ANARTIIROrODA. 129 



any convolutions in its course. In almost all cases the alimentary 

 tube is placed in a distinct perivisceral cavity, which contains a fluid 

 with solid particles in it, corresponding to the blood of the higher 

 animals. In most, if not all, there is further a system of vessels 

 which carry a coloured fluid, which are conti'actile, and which send 

 branches to the respiratory organs, when these exist. This system 

 is believed not to correspond to the blood-vascular system of the 

 higher animals, and it has therefore been termed the " pseudo- 

 hsemal " system (Gr. pseudos, falsity ; and haima, blood). It is 

 believed, on the other hand, to be truly homologous with the water- 

 vasciilar system of the Scoleaida. Kespiration is eflfected by the 

 general surface of the body, or by distinct gills or branchiae. In 



Fig. 81.— Diagrammatic transverse section of an Anneli'^e. d Dorsal arc ; v Ventral 

 arc ; n Branehige ; a Notopodium, or dorsal oar ; 6 Neuropodium, or ventral oar, 

 both carrying setEe and a jointed cirrus (c). 



most cases, also, there exists a series of peculiar tubular involutions 

 of the integument, which are known as the " segmental organs.'' 

 The segmental organs open on the surface of the body by minute 

 apertures, and generally have the form of coiled or folded tubes, 

 which may terminate blindly internally (Leeches), or which open at 

 their inner ends into the perivisceral cavity. In the latter case, the 

 internal openings of the " segmental organs " are usually funnel- 

 shaped and ciliated, and they often serve to convey the generative 

 products to the exterior. The "segmental organs" correspond, in a 

 general way, with the kidneys of the higher animals. The sexes in 

 the Annelida are sometimes distinct, sometimes united in the same 



