396 PHYSIOLOGY OF THE INVERTEBRATA. 



The creeping or burrowing mode of locomotion, wMcli 

 takes place while the animal is on the bottom, is effected 

 partly by the antennae, but more especially by the flexion and 

 powerful extension of the trunk, the caudal plate being thus 

 exserted from the shell inferiorly and moved rapidly behind, 

 as it strikes against the bottom. This mode of locomotion 

 has sometimes a distinctly jerking character. Often, by 

 repeated strokes of the tail, the shell will be turned round 

 several times in succession, and may thus get rather deeply 

 buried in the loose muddy deposit at the bottom of the water. 

 Dr. Sars, in his important paper {loc. cit., p. 33), also 

 describes the movements of the shell, head, trunk, tail, eye, 

 antennulse, antennae, &c., but it is not our object to refer to 

 these separate movements. 



In the higher Crustacea the organs of locomotion or limbs 

 are connected in pairs with the different thoracic segments ; 

 there are frequently seven pairs, as in the Isopoda (e.g., 

 Oniscus), the prawns, and the Talitri (sand-hoppers) ; but in 

 other Crustacea — e.g.., the crabs, crayfishes, and lobsters— there 

 are only five pairs of limbs. The structures of these append- 

 ages differ considerably: in some forms they are wholly 

 foliaceous, membranous, and exclusively adapted for swim- 

 ming ; in others they have the form of small flexed columns, 

 articulated, and disposed only for walking ; in others, still, 

 besides remaining adapted for this mode of locomotion, they 

 become suited to act as so many small spades wherewith to 

 dig the earth, and in that case they are enlarged and lamel- 

 lated towards the extremity ; and still, finally, in others they 

 terminate in forceps, and become prehensile organs, perform- 

 ing at the same time the function of locomotion. In the 

 swimming Crustaxea, such as Astacus, Somarus, Palcemon, &c., 

 the abdomen terminates in a tail-fin, which is the principal 

 organ of locomotion ; but in those individuals which walk 

 more than they swim, the tail-fin is, as a rule, very small, and 

 folded under the thorax : in the crabs, for example, this 



