322 LECTURE XV. 



pyramids on each side in the crabs, two being rudimental and seven 

 forming the exterior of the pyramid in Cancroids and Maioids ; but 

 the number is more variable, and usually greater, in the Macroura, 

 amounting to twenty-two in the lobster. In both Brachyura and 

 Macroura slender appendages called "flabellse" {fig. 132, tn) are 

 adapted to sweep over the surface of the branchiae and free them 

 from foreign particles : in the hermit-crabs and other Anomoura, 

 this office is performed by the fifth pair of legs, which can be 

 inserted for that purpose into the branchial chamber, the carapace 

 being raised to admit them. By this elevation of the carapace a 

 large quantity of water can be admitted to the branchiae ; a membrane 

 connecting the carapace with the inner walls of the branchial 

 chamber and extending along the anterior edge of the first abdominal 

 ring, precludes the admission of water into the cavity occupied by 

 the thoracic viscera. 



So far as the gills are attached to the bases of the ambulatory or 

 natatory legs, they must be influenced by their movements ; and the 

 more active the progress of the Crustacean, the more briskly will it 

 breathe ; and, since the muscular energy directly depends upon the 

 amount of respiration, the two functions are brought into direct 

 relation with each other by this simple connection of their respective 

 instruments. 



The land-crabs have their bi-anchiae always supported by water 

 through special modifications of the apertures of the branchial 

 cavities, which enable them the better to retain fluid, and also by 

 numerous folds or by a spongy structure of the lining membrane of 

 the respii-atory cavity by which the quantity of the contained fluid 

 may be augmented. The moisture contained in the branchial 

 chambers of the land-crabs {Gecarcinus) and tree-crabs (Birgus) is 

 doubtless much more highly aerated than the water which bathes the 

 branchifB of the strictly aquatic species, and thus may explain the 

 fact that the Crustacea which habitually live out of water are 

 drowned by being long immersed in that fluid. 



No other trace of distinct excretory organs has hitherto been 

 detected in the present class than the simple, unbranched, long, and 

 slender tubes which open into the intestinal canal of some of the 

 higher Crustacea. Swammerdam* figures one such tube opening 

 at the hinder end of the intestine in Pagurus. In Maia there 

 are three such tubes ; two inserted on each side of the pylorus, and 

 a third a little further behind. These may be uriniferous tubes, like 

 the corresponding parts in certain insects and spiders. 



* CCXXXIII. p. 87. t. xi. fig. 3. 



