Respiration in Invertebrate Animals. 35 



the limits of the branchial organ, is relatively considerable. 

 Measured by the complex magnitude of the branchiae, the inert 

 oyster is a physiological paradox. It is hard thence to believe 

 that muscularity and respiration are directly proportional. The 

 force generated by the act of breathing is expended in other 

 directions. Cephalopods and air-breathing Gasteropods apart, 

 the branchial structures of every known mollusk are abun- 

 dantly cdiated. In this anatomical particular they contrast 

 strikingly with those of the Crustacea. Both are breathers of 

 water. In one only are cilia provided. The question impli- 

 cating the reason of natural things lies far too deep to be 

 fathomed by a mechanical explanation. In both the purpose to 

 be accomplished is the same ; in both the means employed are 

 intimately similar, and yet in one instance vibratile cilia are con- 

 stituently admitted into the mechanism, in the other they are 

 rejected. Biochemistry at a future sera will elucidate these 

 mysteries. 



The peripheric circulation in the Mollusca is lacunar rather 

 than capillary. This capital fact was first established by Milne- 

 Edwards* and Valenciennes t : these authors describe the blood 

 as effused into the parenchyma of the body. It returns into 

 the veins without the intervention of capillaries. The details 

 upon which rest these general postulates will be afterwards 

 stated. In the anatomical character of the peripheric passages, 

 in the small proportion of fibrine in the blood, the circulating 

 system and the blood of the Mollusca resemble obviously the 

 chylaqueous fluid and its containing system. 



In all moUusks, separate, specially constructed organs are con- 

 secrated to the function of breathing. Even the Brachiopoda 

 are not exceptional to this rule; they are pallio-branchiate. 

 The universal presence of complexly formed and profusely mul- 

 tiplied respiratory organs attests the extreme value of the office 

 which they are designed to fulfill. 



The ultimate vessels of the branchiae in all mollusks, those of 

 Brachiopods andTunicates J excepted, occur in the form of straight 



* Observ.et Exper. sur laCircul. chez les Mollusques. — Comptes Rendus, 

 1845, XX. p. 261. 



t Nouv. Observ. sur la constit. des appareils de la Circul. chez les Mol- 

 lusques. — Ibid. p. 750. See also Ann. des Sc. Nat. 1845, iii. p. 289. 



X I regret that no recent opportunity has occurred to me to test the 

 validity of the anatomical principle expressed in the text. For the present I 

 assume that the ultimate blood-channels in the branchiae of the Tunicate 

 mollusks reticulate (PI. I. fig. 2 & fig. 4) ; that is, that the blood which 

 moves at one moment in one direction courses at the next in another at 

 right angles with the former, the whole being on the same ))lane, and the 

 circumscribed stigmata being water-passages. This assumption conforms 

 with the description of all observers from the time of Savignv. 



3* 



