ANNULOSA : ANNELIDA. 161 



the ' nenropodium,' or ' ventral oar.' The foot- tubercles, 

 likewise, support "bristles, or ' setae,' and a soft, cylindrical 

 appendage, which is termed the 'cirrhus' (fig. 44). 



Fig. 44. Diagrammatic transverse section of an Annelide. d. Dorsal arc ; v. 

 Ventral arc ; n. Branchiae ; a. Notopodium or dorsal oar ; 6. Neuropodium 

 or ventral oar, both carrying setas and a jointed cirrhus. 



The number of the segments varies much, being as many as 

 400 in Eunice gigantea ; and, generally, there is not a distinct 

 head which is separable from the succeeding rings of the body. 

 When such a distinct head appears to be present, it is not 

 comparable with the head of the Artliropoda, but is really a 

 greatly modified praeoral region, or 'prostomium,' as is shown 

 by the position of the mouth. 



The digestive system of the Annelides consists of a mouth, 

 sometimes armed with horny jaws, a gullet, stomach, intestine, 

 and a distinct anus. 



As regards the vascular system, ' no Annelide ever possesses 

 a heart comparable to the heart of a Crustacean or Insect ; 

 but a system of vessels, with more or less extensively contrac- 

 tile walls, containing a clear fluid, usually red or green in 

 colour, and, in some cases only, corpusculated, is very 

 generally developed, and sends prolongations into the respi- 

 ratory organs, when such exist.' (Huxley.) This system 

 has been termed the ' pseudo-haemal system,' and its vessels ary 

 considered by Professor Huxley as being ' extreme modifica- 

 tions of organs homologous with the water-vessels of the 

 Scolecida-,' since the perivisceral cavity, with its contained 

 corpusculated fluid (chylaqueous fluid), is believed by M. de 

 Quatrefages to be the true homologue of the vascular system 

 of Crustacea and Insects. 



Respiration is effected by the general surface of the body, 

 by saccular involutions of the integument, or by distinct 

 external gills, or branchiae. 



The nervous system consists of a double, ventral, gangliated 

 cord, which is traversed anteriorly by the oesophagus ; the 



VOL. I. M 



