236 Relations of the Structure of 



of our most successful microscopists, published, in the Ento- 

 mological Magazine [vol. i. p. 239.1 ^ minute and most inter- 

 esting account of it as observed by him in the larva of 

 an jEphemera. Mr. Bowerbank's observations corresponded 

 very closely with those of Professor Carus, a Continental 

 naturalist, who had published his discoveries a short time 

 previously, but whose descriptions Mr. Bowerbank had never 

 perused. 



The circulation in annulates is not, as it is in vertebrates, 

 entirely confined to limited and well-defined vessels ; neither 

 is there any heart for the reception and expulsion of the 

 blood ; but there is a large wide channel, extending the whole 

 length of the animal, through which the blood is constantly 

 rushing upwards from the posterior towards the anterior ex- 

 tremity. This current is kept in motion by the alternate 

 opening and closing of double valves, distributed at regular 

 intervals throughout its extent : this operation causing a mo- 

 tion, as each portion is afresh impelled upwards, which is 

 precisely equivalent to the pulsations of a heart. The valves 

 open upwards only, and between them, on both sides, are 

 other valves in the great channel, which as regularly open 

 inwards to admit fresh supplies of blood from the lateral 

 portions of the body. On each side of the animal there is a 

 downward current, confined to no distinct or ascertainable 

 channel, but permeating indifferently all parts in the neigh- 

 bourhood of its progress : portions, however, part off from 

 the main current at intervals, and flow through the side 

 valves of the central channel already described. The pulsa- 

 tions in this downward current are more faint, and its pro- 

 gress is somewhat more slow, than that in the upward central 

 current. On the opening of a side valve, the blood may be 

 seen rushing towards it from every part in the neighbourhood, 

 as though it were in a state of freedom among the transparent 

 and muscular parts, and not limited to any vessel. No blood 

 ever leaves the main channel, but through the aperture at the 

 anterior extremity, where it turns inwards towards the oeso- 

 phagus, and cannot be traced, by the microscope, any farther. 

 In the antennae, legs, and nervures of the wings a similar 

 double current is observable. Such is the usual circulation 

 of blood in an insect. 



In the lobster, and other large marine annulates, a some- 

 what more perfect system is observable ; that is, the limits 

 of all the vessels are perfectly well defined, and may be 

 ascertained and figured with confidence and even ease. In 

 these, there is a manifest approach to the circulation of 

 fishes, which possess it in the most simple state of all the 



