ANNELIDA OR ANNULATA 117 



The Nervous System consists of a pair of dorsal ganglia situated 

 above the pharynx and of a double commissural nerve cord with 23 

 ganglia. The dorsal, supra-esophageal ganglia are connected with 

 the sub-esophageal pair by a rather narrow nerve ring surrounding 

 the esophagus. The sub-esophageal ganglia represent five pairs of 

 fused ganglia. From the dorsal ganglia, nerves supply the " eyes " 

 and tactile and gustatory organs. The last ganglion gives rise to 

 seven pairs of nerves. The h-ain gives rise to five pairs of optic 

 nerves. There are ten " eyes " and many olfacto-gustatory and 

 tactile sense organs. 



Allied Injurious Leeches.— Hirudo sanguisaga is found in the 

 nasal passages of man. Haemopis vorax, the horse leech, is taken 

 into the mouth when young by horses and cattle. It lives in ponds, 

 ditches and springs and attacks man, the horse, ox, camel, and dog. 

 It may become attached to the mouth, pharynx or even descend to 

 the trachea. It is also found attached to the conjunctiva. It 

 produces anemia, emaciation and even death. The treatment is 

 strong salt solution, alum, and tar. The tar causes coughing and 

 expulsion of the parasites loosened by the action of the salt and 

 alum. If the water is stocked with fish or filtered through sand, 

 the parasites are destroyed. 



The land leeches^ Haemadipsa Zeylanica^ are wiry, active forms, 

 thin as a knitting needle, i inch long and not more than }4 inch in 

 diameter. They attach to the legs of man and animals. They are 

 found in Ceylon, India, the East Indies, Japan, Australia and South 

 America. 



Adaptation of the Leech to Its Mode of Life. — The leech is re- 

 markably adapted to its habitat. It can swim with great rapidity; 

 it is protected by a rather tough hide. It has a mouth with 3 jaws 

 armed with chitinous teeth, a crop with 1 1 lateral diverticula capable 

 of storing enough blood to last 9 months. It takes in 3 times its 

 own weight at one time. Not only does it have anterior and poste- 

 rior suckers, but it secretes a substance " hirudin " said to be 

 deutero-albumose, which prevents blood from clotting. 



General Considerations 



Distribution. — The Chaetopoda, which include earthworms and 

 aquatic worms found in both salt and fresh water, are distributed 

 widely. Very few are parasitic but a number are commensals. It 



