Respiration in Invertebrate Animals. 309 



granules (PI. XVIII. fig. 4). Neither tubercles, nor setae, nor 

 hooks, nor teeth such as those of the cells of the epidermis of 

 insects, occur on any part of their surfaces or edges. The flat 

 surfaces and edges are perfectly smooth. The laminae are slightly 

 thicker immediately around the vessels than near the margins. 

 In the former situation the intermediate parenchyma is slightly 

 thicker than in the latter. 



The microscope readily resolves these patches of solid tissue 

 to their last elements (PI. XVIII. fig. 5) . They are cells, generally 

 but not always bearing nuclei, and frequently oil-globules. 



They are enveloped by a common capsule, but not always so. 

 They are filled with a colourless fluid, which reaches them imme- 

 diately from the contiguous blood-current. 



Their office is simply and purely mechanical. They connect 

 together the parallel epithelial laminae. They divide the blood- 

 stream. They exhibit every possible variety of size and figure. 

 They are undoubtedly penetrated and surrounded by a plexiform 

 arrangement of neri-e-tubules (PI. XVIII. fig. 13, b). These nerves 

 play no part in the physical act of respu-ation. Nerve-force is 

 here introduced only for the maintenance of the organic struc- 

 tural integrity of the elements. From this condition their right 

 function is inseparable. Oxygen does not therefore really enter 

 and mix with the blood independently of nerve-power. 



In the Brachyurous Crustaceans the flabella (PI. XVII. fig. 9,B) 

 constitute important instruments in the machinery of breathing. 

 They are indispensable in consequence of the closed character 

 of the branchial chamber. They amount to three in number : 

 two small ones which move between the anterior gills, and one 

 external horizontal flabellum, which in its arc of motion sweeps 

 over the external surface of the whole row of branchiae. 



Like those of the Lobster, the flabella of the Crab are profusely 

 studded with appropriate hairs (C, D, E) for agitating and sepa- 

 rating the branchial laminae. During the moment of separation 

 a rush of water takes place between these laminae. So admirably 

 adapted are the setae of the flabella to the purpose intended, 

 that they must be regarded as express provisions. No other 

 sort of instrument would subserve the end in view. How 

 graceful are the contrivances of nature ! how perfect her 

 adaptations ! 



The ultimate setae are multiplied at their distal extremities by 

 several parallel rows of acutely-pointed teeth (C, D, E). Those 

 on the base of the stalk point difi'erently from the former. 

 By this inimitable disposition these exquisite organules are 

 fitted to do double work. They also obtain a better leverage by 

 hooking to the corresponding teeth, which in these orders are 



