CIECULATION IN THE DOKSIBRANCHIATA. 



257 



Fig. 126. 



vessel enveloped in a common muscular (although extremely dia- 

 phanous) sheath. That these vascular sheaths, which are only fine pro- 

 ductions of the integuments, are furnished with voluntary muscular 

 fibres, is proved by the rapid and simultaneous retraction of all the 

 branchiae into the interior of the body which follows when the animal 

 is touched. In Arenicola, as in all Annelida in which the vessels of 

 these organs are naked, the branchiae are destitute of vibratile cilia ; and 

 it will be found that under such circumstances, namely when the 

 branchial vessels occur as naked projections from the external surface, 

 the description here given of these organs in Arenicola will apply in every 

 respect to all other Annelida so furnished. It will be observed that in 

 all the dorsibranchiate genera furnished with branchiae such as those 

 described above, the true blood, circulating in its proper vessels, is found 

 to be exclusively the seat and 

 subject of the respiratory pro- 

 cess ; the fluid in the peri- 

 toneal cavity, abundant in 

 quantity and highly organized 

 though it be, does not in the 

 least degree participate in 

 this great function. The Dor- 

 sibranchiate Annelids may, 

 however, be divided into two 

 great groups, of which one 

 would comprehend those ge- 

 nera in which the function of 

 breathing devolves exclusively 

 on the true blood, while the 

 other would be characterized 

 by the fact that the branchiae 



are constructed so as to permit more or less completely the exposure, 

 in conjunction with the blood-proper, of the chylaqueous fluid of 

 the visceral cavity to the influence of the surrounding aerating ele- 

 ment. Thus it will be seen that when the branchial apparatus is 

 penetrated by two separate and distinct fluids, coordinate probably in 

 organic properties, the vascular system of the body generally will be 

 found so much the less developed, in proportion as the peritoneal fluid 

 supplants the blood in the branchiae. In those races of Dorsibranchiate 

 worms possessing both these kinds of circulation, naked unciliated 

 blood-vessels no longer form exclusively the branchial organs; loose 

 and large-celled tissue (fig. 126, a a} is superadded to the proper blood- 

 vessels, which are far less in relative size than those in the former 

 variety of branchiae ; into the cells of this tissue the fluid of the visceral 

 cavity insinuates Itself, its course being marked by a slow motion. 

 There exists, however, another point of structural difference between the 



