304 Dr. T. Williams on the Mechanism of Aquatic 



or cylindrical processes, which diverge at right angles from the 

 axis of support, in which the main afferent and efferent channels 

 of the blood are lodged. The gill of the Lobster (PL XVII. fig. 8) 

 expresses, typically, the general and minute structure of the 

 branchial organs of all the above- enumerated genera. They con- 

 sist of fourteen separate organs, disposed in two alternate series, 

 and lodged in a thoracic cavity. Each gill, conical in general 

 figure, resolves itself into a multitude of small tubes (fig. 8, a'"), 

 proceeding from the sides of the axis. In a transverse section, it 

 will be seen that the large afferent trunk («), running up one side 

 of the axis of the whole gill, sends off a minute branch to each 

 lateral tubule {a"'). A single tubule (B) has then its afferent 

 vessel, which runs along its central axis (B, b). The sides of this 

 afferent vessel are cribriform, so that the blood readily escapes at 

 evenj point of its course into the 'loose lacunose tissue (C) and 

 (B, c) which forms the circumference of the tubule, and through 

 which the blood returns to the efferent or venous trunk (B, c). 

 The blood-corpuscle thus runs round the circumference of a 

 cylinder. This latter represents the area and period of aeration. 

 It corresponds with a plane surface equal in length to such circum- 

 ference. There is not, therefore, either functionally or structurally, 

 any real difference between the cylindrical branchial filament and 

 the leafy variety of this organ. The walls of the tubule are per- 

 fectly smooth. The hexagonal cells of the epidermis are detect- 

 able at the extreme outermost coat. These cells differ from those 

 of the ordinary epidermis in nothing but in the fact of their 

 greater tenuity. The islets of the included parenchyma are 

 composed of nucleated cells. They are nourished by the 

 branchial blood. The presence of these masses of living solids 

 in the midst of the blood-current, at the very point at which 

 the latter is undergoing aeration, concludes the controversy as 

 to the capacity of the fluid in such place and time, at one and 

 the same period to receive oxygen and to nourish the parenchy- 

 matous tissue. Here it is accomplished. The question why, 

 in the Lobster and its kindred, nature should resort to this 

 curious method of multiplying and subdividing the branchiae, 

 illustrates the unsearchableness of ultimate causes. 



In the Lobster, as in the Crab, two orders of flabellcB exist. 

 The first consists of a whip-like process, moved by powerful 

 muscles, and guarding the outht of the branchial chamber (PL 

 XVIII. fig. 9. shows the minute structure of extreme end). By 

 its regular movements, a determinate current of water flows out- 

 ward. The edges and flat surfaces of this marvellous instrument 

 are profusely armed with secondary instruments (PL XVIII. fig.9), 

 matchless for their beauty, surprising as means to an intendedend. 

 The external or horizontal flabellum is sufficiently long to sweep 



