ANNULOSA: ANNELIDA. 



181 



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 

 Chatophora 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 

 includes the Leeches, and is charac- 

 terised by the possession of a loco- 

 motive and adhesive sucker, poste- 

 riorly or at both extremities, and by 

 the absence of bristles and foot-tuber- 

 cles. The sexes are united in the same 

 individual, and the young do not pass 

 through any metamorphosis. 



The Leeches are aquatic, vermiform 

 animals, mostly inhabiting fresh water, 

 though a few species are marine. Lo- 

 comotion is effected either by swimming 

 by means of a serpentine bending of the 

 body, or by means of one or two suc- 

 torial 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 ap- 

 pendages of any kind. The mouth is 



Suctoria\ This order 



Fig. 55. Hirudinea. a 

 Medicinal Leech (Sang 



a The 

 Sanguisu- 



^ -inalis), natural size ; 

 Anterior extremity of the 

 same magnified, showing the 

 sucker and triradiate jaws; 

 c One of the jaws detached, 

 showing the semi - circular 

 toothed margin. 



