162 MANUAL OP ZOOLOGY. 



4 pras-oesophageal,' or ' cerebral,' ganglia being connected by 

 lateral cords or commissures with the 'post-oesophageal' 

 ganglia. Pigment-spots, or ' ocelli,' are present in many ; 

 and the head often supports two, or more, feelers, which 

 differ from the * antennae' of Insects and Crustacea, in not being 

 jointed. 



The sexes in the Annelida are sometimes distinct, and some- 

 times united in the same individual. The embryos are almost 

 universally ciliated, and even in the adult cilia are almost 

 always, if not always, present, in both of which respects this 

 class differs from the Arthropoda. 



The Annelida ma,y be divided into two sections, characterised 

 by the presence or absence of external respiratory organs or 

 branchiae. The Abranchiate section comprises the Leeches 

 and the Earth-worms ; whilst the Branchiate division includes 

 the Tube- worms (Tubicola) and the Sand- worms (Errantia). 



CHAPTER XXIX. 



ORDERS OF ANNELIDA. 



ORDER I. HIRUDTNEA (Discophora, or Suctoria). This order 

 includes the Leeches, and is characterised by the possession of 

 a locomotive and adhesive sucker, posteriorly, or at both 

 extremities, and by the absence of bristles and foot-tubercles. 



The sexes are united in the same 

 individual, and the young do 

 not pass through any metamor- 



Fig. 45. Medicinal Leech (Sangui- rni ' r i j.' 



suga ojidnaiis). The Leeches are aquatic, ver- 



miform animals, mostly inha- 

 biting fresh water, though a few species are marine. Loco- 

 motion is effected either by swimming by means of a serpentine 

 bending of the body, or by means of one or two suctorial discs. 

 In those forms in which there is only a single sucker (pos- 

 terior), the head or anterior extremity of the body can be 

 converted into a suctorial disc. The body is ringed, as 

 many as one hundred annulations being present in the common 

 Loech ; but it is not divided into distinct somites, and there are 

 no lateral appendages of any kind. The mouth is sometimes 

 edentulous, but is usually armed with teeth. The alimentary 

 canal is short, and is united to the skin by means of a spongy 

 vascular tissue. The pseudo-haemal system consists principally 

 of four great longitudinal trunks, connected by lateral vessels, 



