180 MANUAL OF ZOOLOGY. 



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

 400 in JEtmice 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 com- 

 parable with the head of the Arthropoda, but is really a greatly 

 modified prae-oral 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. Except in the Hirudinea, the alimentary 

 canal is suspended in a capacious perivisceral space, divided 

 into compartments by more or less complete partitions. The 

 alimentary canal is, with one exception, not convoluted, and 

 extends straight from the mouth to the anus ; but lateral di- 

 verticula are often present 



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 respiratory organs, 

 when such exist." (Huxley.) This system has been termed 

 the " pseudo-haemal system," and its vessels are considered by 

 Professor Huxley as being " extreme modifications of organs 

 homologous with the water-vessels of the Scolecida ;" since the 

 perivisceral cavity, with its contained corpusculated fluid (chy- 

 laqueous fluid), is believed by M. de Quatrefages to be the 

 true homologue of the vascular system of Crustacea and In- 

 sects. The pseudo-haemal system, therefore, of the Annelides 

 is to be regarded as essentially respiratory in function. The 

 pseudo-haemal vessels are sometimes wanting, and in these 

 cases respiration appears to be effected by the cilia lining the 

 perivisceral cavity. 



Respiration is effected by the general surface of the body, 

 by saccular involutions of the integument, or by distinct ex- 

 ternal gills, or branchiae. 



The nervous system consists of a double, ventral, gangliated 

 cord, which is traversed anteriorly by the oesophagus ; the 

 " prae-cesophageal," or " cerebral," ganglia being connected by la- 

 teral cords or commissures with the "post-cesophageal" ganglia. 

 Pigment-spots, or " ocelli," are present in many, generally upon 

 the proboscis, sometimes in each segment, or on the branchiae, 

 or on the tail; and the head often supports two or more 

 feelers which differ from the "antennae" of Insects and Crus- 

 tacea in not being jointed. 



