CIKCULATlOiN IN INSECIS. 217 



sides of which the vessels are distributed. The 

 vertical trunk, or aorta, connecting the annular 

 reservoirs, presents a muscular dilatation, which 

 has been considered by Tiedemann as performing 

 the function of a heart. In the Sipunculus, there 

 are two dilated vesicles of a similar kind ; one at 

 the commencement, and the other at the termina- 

 tion of the aorta. 



The Entozoa exhibit only rudiniental vascular 

 systems, consisting in the Ascarides, of two lateral 

 canals extending the whole length of the worm ; 

 and in others, being merely modifications of the 

 intestinal passages. 



Approaches to a more general circulation are 

 made in the class of Insects, the more elaborated 

 nutrient juices of which contain a greater abundance 

 of coloured particles, and assume more of the cha- 

 racter of the blood of the higher animals. This 

 blood, after it has transuded through the coats of 

 the alimentary passages, appears to have a free 

 range through the general internal cavities, and is 

 transferred in a definite course to every part of the 

 system, performing a circuit in its motion. Insects 

 present us, also, with the first trace of a centraliza- 

 tion of the vascular system, in the presence of a 

 tube, already described as the dorsal vessel, the 

 purpose of which is to supply an hydraulic force for 

 pumping up the fluid from the abdominal cavity, 

 and propelling it forwards in a continuous stream 

 towards the head. This vessel extends the whole 

 length of the back ; and after taking a bend down- 

 wards as it advances, tapers into a slender tube, or 

 aorta, which divides into several short branches, 

 terminating abruptly by open orifices. Its cavity is 



