132 FOURTH REPORT — 1834. 



powei", afforded by the contraction of living solids, is necessary to 

 bring the nutrient matter within the influence of this attraction. 

 For in them the organs are gradually collected into distinct 

 masses and individualized, and their life depends upon the reci- 

 procal action of these. That their activities may be maintained, 

 a common nutrient fluid is con^ejed to them on the one hand, 

 and on the other, is brought into near contact with the air. 

 Wherever the heart exists, it is the principal moving power ; but 

 it does not exist until there is a distinct apparatus for aerating 

 the fluid. In the Medusae and some Polypi, the nutrient fluid is 

 the immediate product of digestion ; the digestive cavity supplies 

 the place of a heart, and tubiiles proceed from it through their 

 substance. In insects a single tube represents the heart : it pul- 

 sates from behind forwards, and absorbs at every diastole a por- 

 tion of a fluid mass which surrounds the intestine, and is the 

 product of digestion. Here the vessel scarcely ramifies, that 

 form being assmned by the respiratory apparatus which con- 

 ducts the air to every part of the body. In the Crustacea the 

 heart begins to be concentrated; there are arteries and veins : the 

 single cavity of the heart receives the blood from the gills and 

 distributes it to the body. In the ]Mollusca the heart consists 

 of an auricle and ventricle. In the highest of this class, there- 

 fore, it resembles the systemic heart of Mammalia ; but in 

 some of the lowest it surrounds a part of the intestinal tube, 

 and in this respect they form a link between the others and those 

 animals in which the heart and digestive cavity are one and the 

 same organ. In the Vertebrata, wherever the heart is single it 

 belongs to the lungs; and here first the nutrient matter is ab- 

 soi'bed from the digestive cavity by peculiar vessels, and then 

 conveyed to the circulating system. As the pulsating vessel is 

 gradually concentrated, so also is the nervous system. When 

 the heart has the form of an oblong sac, the nervous system pre- 

 sents a series of swellings connected by a single or double cord. 

 In the Mollusca the heart becomes more globose, whilst a cen- 

 tral mass of nervous matter represents the brain, and in many 

 instances sui*rounds the oesophagus as the heart does the intes- 

 tine in some. In vertebral animals the ganglionic chain becomes 

 a spinal cord, the ring a brain of a spheroidal shape, whilst the 

 heart is a more perfect central organ ; and here first a peculiar 

 system of nerves is appropriated to the nutrient apparatus (the 

 sympathetic), whilst there is a peculiar set of vessels for absorp- 

 tion. Thus do the nervous and circulating systems appear to 

 advance simultaneously towards perfection, the one being a con- 

 dition, but not a cause, of the other's existence. In all the vei'- 

 tebral animals the heart lies below the spinal cord and abdomi- 



