156 THE BIOLOGY OF THE SEA-SHORE 



interesting to consider how far shore conditions have opened 

 the way to ectoparasitic habits. For instance, Pycnogonum, 

 a rather stouter form than most, is frequently found clinging 

 in close contact with some large anemone such as Tealia, 

 " whose living juices it very probably imbibes " (D'Arcy 

 Thompson, loc. cit.). It is rarely, as we shall see, that a 

 particular animal is restricted to a sole mode of locomotion ; 

 thus in addition to hanging on to seaweeds and hydroids 

 some Pycnogonids are able to perform gentle swimming 

 movements. 



Without a doubt, the most highly specialised form of 

 locomotion on the shore is that characteristic of Echinoderms, 

 e.g. Asterias nihens. It is one pecuHarly adapted both to a 

 rocky substratum and to progress between tide-marks where, 

 in addition to climbing up vertical surfaces or beneath over- 

 hanging ledges, the animal is obliged to keep a firm grip to 

 avoid dislodgment by wave action. The organs of locomo- 

 tion are, of course, the tube feet, and the power capable of 

 being exerted by these is so great that, when an attempt is 

 made to remove a starfish, they will break rather than release 

 hold. The customary explanation of the locomotion of the 

 starfish is very much as follows : the tube feet, distended 

 with fluid, are swung forward in the direction of movement 

 and placed upon the substratum. By muscular contraction 

 in the lower part of the tube feet the water is forced back 

 into the ampullae and a vacuum is thus created between the 

 under surface of the foot and the surface of attachment. By 

 means of the hold so obtained the starfish pulls itself a short 

 distance forward. Contraction of the ampullae then forces 

 the water into the lower part of the foot, destroys the 

 vacuum, and the hold is released. The tube feet are now 

 swung forward again and the cycle of movements repeated. 

 With this view, namely that the starfish is supposed to haul 

 itself along as one might do by attaching ropes and then 

 shortening them, Jennings (1907) is in disagreement. Loco- 

 motion has been shown by this writer to proceed normally 

 when the animal is placed either on a greased surface, the 

 attachment of the suckers being thereby impeded, or on 



