Respiration in Invertebrate Animals. 37 



vertebrated animals. They are bounded by skilfully configurated 

 cartilages, as will be afterwards explained. In the Tunicata, as 

 in other Acephala, the blood leaving the open ends of the arteries 

 passes into the interstices — lacuna — of the parenchyma of the 

 body ; thence it is taken \ip by the open mouths of the venous 

 radicles. The solids are thus literally soaked in the fluids. The 

 former are everywhere bathed by the latter. It may be affirmed 

 in a general sense, that the higher the serial position of the ani- 

 mal, the smaller the breadth of the ultimate blood-currents, and 

 conversely. The degree of subdivision which occurs in the 

 blood-streams represents a numeric measure of the nutritive 

 actions. The area comprehended by the mantle is divisible in 

 the Tunicata, as in Acephala, into two sub-areas. The one is 

 either bounded, lined, or traversed by the branchiae, and contains 

 the mouth ; the other embraces the viscera and includes the anal 

 outlet. This fact is absolute. That space into which the mouth 

 opens is homologous with the pallial extra-branchial or general 

 cavity of the mantle in the Acephala. That in which the intes- 

 tine terminates coincides with the intra-branchial or visceral en- 

 closure in all bivalves. An exact conception of these primary 

 divisions of the body in the inferior mollusk is really indispen- 

 sable to the perfect understanding of those respiratory and ali- 

 mentary currents of the water, the direction and relative bearing 

 of which have perplexed anatomists from the epoch of Cuvier to 

 the sera of Messrs. Hancock and Clark. 



In the ceconomy of the Tunicate and Acephalan mollusks this 

 principle is inviolable — that nothing, neither water nor aliment- 

 ary particles, is conducted to the mouth, which has passed 

 through the gills. Water charged with carbonic acid is never 

 swallowed. 



The feculent pellets are never and cannot be mixed with the 

 alimentary. The current which conveys fresh water to the 

 branchiae is convective also of food to the mouth. The stream 

 which carries away the effete product of respiration bears off the 

 feculent rejectamenta. 



There are then, in truth, but two chief oeconomic water move- 

 ments in these animals — that which enters the pallial or extra- 

 branchial space, and that which leaves the visceral and intra- 

 branchial inclosure. This is simple and intelligible. It resem- 

 bles a ray of light shining amidst a darkness which for half a 

 century has brooded over a vexed and perplexing controversy. 



1 1 is impossible to perform one step in advance towards a more 

 satisfactory knowledge of this subject, unless the meaning of the 

 '•siphons" (PI. I. fig. 1, a, h) be first brought into the light of clear 

 definition. They are commonly distinguished into the branchial 

 and the anal. The terms in the ordinary signification would indi- 



