SYSTEM OF AQUEDUCTS. 173 



muscles of the trunk (&), passes out of the thorax, nearly in 

 contact with, and on the right side of, the oesophagus. It 

 terminates nearly mid- p. 



way between the heart 

 and the rectum (m), 

 opening into a consi- 

 derable cavity, which 

 has the liver under- 

 neath, and the mem- 

 brane enveloping the 

 spire above it. When - 

 the animal contracts 

 the distended foot, the 

 water is seen to flow 

 out between the man- 

 tle and the shell on 

 the right side. The tube and cavity are easily inflated by 

 a blowpipe introduced into one of the tubes of the foot." 



It seems obvious enough that this apparatus must be 

 intended, principally, in aid of the locomotion of the mol- 

 lusca. Delle Chiaie does not hint at this its use, but seems 

 to have considered it as more closely connected with their 

 respiration and nutrition, for he introduces his essay by 

 reminding us, that water is to the mollusca in general what 

 air is to the land animals ; and he explains the fact that the 

 former can sustain a very long privation of food, on the sup- 

 position that life is supported by the water retained in the 

 aqueducts, and endures only until this is consumed by ab- 

 sorption or evaporation.* There is no doubt that the circu- 

 lating fluids brought into contact with the water in the 

 ducts will be aerated in some degree, but the main purifica- 

 tion of the blood is left to other structures, and the system 

 in question must have more than a secondary office, being 

 indeed primarily designed to give buoyancy and enlargement 

 to the body, and a greater aptitude to the parts by whose 

 actions it is moved. When shrunk within its shell, you 

 might well deem any animal that could hide itself there all 

 too small and weak to carry about a burden larger and 

 heavier than itself, and that safety might be here advan- 

 tageously exchanged for relief from so much heaviness of 

 armour, and from such an impediment to every journey. 

 There is in my small cabinet a fine specimen of Cassis tuber- 



* Van Beneden, with whom Milne-Edwards agrees, supposes that the 

 circumfluent water gains direct access into the blood-vessels by the medium 

 of these passages. Jinn, des Sc. Nat. (1845) iii. 277. 



