298 Dr. T. Williams on the Mechanism of Aquatic 



valve-like organ, placed in the efferent canal, wliich by its move- 

 ments drives a continual current from behind forwards, or from 

 within outwards, and thus occasions a constant ingress through 

 the afferent opening : this organ is the flabellum. It is the 

 modified appendage of the second pair of feet-jaws, specially- 

 developed to answer this purpose. The perfect contact of the 

 water with the respiratory surface is further ensured by the 

 actions of the flabelliform appendages of the other maxillary or 

 ambulatory member, which in most Decapods penetrate into the 

 branchial cavity, and incessantly sweep and comb over the surface 

 of the branchiae. The membrane lining the branchial chamber 

 in some land Crabs, which not only habitually live out of water, 

 but are infallibly drowned if immersed in that fluid, is sometimes 

 disposed in folds capable of acting as reservoirs for a considerable 

 quantity of water, and sometimes presents a spongy texture 

 equally well adapted for storing up the fluid which is necessary 

 to keep the organs of respiration in the state of humidity re- 

 quired for the performance of their functions*. 



The preceding cursory account is offered only as an introduc- 

 tion to that narrative of original details which is now to follow. 



An exact inquiry into the circumferential circulation of the 

 Crustacea will serve to elucidate the apparatus of the blood- 

 system as it exists in the Insect organism. It is only by a minute 

 scrutiny into the last extreme of the blood-current, that the 

 physiologist can penetrate the mystery of the nutrimental act 

 and the ultimate mechanism of the respiratory process. No 

 opportunity is more favourable than that offered in the example 

 of the Crustacea ; — the structures are transparent ; the blood- 

 current is obvious to the eye; every element of structure may be 

 readily reduced to its last analysis. 



Minute Anatomy of the Peripheral Blood-channels and Branchial 



Organs. 



In nearly all species the primary blood-channels, both venous 

 and arterial, are circumscribed by a special membrane which is a 

 distinct and separable structure. The arterial trunks are con- 

 tractile ; they embrace closely the contained fluid. The muscle- 

 tissue present in the parietes of the heart extends evidently to 

 those of the arteries. The veins are non-contractile ; their walls 

 adhere externally to the solid parts, amid which they lie ; they 

 cannot therefore contract in transverse diameter ; they are pas- 

 sive conduits, the arterial are active. The arterial and venous 

 trunks are lined internally by an extremely fine hexagonally- 

 celled epithelium (PI. XVIII. figs. 2, 3, 4). The cells present re- 



* See Carpenter's Principles of Comparative Physiology. 



