164 THE BIOLOGY OF THE SEA-SHORE 



of shore animals that they are by no means tied to a par- 

 ticular mode of locomotion. For instance, the Planarian 

 worm already mentioned, Leptoplana tremellaris, when 

 alarmed or in search of prey, may exchange the gliding habit 

 for that of swimming, which it performs somewhat in the 

 manner of a skate except that the undulations are more 

 rapid. According to Gamble {loc. cit.), in swimming " the 

 expanded fore parts of the body act as lobes which are 

 flapped rapidly up over the body and then down beneath it, 

 undulations running rapidly down them from before back- 

 wards." Again, as pointed out by Eliot (1910), some 

 Nudibranch molluscs which are provided with lateral wing- 

 like expansions or with flat tails have some powers of 

 swimming, e.g. Elysia, and even clumsy- looking Dorids 

 when dropped into a few feet of water may find their way 

 to the bottom with movements like those of a flat fish. 



We owe to Eisig (1906) a very thorough-going analysis 

 of the methods of locomotion in Annelids. The pre- 

 dominant method of locomotion is described by this writer 

 as " podial — undulatory," and consists of a horizontal undu- 

 lating movement of the body in conjunction with the move- 

 ments of the parapodia. In this form of locomotion, 

 successive parapodia on the same side of the animal are 

 working in opposition. When the undulations consist 

 of numerous small waves of low intensity, each individual 

 wave embracing only a small number of segments, the 

 resultant progress is described as a swimming gait 

 (" Schwimmgang," or " Marche natatrice " of Bohn). With 

 an increase in the amplitude and intensity of the waves the 

 animal rises from the bottom and begins to swim {e.g. Nereis). 

 In typically swimming forms (e.g. Nephthys) the undulatory 

 element predominates. Cases of purely undulatory locomo- 

 tion occur in forms such as Nemerteans but not in Annelids, 

 although undulatory movements (albeit in the vertical plane) 

 are performed by tubicolous Polychaets in connection with 

 respiration within the tube. 



F ' Crustaceans of the genus Portunus have the last two 

 joints of the last pair of thoracic appendages considerably 



