172 THE BIOLOGY OF THE SEA-SHORE 



swimming, a burrowing, or a jumping form, as the case may 

 be, is not by any means restricted to a sole method of locomo- 

 tion. 



In addition to ploughing its way through the sand, 

 the cockle makes use of its powerful foot to execute a kind 

 of jump. This method of progress is brought about by 

 bending the foot under the shell and suddenly straightening 

 it, when the animal makes a clumsy kind of leap and rolls 

 over and over. A similar form of progress is seen in the 

 tropical Stromb shells and their relatives the Scorpion 

 shells, Rostellaria, etc. (Woodward, 19 13). 



Locomotion involving Use of the Surface Film. — Shore 

 animals with their marked resourcefulness have not neglected 

 to make use of the elastic properties of the " surface film." 

 Locomotion, with inverted body, on the under side of the 

 surface film is particularly prevalent among Gasteropods 

 {e.g. Rissoa, and many Nudibranchs), and is made possible 

 by the flattened foot with its mucous secretion, the usually 

 large amount of surface in relation to mass, and light weight. 

 As previously mentioned, it also occurs in Planarian worms 

 such as Leptoplana. 



According to Colgan (1909), " Nudibranchs can float 

 with the ventral surface uppermost and in confinement 

 many of them show a great love of this position. ... As 

 far as is known, they are obliged to make their first excursion 

 to the surface with the aid of a piece of seaweed or other 

 material support and possess no power of levitation of their 

 own. On reaching the surface they secrete from the foot a 

 layer of mucus and remain in an inverted position with the 

 dorsal region downwards and the foot and mucus-layer 

 upwards. By bringing the margins of the foot together they 

 can drop from this position at will and frequently remain 

 suspended some inches below the surface by a thread of 

 mucus connected with the superficial layer." The same 

 writer says that reascent of the thread is probably made in 

 the way described by Taylor for the arboreal form Limax 

 arbor um (1894-19 14), that is to say , " by bringing the extremi- 

 ties of the body together and transferring the point of 



