86 RESPIRATION. 



and cold climates are subject to a winter hibernation, and 

 whilst in this state the heart ceases to beat, respiration is nearly 

 suspended, and wounds do not cicatrize. The epiphragm or egg- 

 skin with which the animal at this time temporaril}^ closes the 

 aperture of its shell is slightly permeable by air; like the shell 

 it is a production of the mantle, and is in some species thin and 

 cartilaginous (American and most European species), in others 

 opaque in consequence of calcareous elements (subtropical). 

 It is only formed by non-operculate pulmonates, and occasionally, 

 when the mouth of the shell has been closely applied to another 

 shell, or to a piece of wood or rock, sufficient protection is thus 

 obtained and the epiphragm is not developed. In case the 

 weather is very severe, the mollusk gradually retires farther 

 within its shell, constructing additional epipliragms at intervals — 

 alwaj's thinner than the first one. Cold is not the only cause of 

 hibernation ; dryness and want of food also produce it, and 

 Fischer,* by placing a Bulimvs decollatus alternatel}^ upon very 

 dry and humid ground found that it would form and destroy 

 epiphragms at the rate of ten or fifteen per month. Heat 

 causes a summer sleep or aestivation, but in this the animal 

 functions are much less interrupted. 



In the nudibranchiate mollusca the respiratory plumes form 

 tufts or rosettes exposed on the back or sides of the animal 

 (viii, 92) ; or they are protected b}^ a fold of the mantle (infero- 

 branchs and tectibranchs of Cuvier). Finally Eolis, Elj^sia, 

 Pontolimax appear to have no specialized breathing organ, 

 unless this office is performed by the dorsal papilla? of Eolis: — 

 it has been doubted, because the animal appears to survive their 

 loss without inconvenience. 



Pte7^opoda. The respiratory apparatus is branchiate when it 

 exists, but is ver^?- diverse in its disposition in the different 

 genera. In Clio there is no distinct organ ; in Euribia it consists 

 of two naked appendages of the anterior part of the bod}^ ; in 

 Fneumodermon of a posterior organ, having some analogy to 

 the branchial rosette of Doris ; in the thecosomata ( Hyalsea, etc. ), 

 it is large, describing a regular curve, with anterior concavity , 

 and contained in a cavity of the mantle. 



In the Scaphopoda there are no specialized respiratory^ organs. 



Pelecypoda ( viii, 89; xxii, 65). In the bivalve mollusca, or 

 lamellibranchiata, so called from the form of the breathing organs, 

 the branchiae are double, placed on either side of the bod}^, and 

 enclosed between the mantle-borders and the interior visceral 

 mass, the latter interposed.- Behind the branchial cavity are two 

 tubes (Siphonida) or merely two orifices (Asip>honida) for the 

 introduction or exclusion of the water destined for respiration. 



* Melanges Conch., 35. 



