CIRCULATION IN AMPHINOME. 249 



respect to the former, an exterior concentric or embracing cylinder. 

 These two cylinders are in no instance in agglutinated contact : a space 

 intervenes, varying in capacity in different species, to designate which 

 the term ' peritoneal/ or splanchnic, may be used with perfect propriety. 

 This space is occupied by a vital or organized fluid charged with cor- 

 puscles, which exhibit, under the microscope, characters distinctive of 

 species. Independently of its physiological uses, this fluid enacts me- 

 chanical functions indispensable to the well-being of the animal : on 

 it, as upon a pivot, the vermicular motions of the intestinal cylinder 

 are performed. 



(651.) Although, as a whole, forming a cylinder, in no instance does 

 the alimentary canal of the Annelida present the figure of a smooth- 

 walled tube. The parietes are invariably sacculated, and often super- 

 ficially multiplied in the most elaborate manner. In the Lumbriciform 

 species, each segment of the body has its own independent stomach. 

 Those of contiguous segments communicate through an opening con- 

 siderably more contracted in diameter than that portion of the intes- 

 tine from which it leads. Thus the intestine of the Errant Annelids, 

 especially, may be compared to a line of pears, the apex of each suc- 

 cessive pear being applied to the base of its predecessor in the series : 

 if these bases were prolonged on each side, the stomach of the Leech 

 would be the result ; if compressed, that of those species in which the 

 tube is nearly straight. 



(652.) The course of the principal trunks of the circulating system 

 in the Dorsibranchiata bears a general resemblance to what we have 

 already seen in the Abranchiate order, modified, of course, by the 

 variable position of the branchial tufts. The annexed figure (122) of 

 an elaborate dissection of an Amphinome (A. capillata}, copied from 

 one of the beautiful drawings contained in the Hunterian Collection *, 

 affords an example of a circulating system in which the propulsion of 

 the blood is effected entirely by vessels, without the intervention of any 

 muscular cavities or heart. In this animal the respiratory organs are 

 penniform appendages placed along the back, and these external 

 vascular tufts communicate with delicate plexuses of vessels, situated 

 in the interior of the body, called the branchial plexuses. In the 

 figure the branchial plexuses of the left side only are represented 

 (qqq)\ and of these, one marked q' has been turned aside. The blood 

 and nutritious fluids derived from the whole alimentary track are col- 

 lected by the large ventral intestinal vein (n n n) and conveyed to the 

 branchial plexuses through the numerous vessels (o o o), some of which 

 (o' o' o') are displaced in the drawing in order that their connexions may 

 be better seen. Besides the blood and nutriment thus derived from the 

 intestine, the branchial plexuses receive the circulating fluid from all 



* Descriptive and Illustrated Catalogue of the Physiological Series of Compara- 

 tive Anatomy in the Mus. Roy. Coll. Surg. of England, vol. ii. pi. 14. 



