258 MANUAL OF ZOOLOGY. 



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

 appendage, which is termed the "cirrhus" (fig. 126). 



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 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 "prostomium" or "cephalic, 

 lobe " is placed in front of the mouth, and often carries " ten- 

 tacles " above and tactile processes or " palpi " below. 



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 diver- 

 ticula 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 " pseudohaemal system," and its vessels are considered by 

 Professor Huxley as being " extreme modifications of organs 

 homologous with the water-vessels of the Scoleada:" since the 

 perivisceral cavity, with its contained corpusculated fluid (chyl- 

 aqueous fluid), is, as shown by M. de Quatrefages, the true 

 homologue of the vascular system of Crustacea and Insects. 

 The pseudohaemal system, therefore, of the Annelides is to be re- 

 garded as essentially respiratory in function. The pseudohaemal 

 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, or 

 by distinct external gills or branchiae. 



The excretory organs of the Annelides are the so-called 

 " segmental organs." In their simplest form (as in the ordi- 

 nary Leeches), each segmental organ is in the form of a much- 

 folded tube, partly labyrinthic, partly vesicular, often with an 

 appended caecum, and opening externally by a distinct aper- 

 ture or " stigma," but having no internal communication with 

 the body-cavity. In these cases, the segmental organs may be 

 regarded as representing the kidneys of the higher animals. 



