THE MOVEMENTS OF SHORE ANIMALS 165 



expanded and flattened, and are known as swimming 

 crabs. Stebbing (1893) quotes Gosse as having observed 

 species of this crab in the Caribbean Sea and among the 

 gulf-weed in the tropical Atlantic, shooting through the water 

 almost like a fish, " with the feet on the side that happens 

 to be the front all tucked close up and those on the opposite 

 side stretched away behind, so as to hold no water, as a 

 seaman would say, and thus offer no impediment to the 

 way." The swimming powers of British species of the 

 genus are not nearly so well developed and the paddle-like 

 last pair of thoracic limbs are used as much for shovelling 

 sand as for swimming. 



A peculiar form of swimming may be observed in the 

 common scallop Pecten opercularis, and to a lesser degree in 

 P. maximus. The mechanism of this curious jerky motion 

 has been described by Vies (1906) and by Dakin (1909). 

 Briefly stated, the movement is due to the sudden expulsion 

 of water (brought about by the vigorous clapping of the 

 shell-valves) in a narrow jet at each side of the dorsal edge 

 of the shell, the pallial folds acting as a valve and preventing 

 the escape of the water elsewhere. The opening of the shells 

 is normally in the direction of movement, the hinge behind. 

 As in many other forms, however, a sudden danger or 

 stimulus gives rise to what may be called a special flight- 

 reaction, the animal reversing its normal procedure, and 

 darting away with its hinge-line foremost. Dakin {loc. cit.) 

 suggests the following structural modifications as being in 

 close relationship with the swimming habit : the equilateral 

 character of the shell, the evolution of a muscular mantle- 

 fold or velum, the large single adductor muscle with its 

 adaptations for rapid contraction, and also the large internal 

 cartilage for opening the shell. 



Burrowing. — The number of burrowing forms occurring 

 on the shore is very large, and, as one might expect, the 

 majority are Annelids. The advantages of this habit on the 

 shore are, as we have seen, protection both from enemies 

 and from drought. In addition, the sand or mud of most 

 shores contains a large amount of organic debris, decaying 



