198 



THE RISE OF ANIMAL LIFE 



out of the seminal receptacles and unite 

 with the eggs. After the mucous ring slips 

 over the anterior end, the ends contract, 

 forming a closed capsule, or cocoon, full of 

 fertilized eggs, which is left behind in the 

 burrow. There is no larval stage and the 

 eggs hatch into small earthworms which 

 penetrate the wall of the mucous capsule 

 and begin to shift for themselves. 



Relatives of the earthworm 



Earthworms have many relatives which 

 range in size from microscopic forms to 

 species which may reach 10 feet in length. 

 There are well over 2,000 species and most 

 of them are smaller in size than the com- 

 mon earthworm. Although nearly all live in 

 damp soil, some are found in fresh or 

 polluted waters. One form, Tuhifex, is en- 

 couraged to grow in filter beds of sewage 

 disposal plants in order to keep the filter 

 open. They are considered very valuable 

 for this purpose and specimens are often 

 shipped to new filters to start the "culture" 

 going. The common blood worm, which is 

 a species of Tubifex, is found in tvibes at 

 the bottom of fresh-water ponds where it 

 feeds on the muck and perhaps aids in the 

 purification of such waters when they are 

 polluted. Another small form, Enchytraeus, 

 is sold in pet shops as a source of food for 

 small fish. 



Certain of the small forms, such as 

 Chaetogaster, reproduce asexually by trans- 

 verse fission and sometimes several cling 

 together, resulting in a chain of individuals. 

 In some species this method has apparently 

 replaced the sexual method altogether. 



Leeches 



After a swim in the old swimming hole, 

 boys often find small black leeches that 

 cling tenaciously to the skin. When re- 

 moved, they leave a stream of blood flow- 

 ing from the wound. It is also common for 

 the fisherman to see a large ( 12 inches 

 long) leech, Haemopsis grandis, swimming 



in beautiful undulating movements near 

 the surface of the water. Another leech, 

 Hirudo medicinalis, was once cultured in 

 Europe for the specific purpose of blood- 

 letting when that practice was in vogue. It 

 is interesting to note that during the last 

 century, and before, it was considered 

 beneficial to remove blood in certain ill- 

 nesses. Today the procedure is the reverse, 

 as is indicated by the many blood banks 

 over the country. 



In some regions, particularly the tropics, 

 leeches live in watering places where large 

 vertebrates come to drink. They attach 

 themselves to the buccal cavity, sometimes 

 in such numbers as to cause serious injury 

 to horses as well as other animals, including 

 man. Some leeches live on land and are 

 occasionally so numerous that they are 

 a serious hazard to human beings. Army 

 commanders have been known to provide 

 their men with leech-proof stockings in 

 order to get through such infested areas. 



The leech has many of the annelid 

 characteristics, but it lacks setae, and it pos- 

 sesses copulatory organs, which other an- 

 nelids lack. What appears to be segmenta- 

 tion externally does not correspond with the 

 actual segments, for there are fewer seg- 

 ments internally than appear from the out- 

 side. The body has remarkable powers of 

 extensibility and contractibility, enabling it 

 to move like the measuring worm. The 

 hold-fast organs are two suckers, one on 

 each end, which are used both in locomo- 

 tion and in feeding. In the center of the 

 anterior sucker is the mouth, which is 

 usually provided with three small cutting 

 teeth that inflict a wound when the leech 

 is feeding upon its victim. The anus is 

 located in the center of the posterior sucker. 

 The digestive tract is sacculated so that it 

 can retain a large meal of blood. Appar- 

 ently this is provided because meals are 

 usually few and far between, some leeches 

 being able to live a year between feedings. 



When securing a blood meal the leech 

 becomes attached to the skin, which it 



