594 Analysis of Books, fyc. 



The circulating system consists of an artery situated on the upper 

 surface of the alimentary canal, beset with numerous little glandu- 

 lar bodies, and having attached to it numerous minute pulmonary 

 branches, extending to the external integuments. Immediately 

 behind the head, the artery bends downwards in a serpentine form 

 towards the under surface of the alimentary canal, and there termi- 

 nates in the vein, which extends in the same direction to the extre- 

 mity of the body. The serpentine curve of the vascular system, from 

 the upper to the lower surface of the alimentary canal, must be 

 viewed as the equivalent of a heart, with which it agrees in having 

 a pulsatory motion. 



The artery, by its successive contractions from the posterior to- 

 wards the anterior part of the body, impels the blood, partly by 

 lateral branches into the vein, and partly into its own pulmonary (re- 

 spiratory) branches, which again re-act, and return to it their aerated 

 blood. The vein, on the other hand, at each diastole of the artery, 

 sends back a portion of its blood, by means of the lateral branches 

 of communication between the two vessels, in which it is assisted 

 by the successive impulses of the blood driven into it by the heart. 

 Hence, the circulation consists partly in an oscillation of the blood 

 between the artery and vein, by means of lateral anastomoses, and 

 partly in a direct current impelled by the heart from the artery into 

 the vein, and from the latter again into the artery. 



The mode of increase in these animals, as is well known, from 

 the observations of Reaumur, Bonnet, and Mueller, is by the spon- 

 taneous separation of segments of the body, which are rapidly de- 

 veloped into new and self-existent individuals. The astonishment 

 created by the first idea of such a process, is diminished when we 

 consider the simplicity of the whole structure, and the identity of 

 character of all the parts of its most important systems. Thus, the 

 integuments, the subjacent muscles, the simple alimentary canal, 

 the central nervous cord, the artery, and the vein, have nearly the 

 same form and character in each segment of the body. The heart 

 and brain alone are newly-developed parts in the new-formed animal, 

 and even these may perhaps be considered as essentially pre-exis- 

 tent to the change, and merely advanced by it to a higher degree of 

 activity. 



The heart first presents itself before the detachment of the new 

 being from the parent stock, as a dilatation of one of the capillary 

 anastomoses between the arterial and venous trunks. The brain, 

 in the same manner, is nothing more than the development of one 

 of the loops or collars surrounding the alimentary canal of the ori- 

 ginal animal, and so small on its primary form as almost to elude 

 observation. 



