ANNULOSA. 537 



coats are liberaliy provided with nervous filaments. This 

 vessel is kept in its position by the tracheal tubes, and by 

 muscular fibres, which, in general, are disposed in triangles, 

 and increase in breadth from the superior part of the body 

 to its inferior extremity. 



The contents of the dorsal vessel are fluid, but of such 

 consistence, that, when its coats are punctured, no liquid 

 flows out. The colour is usually similar to the adipose 

 matter which is collected on or near its surface, and differs 

 according to the species. When placed under a micros- 

 cope, this humour appears to consist of grains or globules, 

 containing other globules. It mixes readily with water, 

 and when dried, resembles gum. 



The humour of the dorsal vessel is subject to some de- 

 gree of motion, arising from the contractions which it ex- 

 periences. These are irregular as to time, and proceed 

 from the one extremity to the other, by stages usuaUy cor- 

 responding with the rings of the animal, and strongest in 

 the abdomen. The nerves do not appear to exercise any 

 influence on these contractions. The muscular fibres and 

 tracheal filaments appear to exercise the greatest control. 

 These contractions, which, in some species, are little more 

 than thirty, in others amount to one hundred and forty in 

 a minute, according to the species, have been denominated 

 pulsations, and the organ itself has been termed a heart. 

 To that viscus, however, it bears no resemblance, except in 

 its contractions, which, however, are irregular. It neither 

 receives nor gives motion to any circulating fluid. Its use 

 appears to be to imbibe and convert the fluid of the abdomen 

 into fat, to serve as a supply in those numerous cases where 

 much nourishment is suddenly required, as, during the me- 

 tamorphoses of youth, and the production of eggs in matu- 



