440 SANGUIFEROUS SYSTEM. 



peripheral parts to form plexuses around the internal nutri- 

 tive organs, and thence to extend to the internal respiratory 

 apparatus ; but many of the supposed vessels of these ani- 

 mals are probably, like the median dorsal, only transparent 

 muscular fasciculi. In the cirrhopods the sanguiferous sys- 

 tem, like most other parts of their economy, is constructed 

 on the plan of that of the higher articulata, and especially of 

 the Crustacea. A wide pulsating dorsal vessel, as observed 

 by Poli, extends forwards along the median line, apparently 

 receiving the blood from the branchial and the inferior sys- 

 temic veins, and transmitting it by large trunks into the 

 articulated feet and other parts of the body. Simple cur- 

 rents are observed to flow outwards and inwards through 

 the articulated feet, as in insects and the lower Crustacea, 

 without ramification of the two bounding canals, and the 

 venous blood appears to return to the fore part of the abdo- 

 men, as in crustacea, before being sent to the small branchial 

 laminoe attached to the haunches of the articulated mem- 

 bers. 



In the annelides, as in the larva state of the higher ento- 

 moid animals, the blopd is extensively circulated through the 

 system by minute arteries and veins ; but the great centre of 

 the sanguiferous system is still in an inferior condition of 

 development, presenting only the form of an elongated, 

 simple, pulsating, median, dorsal artery, provided with dis- 

 tinct, circular, muscular fibres, and conveying a thin, red- 

 coloured, serous blood, with comparatively little fibrin or 

 globules, and without distinct cordiform enlargement in its 

 course. The venous blood of these animals is commonly 

 returned from the system to the posterior extremity of the 

 dorsal artery, by a median vein commonly termed vena 

 cava, or by two inferior lateral veins, which collect the 

 blood from the capillaries of the arterial branches, as they 

 pass backwards along the sides of the body. So that the 

 general plan of the circulation in the adult forms of the sim- 

 pler annelides (represented in Fig. 133. A, where (a) is the 

 median dorsal artery conveying the blood forwards, and (b.b,) 

 the two returning lateral veins,) not only resembles that of 

 insects and other entomoid animals, but also the earliest 

 embryo condition of this system in the vertebrated classes. 

 This is nearly the course followed by the red blood, observed 



