APLYSIA. 215 



BLOOD VASCULAR SYSTEM. 



(Figs. 13-15.) 



The blood vascular system in Aplysia consists of a two- 

 chambered heart, arteries, venous sinuses and lacunae. The 

 heart is enclosed within a pericardial chamber, and is situated 

 on the left side of the animal, ventral to the kidney and over- 

 lying the visceral mass somewhat obliquely. It is in close 

 proximity to the ctenidium as in all Molluscs. The pericardium 

 is a portion of the coelom and communicates with the renal 

 cavity, which is also coelomic, by a reno-pericardial aperture. 

 The arteries constitute an asymmetrical system of vessels 

 supplying the tissues with blood from the heart. There is no 

 capillary system, but the arteries discharge into large spaces 

 within the tissues known as lacunae. The lacunae are devoid 

 of epithelial lining, the blood coming into apparent contact with 

 the muscles and connective tissue fibres. From the lacunae 

 the blood travels back to the general body cavity surrounding 

 the gut and other organs. This body cavity is not, therefore, 

 a true coelom, since it contains blood, but a haemocoele. From 

 the haemocoele the blood is collected in wide vessels with 

 definite walls, the venous sinuses. These occur only near the 

 ^ ctenidium and kidney, with which they communicate, and are 

 not to be found in other parts of the body. The blood traverses 

 either the kidney or the ctenidium on its way back to the heart, 

 so that the blood entering the heart is mixed blood, the greater 

 part being oxygenated blood from the ctenidium and a small 

 portion being blood purified from nitrogenous waste, but not 

 oxygenated, from the kidney. 



The blood is colourless and contains amoeboid corpuscles. 

 The oxygen carrier is haemocyanin dissolved in the plasma. 



It is at first sight difficult to see how a constant circulation 

 can be maintained on account of the great spaces in the body. 

 The blood fills them very rapidly causing sudden turgidity, 

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