MOLLUSCA OF British Guiana. 41 



specialised portions of the mantle, or that integumentary 

 layer that covers over the viscera of the body and 

 secretes the shell of the Mollusca. By the folding 

 of the mantle over the neck of the animal, a large 

 chamber is formed for the gills, and water is ad- 

 mitted and driven from this chamber for respiratory 

 purposes. 



A most noteworthy feature in connexion with the 

 respiratory system of the Apple-snails, is the existence 

 of a capacious pulmonary air-chamber situated above the 

 branchial chamber, and opening dire6lly into it by a 

 small hole in the front towards the left side. By certain 

 writers this chamber has been treated as equivalent to 

 a swim-bladder or float, but as its walls are richly 

 supplied with vessels leading to the expanded auricle 

 of the heart, its funftions are evident. In the 

 large specimens of the Ampullaria glauca, the 

 details of its stru6lure are easily made out. If the 

 animals are kept under observation, they will frequently 

 be seen ascending to the surface and pumping in air by 

 means of the siphonal neck-lappet, even though the 

 water in which they are living be well aerated for 

 branchial respiration. This additional pulmonary cham- 

 ber is extremely suggestive in its relation to the develop- 

 ment of that of the true air-breathing forms, such as the 

 land-snails and slugs etc., in which gills are never de- 

 veloped; while it helps to explain the survival of the 

 animals when purposely kept for long periods, in some 

 cases even for years, out of water. 



Both in the Melanias and Ampu.larias, when the body 

 is retraced into the shell, the aperture is closed by a 

 horny plate or operculum which is secreted by the 



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