222 SNAILS AND SLUGS [CH. VII 



vessels over the sides and roof of the pulmonary chamber 

 which is in direct communication with the outer air. 

 These afferent pulmonary vessels are connected by series 

 of delicate vessels with corresponding efferent vessels, 

 afferent and efferent occurring in regular alternation. 

 The blood becomes aerated in passing through the 

 delicate connecting vessels. The efferent pulmonary 

 vessels gradually unite on the roof of the chamber to form 

 the pulmonary vein which, as already mentioned, conveys 

 the blood into the auricle of the heart. A portion of the 

 blood has, however, a different course, passing into the 

 glandular tissue of the nephridium and thence into 

 the pulmonary vein without fully circulating within the 

 pulmonary network of vessels. From this blood nitro- 

 genous waste substances are eliminated by the nephri- 

 dium. 



The blood of Helix contains amoeboid corpuscles in 

 relatively small numbers and, dissolved in the plasma, 

 an oxygen-carrier, hsemocyanin. This substance contains 

 copper in combination with a proteid ; when deoxidised it 

 is colourless, but blue when oxidised. The blood ofPlanorbis 

 contains haemoglobin dissolved in the plasma. This sub- 

 stance is also found in the muscles of the pharynx and 

 jaws of Limncea and Vivipara 1 . The air within the 

 pulmonary cavity is renewed chiefly by diffusion when the 

 respiratory aperture is open. A more complete change 

 of air is brought about when the snail is retracted into 

 its shell by the columellar muscle; the resulting com- 

 pression forces air out of the chamber ; on the other hand 

 1 Lankester, Proc. Roy. Soc. 1873. 



