306 Dr. T. Williams on the Mechanism of Aquatic 



office to be discharged can only be appreciated by studying them 

 in their connexions. 



The flabellum, examined in its general structure, is a machinery 

 no less remarkable. It consists of two horny parallel plates 

 (Plate XVIII. fig. 9, h, c). Between these plates there travel with 

 great regularity radiating currents of blood (/). The currents 

 are separated by muscular fascicles (e), observing a similar dispo- 

 sition. The latter are levers of great power. Along the margins 

 of the flabella the blood returns in large obvious channels. It 

 is worthy of the special attention of the physiologist, that from 

 these currents there proceed outwards at right angles minute 

 streams ti^aversing the axis of each microscopic seta {a). The 

 fluid in these exists as a single column, sometimes as a flux 

 and reflux stream moving in lacunae (fig. 7, a). It does not ad- 

 vance and return along difi^erent conduits. In other words, the 

 hollow axis of the seta is not divided by a longitudinal partition 

 into two channels. It is a single tube. The blood in these parts 

 moves in a flux and reflux manner. These little appendages, the 

 structure of which is so accessible to the eye, express with great 

 clearness the method in which constantly, in all invertebrated 

 animals, the nutritive fluids reach the collateral recesses, the by- 

 parts of the solids. In the Lobster, the biliaiy tubules (PI. XVIII. 

 fig. 10) resemble strikingly in structure the branchial. To esta- 

 blish further the manner in which, in the crustacean organism, 

 the blood is related to the organized parts, it were at this place 

 not inappropriate to allude incidentally to the minute anatomy 

 of the liver-follicle of the Crustacea. It is a simple tube (fig. 10), 

 having two coats embracing one another concentrically (/"). Be- 

 tween these coats the blood flow^s in irregularly-bounded channels, 

 forwards on one side, or half, and reversely on the other. 



The hollow axis is filled by the secreted product [c, g, h) . It 

 is the commencement of the excretory duct. The epithelial cells 

 of the outer coat are the same at every stage of the tubule {a). 

 Those of the inner (e) begin to change in character as the ex- 

 treme caecal end of the tubule is approached. At first they are 

 hexagonal and flat ; by degrees they bulge ; they become filled 

 more and more with fluid, until at length they become sphe- 

 roidal {b). They form a layer of several series deep. This end 

 of the follicle, as supposed by Mr. Goodsir, is really the pro- 

 ducing or secretive centre. The blood pervades irregular pass- 

 ages tunnelled between the spheroidal cells {b). The fluid 

 elements of the blood pass from the blood-channels into the 

 interior of the modified epithelial cells. From the latter it 

 exudes into the hollow axis of the tubule (c). This is the act 

 of secretion ! The fixed cells impress upon the fluid by which 

 they are traversed a tendency/ to change, a disposition to combine 



