by making a short circle, turning in the shortest direction 

 towards the light. Now for any animal to walk in a circular 

 path it is necessary for those legs on the outside of the circle 

 to act with a greater force than those on the inside, and thus 

 shove the body around ; in the case of an animal orienting 

 itself so as to head towards the source of light, this means that 

 those legs away from the light act stronger than those toivards 

 the light, they being the legs on the outside of the circle which 

 the animal describes in coming around. If this rule holds true 

 when the Pycnogonid swims, and I see no reason why it should 

 not, we have a simple explanation of its orientation with refer- 

 ence to light at all times. This can perhaps best be made 

 clear by taking a particular case and following it through. 

 An animal is placed on the bottom with the long axis of the 

 body at right angles to the rays of light. This is represented in 

 the diagram A (Fig. 3), in which we are supposed to be looking 



