PHYSIOLOGY OF THE INVERTEIiRATA. 395 



The ArachnidaRro entirely devoid of wings, and the organs 

 of locomotion are never inserted on the abdomen. 



The Arancina may be conveniently divided into two groups 

 — the wandering and the sedentary spiders. In the former 

 group belong the swift-runner, the side-walker, and the vault- 

 ing or leaping spiders. All the wandering spiders trust to 

 their swiftness of movement in securing their food ; and some 

 of them can run in any direction. 



The Crustacea. 



In the lower Crustacea, represented, for example, by the 

 PJti/llopoda, Dr. G. 0. Sars states that there are two modes of 

 locomotion. In the case of CydcstJici^ia hislojn, one of these 

 is accomplished when the animal is freely suspended in the 

 water ; in the other it takes place while it is at the bottom 

 of the water ; in the former case, it is a swimming motion ; 

 in the latter, a creeping or more generally a burrowing, 

 motion. The swimming motion is performed exclusively by 

 the aid of the antenna?, the repeated strokes of which propel 

 the animal through the water. During this motion, the 

 antennae, together with the anterior part of the head, remain 

 exserted from the front part of the shell, being moved laterally 

 to a greater or less extent. The locomotion effected by this 

 means is not very rapid, nor abrupt or jerking, but a perfectly 

 even run through the water, whereby the animal as a rule 

 turns the dorsal part uppermost. Not rarely, however, this 

 attitude becomes changed, and the animal is often observed 

 to revolve several times before breaking off the motion and 

 sinking to the bottom. On the whole, the swimming motion 

 appears to be effected with considerable effort, especially 

 when the individuals arc carrying a young brood ; and hence 

 this motion is never continued for any length of time, but 

 takes place at intervals, the animal being more frequently 

 found resting on the bottom or affixed to some submerged 

 object. 



