122 



INVERTEBRATE ANIMALS. 



cavity. Locomotion is effected either by means of the alter- 

 nate fixation and detachment of the suckers, or by a serpentine 

 bending of the body. 



The body is obviously ringed or annulated, but none of 

 the rings carry lateral appendages of any kind. The mouth 

 is sometimes destitute of teeth, but is occasionally armed with 

 complex jaws. The alimentary canal 

 is short, with lateral dilatations, and 

 united to the skin by means of a 

 spongy vascular tissue, so that the body- 

 cavity is obliterated. The pseudo- 

 hasmal system is well developed, and 

 consists essentially of four great longi- 

 tudinal vessels. Respiration appears 

 to be effected, in part, at any rate, 

 by means of the segmental organs, 

 which have the form of little sacs 

 opening externally by minute aper- 

 tures. The nervous sj'stem has its 

 usual form, and the ganglia in front of 

 the gullet {^'' prce-cesophageaV ganglia) 

 give off branches to a number of simple 

 eyes which are placed on the head. 

 The sexes are united in the same in- 

 dividual. 



The most familiar of the leeches are 

 '^o^iLau)%i^t\^\h the common horse-leech (Hceinopsis), 

 Anterior extremity of the and the medicinal leech (Sanguisuga 



same magnified, showing the ^r. . _. , mi i- i 



sucker and triradiate jaws; ojjicinahs, -Pig. 47). ihe lormer has 

 showing "IhiTemfeSa; Small and blunt teeth, but the latter is 

 toothed margin. provided with three semicircular tooth- 



ed jaws (Fig. 47, h, c), which meet in a 

 point, and are sufficiently powerful to cut through the human 

 skin. The medicinal leech is a native of fresh waters through- 

 out the south and east of Europe, and it is imported in large 

 numbers from Hungary, Bohemia, and Russia. 



Fig. 47. — Himdinea. a The me- 

 dicinal leech (Sangwisuffa 



Okdee II. OLiGOCH..iETA. — In this order are included the 

 earth-worms (ZMmhricidcB), and the water-worms (Naididce). 

 They are all distinguished from the preceding by the fact 

 that the body is furnished with rows of bristles which take 

 the place of the foot-tubercles of the higher Annelida, and 

 which are the organs of locomotion. They are distinguished 

 from the higher forms by the fact that the locomotive bristles 



