LEECHES 



451 



worm, viz., the variously-shaped Polychaeta, are for the most part marine. 

 One species, the Lug- Worm {Arenicola marina) is found and caught in 

 millions off our coasts, and much used for bait. This worm burrows in 

 the sand of the sea-shore, and resembles the earth-worm, being, how- 

 ever, provided with red external gills. 



Anyone who has visited a sea-water aquarium must be familiar with 

 the tubes found encrusted upon mollusc-shells, stones, etc. These are 

 constructed by Tubicolous Annelids (Serpula; see illustration, p. 462) 

 for the protection of their soft bodies. (Compare with caddis flies.) The 

 animals climb nimbly up and down these tubes like an earth-worm in its 

 run, while above the aperture of the tubes they elevate their plume-like 

 and usually beautifully-coloured gills, which are also used for producing 

 a current of water (see p. 438, Section d). 



ORDER II. : LEECHES (HIRUDINEA). 



The Medicinal Leech (Hirudo medicinalis). 



(Length up to 8 inches.) 



The medicinal leech lives at the bottom of ponds and lakes. Its colour is 

 very variable, but mostly greenish-brown. (Compare with other bottom- 

 living animals.) Like the majority of free-living worms, it possesses 

 eyes, which are visible as dark dots upon the anterior body rings. It is 

 a parasite, its food consisting of animal juices, which during its youthful 

 stage it derives from snails, tadpoles,- and other cold-blooded creatures, 

 but in its adult condition from warm-blooded animals as soon as these 

 enter its habitat (name). As, however, the former of these animals 

 are soon Bucked dry, whilst the latter leave the water soon after entering 

 it, it is necessary that the leech, unlike sessile parasites (see tape- worm), 

 should be endowed with the power of free locomotion. It swims very 

 nimbly by undulatory movements of its much-flattened body, and creeps 

 by means of the two suckers (see cuttle-fish) developed on the anterior 

 and posterior ends of the body, somewhat after the manner of the Looper 

 or Geometer caterpillars (which see). These suckers are also indispens- 

 able to the animals as organs for retaining its captured prey. At the 

 bottom of the anterior sucker, which during suction acts like a cupping- 

 glass, is placed the three-cornered mouth. By opening this slightly we 

 obtain a view of the apparatus by which the leech gains access to the 

 body juices of its host, viz., three plates, ox jaws, radiating from a point, 

 the curved outer edges of which are sharply serrated. Thus, each jaw 

 represents, as it were, a portion of a minute circular saw, and also acts 



30 



