260 MANUAL OF ZOOLOGY. 



CHAPTER XXVIII. 



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 extremi- 

 ties, and by the absence of bristles and foot-tubercles. The sexes 

 are mostly united in the same individual, and the young do not 

 pass through any metamorphosis. 



The Leeches are vermiform, mostly aquatic animals, chiefly 

 inhabiting 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 rare exceptions 

 (Branchellion}, there are no lateral appendages of any kind. 

 The mouth is placed at the front end of the body, and may or 

 may not be furnished with teeth. The pharynx is muscular ; 

 the gullet leads into a stomach with, usually, capacious lateral 

 caeca (fig. 128, B) ; and the anus is placed in front of or at the 

 bottom of the posterior sucker. The alimentary canal is 

 united with the integument by a spongy tissue, formed of 

 vascular sinuses, which more or less completely obliterate the 

 body-cavity, and in which the blood circulates. The pseudo- 

 haemal system generally consists, in addition to the sinuses 

 just alluded to, of four principal longitudinal trunks, devoid 

 of special dilatations. Respiration is carried on mainly by 

 the soft integument, possibly assisted by the "segmental 

 organs," and, in the case of Branchellion, by the vascular 

 leaf-like appendages on the sides of the body. The "seg- 

 mental organs " are in the form of a larger or smaller num- 

 ber of sacs (fig. 128, B), which open upon the abdominal 

 surface by so many pores or "stigmata." The function of 

 these sacculi appears to be excretory, and in the majority of 

 the Hirudinea they are closed internally, and only open ex- 

 ternally by the "stigmata." In some of the Hirudinea, how- 

 ever, the "segmental organs" agree with those of the great 

 majority of the Annelides in not only opening externally, but 

 in also communicating internally with the perivisceral cavity. 



