536 PHILOSOPHY OF ZOOLOGY. 



glomerate gland attached to the intestines, in others, as a 

 collection of intestinula cceca. The hepatic vessels of In- 

 sects (for there is no secreting organ which can be compar- 

 ed to a liver), have their origin in the fluid contents of the 

 abdomen, and even send their capillary extremities to the 

 remotest part. They vary greatly in number and size. 

 The walls are dense and cellular, and 'the bile which they 

 secret is yellow, brown, or white, according to the species. 

 These vessels, in some, terminate separately, in others, 

 they unite into a common duct. The terminations vary 

 greatly, being in different species at the pyloric extremity 

 of the stomach, the middle of the intestine, or at the rec- 

 tum. 



The circulating system appears in this group, in its most 

 perfect form, among the larger Crustacea. There is a pul- 

 monic ventricle situate dorsally, and a systemic ventrical 

 situate ventrally, thus executing a complete circulation in 

 which all the blood in its course is aerated. In the arach- 

 nida with pulmonary sacs, the heart seems to be lengthen- 

 ed, placed along the back, and to give out vessels laterally. 



In insects, neither absorbing nor circulating vessels have 

 been detected, although anxiously looked for by many ce- 

 lebrated anatomists and microscopical observers. The nu- 

 tritious portion of the food appears to be absorbed by the 

 walls of the intestines, and discharged into the cavity of 

 the body, where there are neither veins, arteries, nor heart. 

 Towards the back, indeed, there is an obvious vessel, pla- 

 ced longitudinally, to which some have given the name of 

 heart, but which is more generally denominated the Dorsal 

 Vessel. This vessel is widest in the middle, and diminishes 

 in size at each extremity. Its walls consist of two mem- 

 branes, the internal one muscular, and the external one 

 cellular. This last is so much crowded with tracheal ves- 

 sels, as to appear to be entirely composed of them. All the 



