258 



ANNELIDA. 



Fig. 127. 



branchial organs of this group and those of the former, viz., that wherever 

 the fluid of the peritoneal cavity is admitted into the interior of the 

 branchial organs, the latter are invariably supplied more or less pro- 

 fusely with vibratile cilia. 



(667.) In the- branchial structure of worms thus constituted, the 

 branchial appendages are found, instead of being composed of naked 

 vessels, to present the appearance of round or laminated organs, into 

 which the fluid of the visceral cavity freely penetrates. 



(668.) The blood-system is more concentrated in Arenicola than in 

 any known Annelid. A large dor- 

 sal trunk (fig. 127, a; fig. 128, i), 

 at the anterior three-fourths of 

 the body receiving exclusively the 

 efferent vessels of the branchiae, 

 proceeds forwards from the tail 

 and empties itself into the cardiac 

 cavities, of which one is situated 

 on either side of the oesophagus 

 (fig. 127, b V ; fig. 128, b b). An- 

 other vessel, proceeding from the 

 head towards the heart, empties it- 

 self into the same cavity with the 

 former. The blood then enters a 

 second cavity (fig. 127, c' c'), more 

 ventrally situated, by which it is 

 partly propelled forwards into the 

 suboesophageal trunk, but princi- 

 pally backwards into the great lon- 

 gitudinal trunks of the alimentary 

 canal. The blood returning from 

 the intestinal system of vessels 

 reaches the dorsal intestinal (g) 

 (lying in the median line under- 

 neath the dorsal trunk), from 

 which the current diverges laterally 

 at right angles into the branchias 

 (//). This conformation differs 

 from that prevalent in all other 

 Dorsibranchiate Annelidans, in which the great ventral trunk is the 

 source of the branchial arteries. But the typical plan of the circulation 

 is observed in the system of Arenicola at the posterior half of the bran- 

 chial division of the body, where the afferent vessels of the branchiae 

 emanate from the ventral trunk. It may be necessary to explain that 

 the motion of the blood in that part of the circulating system which is 

 anterior to the heart is the reverse of that posterior to this centre. The 



Plan of the circulation in Arenicola. 

 (After Dr. Williams.) 



