STRUCTUEE AND PHYSIOLOGY OF THE MOLLUSCA. 31 



sinuses ; in the cuttle-fishes the oesophagus is partly or entirely 

 surrounded by a venous smus ; and in the acephala the viceral 

 cavity itself forms part of the circulating system. 



The circulation in the tunicaries presents a most remarkable 

 exception to the general rule, for their blood ebbs and flows m 

 the same vessels, as it was supposed to do in the human veins 

 before the time of Harvey. In the transparent salpcB it may be 

 seen passing from the heart into vessels connected with the 

 viscera and tunics, and thence into the branchial vessels ; but 

 when this has continued for a time, the movement ceases, and 

 recommences in the opposite direction, passing from the heart 

 to the gill and thence to the system. {Lister.) In the compound 

 tunicaries, there is a common circulation through the connecting 

 medium, in addition to the individual currents. 



Aquiferous canals. Sea-water is admitted to the visceral 

 cavity of many of the mollusks (as it is also in radiate animals), 

 by minute canals, opening externally in the form of pores. 

 These aquiferous yores are situated either in the centre of the 

 creeping disc, as in cyprcea, conus, and aticillaria ; or at its mar- 

 gin, as in haliotis, doris, and aplasia. In the cuttle-fishes, they 

 are variously placed, on the sides of the head, or at the bases of 

 the arms ; some of them conduct to the large sub-orbital 

 pouches, into which the tentacles are retracted. 



Respiratory system. The respiratory process consists in the 

 exposure of the blood to the influence of air, or water con- 

 taining air ; during which oxygen is absorbed and carbonic acid 

 liberated. It is a process essential to animal life, and is never 

 entirely suspended, even dming hybernation. Those air- 

 breathers that inhabit water are obliged to visit the surface fre- 

 quently ; and stale water is so inimical to the water-breathers, 

 that they soon attempt to escape from the confinement of a glass 

 or basin, unless the water is frequently renewed.* In general, 



• When aquatic plants arc kept in the same glass with uater-breathiug 

 snails, a balance is prodiu-cd ; which enables both to live without change of 

 water. 



