THE GASTEROPODS. 



157 



the freshwater kinds are obliged to rise to the surface every 

 time they require to take in fresh air. Such a mode of aerating 

 the blood would obviously be unsuited to marine gasteropoda 

 which are consequently all furnished with branchiae or gills 

 differently placed. In the naked sea-slugs they expand freely 

 in the water, like the tentacula of the sea anemone, and nothing 

 can be more elegant than their forms or arrangement. In the 

 glauci and scyllaaa we see at each side of the elongated body long 



Scylh 



Glaucus. 



arms branching out into tufted filaments ; while in the briarei a 

 hundred furcated stems serve for the aeration of the blood. In 

 the eolides they assume the shape of long riband-like lamellae, 



Eolis. 



disposed in imbricated rows ; in the dorides they form a wreath 

 or garland round the lower intestinal aperture. But whatever 

 their form, their structure is essentially the same, each tuft or 

 lamella containing the ramifications of the branchial vessels, 

 and effecting the oxygenisation of the blood by the extent of 

 surface which they expose to the water. 



In the far more numerous gasteropoda provided with covered 

 gills, we find the same variety of arrangement as in the nudi- 

 branchiate genera. In some they are placed on one side of the 



