ANAETHEOPODA. 



129 



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

 tube is placed in a distinct perivisceral cavity, wliicli ccmtains aiiuid 

 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 contractile, and which send 

 branches to the I'espiratory organs, when these exist. This system 

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

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

 liiemal " system (Gr. j^seudos, falsity ; and haima, blood). It is 

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

 vascular system of the Seolecida. Respiration is eft'ected by the 

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



Fig. SI.— Diagrammatic ti'ansverse section of an Anneli 'e. li Dursal arc; v Ventral 

 Jirc ; n Braiicliiii^ ; a Notoixtrlium, or dorsal oar ; h Nenrnpnilium, or ventral oar. 



Ijoth carrying sctu' and a Jointed cirrus (<■). 



most cases, also, there exists a series of ])cctdiar tubular involutions 

 of the integument, whicli 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 ojienings of the " segmental organs " are usually funnel- 

 shaped and cili;ited, 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 



