XVlil INTRODUCTION. 



averaging -0005 inch, but some attaining -0003. These 

 are very numerous in the mass. 



5. Circidatoy and Respiratory nystems. These exist in so 

 simple a condition that we can scarcely separate them in 

 our investigations. Dr. Williams has distinguished by the 

 term Chylaqueous fluid, " that fluid which occupies the 

 gastric and perigastric cavities of all animals below the 

 Annelida."* It is far less vitalized than true blood, but 

 still it is not mere water, being impregnated with organized 

 corpuscles and slightly albuminized. In the animals of 

 the class before us there is no blood, and no vascular system, 

 but the cavity of the body is ample, and is copiously 

 occupied by a transparent fluid, which has by some been 

 mistaken for sea-water. I have, however, proved^ by ex- 

 periments, recorded elsewhere,t on numerous species, that 

 this fluid is copiously provided with organic corpuscles, 

 circular or ovate disks, granulose in character, of a clear 

 yellow colour, varying from '0001 to -0008 inch in diameter, 

 the larger ones inclosing oil-globules. The fluid coagulates 

 on the addition of nitric acid, showing that it holds albu- 

 men in solution. 



It would appear that the action of the stomach is confined 

 to the solution and extraction of albumen and oil, which 

 are carried with sea-water into the general cavity, the com- 

 pound being a chylaqueous fluid; and that it is in the 

 upper part of the interseptal chambers that it is acted upon 

 by the biliary secretion. 



For the free circulation of this fluid to every part of the 

 interior, the whole body is lined with a delicate, strongly 

 ciliated epithelium. The ciliary current is upward : when 

 a pellucid diantlius has its fosse much exposed, it is quite 

 easy to see the current driving up from every part of the 

 interior along the whole inner wall, and passing into the 

 tentacles, up which the atoms are then hurled. I believe 

 there is no change in the set of this current : for though 

 atoms are seen, especially at the bottom of the tentacles, 

 occasionally to pass annularly or diagonally; and though 

 of course there must be a return of the fluid driven up- 

 wart [ — f or there does not appear, with the closest watching, 

 a trace of exit at the tip of the tentacles; and though, 

 indeed, atoms are seen, though rarely, to pass downward, — 

 I think these irregular and retrograde movements are 



* Phil. Trans. 1852. t Annals of Nat. Hist.; March, 1858. 



