188 ARTHROPOD A [CH. 



that is, the space enclosed between the branchiostegite and the 

 side walls of the thorax. Thus as mentioned above, the Crabs 

 having a high branchial chamber of shorter length than that of the 

 crayfish have preserved more of the pleurobranchs and lost more 

 of the podobranchs. 



In order that the gills may be able to carry out their functions 

 properly the water which bathes them must be constantly changed. 

 In other phyla of the animal kingdom, notably the Mollusca, this 

 is effected by means of cilia but cilia are an impossibility to an 

 Arthropod since the ectoderm everywhere seems to have an irre- 

 sistible proclivity to produce cuticle. 



In the absence of cilia a current of water is caused to pass over 

 the gills by the action of the scaphognathite, that is, the exopodite 

 of the second maxilla. This is a fan-shaped plate fitting into the 

 extreme front corner of the branchial chamber. During the life of 

 the animal it is in constant vibration, and it literally bales the 

 water out of the front end of the branchial chamber, causing a current 

 of fresh water to enter the hinder and l<^MBfcd of the chamber 

 between the edge of the branchiostegite anclH Rises of the legs. 

 The whip-like exopodites of the maxillipedes ai^Wpcaphognathite ; 

 they whisk away from the sides of the animal thewater which the 

 scaphognathite has baled out. The constant shaking of the gills in 

 consequence of the motion of the legs must aid in ridding them of 

 the film of water which might cling to their surfaces. 



Turning now to the internal anatomy, if the carapace be clipped 

 away from the mid- dorsal region of the head and the thorax, and 

 the terga removed from the segments of the abdomen, a general 

 view of the internal organs will be obtained. In the extreme front 

 is seen the brain at the sides of which two roundish glandular 

 masses, the kidneys, are situated: behind these the so-called 

 ''stomach" with its attached muscles. Behind the stomach we find 

 the heart floating in a sac termed the pericardium, whilst in the 

 abdomen we find a mass of muscle which, as we approach the peri- 

 cardium, breaks up into strands which are attached to the sides of 

 the thorax. 



Although the heart and its vessels are the structures which 

 naturally present themselves first on commencing dissection, yet it 

 will be more convenient to describe the alimentary canal first of all. 

 This canal differs sharply from the type found in most other groups 

 of animals in the large part of its length which is constituted 

 by the stomodaeum and the proctodaeum. The part lined by 



