THE MOVEMENTS OF SHORE ANIMALS 171 



diameter. Further data with regard to these boring forms 

 can be obtained in the valuable paper by Caiman so frequently 

 referred to above. 



Jumping. — Progress by means of leaps is best seen in the 

 sub-aquatic (and terrestrial) forms or forms such as certain 

 Amphipods {e.g. Talitrus, Orchestia, etc.) and the shore- 

 frequenting CoUembola. The former include the common 

 " beach-fleas " or '* sand-hoppers " in which the driving power 

 for the spring is obtained by flexing the posterior portion of 

 the abdomen and driving it swiftly backwards, the necessary 

 purchase on the substratum being obtained by means of 

 the last three pairs of thoracic Hmbs. In this way the animal 

 can jump to a distance of several feet. 



In the CoUembola a special jumping organ is usually 

 present in the shape of a long style or spring, consisting of an 

 elongated basal portion and two small appendages, situated 

 on the under side of the abdomen close to the posterior end. 

 This appendage is carried flexed beneath the abdomen where 

 in some genera it is retained in position by means of a 

 " catch " projecting from the under surface of the third 

 abdominal segment. The spring is considered to be 

 elastic, so that when the catch is removed the spring sweeps 

 backward with considerable force, and striking the ground 

 jerks the animal forward. 



In deaUng with the CoUembola (Poduridae) of Cold 

 Spring Beach, Davenport (1903) shows that the variety 

 of movements in these forms is considerable. The Podurids 

 of the beach live between tide-marks, burrow in the sand 

 at high tide and rise to the surface when the tide is out. 

 They run up surfaces in the face of the wind and leap when 

 they reach the top, being blown back to the starting point. 

 In addition to the normal movements of running and 

 springing, Davenport distinguishes such special locomotor 

 movements as whirling, descent and ascent in sand, running 

 up stones, and leaping into the air. A similar variety marks 

 the movements of most shore forms, and is no doubt accounted 

 for by the varied nature of the habitat. In the majority of 

 cases a particular shore animal, though predominantly a 



