442 SANGUIFEROUS SYSTEM. 



the lateral vessels ; and posteriorly these vessels send each a 

 large branch to the ovary and cloaca. In the nais, the usual 

 median dorsal artery, propelling the blood forwards, divides 

 at the anterior part of the neck, and descending on each side 

 of the cesaphagus, forms, as in many other elongated narrow 

 species, a single inferior median epi-neural vein, by which the 

 blood is conveyed posteriorly. This arterial oesophageal ring 

 of the nais exhibits the same regular systole and diastole, which 

 are seen in the dorsal artery, and which are obvious in the 

 corresponding sacculated arterial rings passing down from the 

 anterior part of the great dorsal artery to the ventral vein of 

 the earth-worm, (133. C. c, d.) Where there are two median 

 trunks, a dorsal and a ventral, in the sanguiferous system of the 

 annelides the inferior is commonly a smaller returning vessel, 

 extending along the motor surface of the nervous columns, as 

 in the entomoid classes, and analogous to the descending ab- 

 dominal aorta of fishes, and the embryos of higher vertebrata. 

 So that the position of the heart-forming dorsal trunk, the 

 median ventral vessel, and the vascular rings or branchial 

 arches which connect them, in many of the articulated classes, 

 correspond precisely with the inverted position which I have 

 shown in their moto-sensitive nervous columns, and other 

 important organs. 



The simplest condition of the circulating system is seen 

 in those which have no perceptible respiratory organs, as 

 in the nais and planaria ; but considerable modifications 

 of the general plan are induced in higher genera, by the de- 

 velopment of organs for this function, in form of internal air 

 sacs, or of external cephalic or dorsal branchiae. The high cu- 

 taneous vascularity appears to supply their place in the abran- 

 chiate species, where the ramifications of the blood vessels 

 are quite distinct from those of the digestive apparatus. The 

 long dorsal artery of the nereis cuprea appears to be slightly 

 dilated in each segment of the body, and receives or gives 

 off the branchial vessels from the arterial arches which en- 

 compass the oesophagus ; but in other species the branchial 

 vessels are given off to these organs from each side of the 

 dorsal vessel in its whole course forwards, and small pul- 

 sating vesicles are generally perceptible on the lateral syste- 

 mic branches of the aorta. The little tubicolous clymene 

 likewise gives off from the great dorsal artery conveying red 

 blood, numerous lateral branches extending like branchial 



