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LETTER XVI. 



ON THEIR ORGANS OF RESPIRATION. 



The respiratory organs of the mollusca have peculiar 

 claims to the attention of the conchologist, not solely because 

 of their function, which, indeed, is one of chief importance, 

 but because they have furnished the principal characters on 

 which modern systematists have proceeded to svibdivide tlie 

 cUiss into orders and families. Cuvier, of whom, among 

 recent naturalists, it may most truly be said that he was 



" Ordain'd to light with intellectual day 

 The mazy wheels of Nature as they play," 



was the first to perceive their utility in this respect ; and 

 when it is considered that their position mainly determines 

 the arrangement of the other viscera, and must consequently 

 exert a powerful influence over the habits of the animals, 

 you will feel disposed to admit that a happier choice could 

 not have been made, the more particularly as the organs in 

 question are in general easy to detect, and exhibit suflicient 

 variety in location and form for every systematic purpose. 



Molluscous animals are either Puhno7iiferous ,* and breathe 

 atmospheric air only, or they are Branchifcrous, and respire 

 it through the medium of water. In the former the respi- 

 ratory organ is a simple cavity, commonly situated on the 

 anterior part of the back ; but sometimes, as in Testacella, 

 near the tail. The air is admitted by a small circular aper- 

 ture that opens outwards on the neck under the margin of 

 the cloak, and which the animal opens and shuts at pleasure. 

 Externally the cavity is protected either by a thick fold of 

 the cloak, often strengthened with a horny or calcareous 



* Lamarck ohjocts to the use of this term, as apjilied to the mollusca, 

 Anim. s. Vert. vi. ii. 44 ; and hence some have called the order Pneianono- 

 branchous. But there is no good reason in the objection. The distinction 

 between a lung and a gill, rests on anatomical structure. When the air is 

 operated on through the medium simply of an internal cavity or cavities, 

 the respiration is piilmonary ; when, on tiic contrary, processes or gills pro- 

 ject from the cavity or from the surface, and expose tlie blood to the air by 

 the vessels which arc distributed on these processes and folds, the respira- 

 tion is branchial. — Milne-Edwards, Hist, Nat. des Crustucis, i. 91. 



