ORGANS OF RESPIRATION. 541 



means of examining this unexplained phenomenon. In marine 

 species, their movements are quickly arrested by immersion 

 in fresh water, and by sea water in the palustrine forms. 



Nearly all the gastcropods breathe, like the other mollus- 

 cous classes, by means of branchiae, which are disposed 

 externally in the naked species, and are generally concealed 

 under the mantle in the testaceous forms ; a few species only 

 breathe air by a pulmonic cavity opening on one side, some 

 of which are aquatic in their habits, others terrestrial. The 

 naked gasteropods, like the naked annelides, have greater 

 latitude of surface than the testaceous forms, for the distri- 

 bution of their external ciliated branchiae, and of their 

 general ciliary currents ; and, indeed, their whole surface 

 is often ciliated and respiratory. The branchiae in these 

 species are generally disposed, as in the dorsibranchiate 

 annelides, in ramose tufts, symmetrically arranged over a 

 greater or less extent of the dorsal or lateral parts of the 

 body; but the modifications of form and structure which 

 they present are too numerous and diversified to afford satis- 

 factory means for the zoological subdivision of this class. 

 In scyllaa, they are disposed in isolated, ramified, small tufts 

 on the outer surface of the extended dorsal folds of the 

 mantle, and on the surface of the back ; in tritonia, as in 

 arenicola among the annelides, they rise more directly 

 in elegant ramifications, along each side of the back; in 

 eolidia and cavolina, they form numerous rows of small 

 simple clavate projections, similarly disposed along the back; 

 and in tergipes, a single row on each side; in tethys, the two 

 dorsal rows are composed of alternately disposed tufted and 

 crested branchiae ; and in glaucus, they form long filiform 

 extensions from the ends of the lateral palleal appendices. 

 In the tritonia and similar forms, the number of branchial 

 pairs varies with the degree of development of the body, 

 and they are developed successively from the front pair back- 

 wards. 



In the doriSy the branchiae form elegant plumose or ramose 

 tufts around the anal opening on the posterior part of the back ; 

 they constitute a single penniform organ, under the margin 

 of the mantle, on the right side, in pleurobranchus and 

 pleitrobranchaa ; they are protected by a thin pellucid shell, 

 on the right side of the body in aplysia, and on the middle 



