MOLLUSCA. 129 



in the digestive gland. The hsemocyanin of the blood is a copper- 

 containing proteid which readily takes up oxygen from the air in 

 the lung into loose chemical combination, and parts with it as 

 readily to the tissues. It acts, therefore, as an oxygen-carrier. 



5. The Respiratory Organs are represented by a true air- 

 breathing lung (pulmonary sac) placed on the upper side of the 

 visceral hump, behind the collar, and communicating with the 

 exterior by a rounded valvular opening on the right side of the 

 body. The delicate roof and side- walls of the lung are formed 

 by the mantle, which presents internally a network of ridges, in 

 which the vessels already described ramify. The floor is thin, but 

 muscular, and immediately overlies the crop and the bulk of the 

 reproductive organs. When at rest it is strongly convex upwards, 

 but it becomes flattened by contraction so that the lung-cavity is 

 increased in size, and air consequently passes in. This is inspiration, 

 the converse of which, expiration, is effected by the floor simply 

 ceasing to contract. The pulmonary opening is valvular, and 

 thus the supply of air is regulated and desiccation prevented. 

 The essential part of respiration consists in carbon dioxide 

 diffusing out of and oxygen diffusing into the vessels ramifying 

 in the lung-roof. The part played by haBmocyanin is explained 

 above. 



6. Excretory Organs. A large, cream-coloured kidney, some- 

 what triangular in form, with the apex forwardly directed, is 

 closely united to the posterior part of the lung-roof. It contains 

 a cavity (the surface of which is increased by the projection into 

 it of numerous lamella) which communicates with the pericardium 

 by a minute opening, and with the exterior by an ureter. This 

 arises from the anterior end of the kidney, passes along its right 

 side, and then runs along the side of the lung above the rectum 

 to open close to the anus. The kidney is equivalent to one of 

 the renal organs in the mussel, and is perhaps to be regarded as 

 homologous with one of the nephridia in such a form as the 

 earth-worm. It is plentifully supplied with blood, from which its 

 glandular epithelium separates nitrogenous waste in the form of 

 ammonium and calcium urates. The digestive gland has also 

 been shown to take part in the work of nitrogenous excretion. 



7. Reproductive Organs (Fig. 37). The snail possesses a very 

 complicated set of hermaphrodite reproductive organs, mostly of a 

 whitish colour. (1) The hermaphrodite gland is a small kidney- 



2 9 



