ANNULOSA: ANNELIDA. 1 89 



lateral 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. 



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 may 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). 

 The Annelida are also often divided into two sections, called 

 ChcBtophora and Discophora, according as locomotion is effected 

 by chitinous setae (Earth-worms, Tube-worms, and Sand-worms) 

 or by suctorial discs (Leeches). 



CHAPTER XXIX. 



ORDERS OF ANNELIDA. 



Order I. Hirudinea {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 metamorphosis. 



The Leeches are aquatic, vermiform animals, mostly in- 

 habiting 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 (posterior), 

 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 Leech ; but 

 it is not divided into distinct somites, and, with one exception, 

 there are no lateral appendages of any kind. The mouth is 

 sometimes edentulous, but it is usually armed with teeth. The 



