196 



THE RISE OF ANIMAL LIFE 



worms which have been "drowned" out 

 of their burrows. The apparently fantastic 

 stories of earthworms falhng from the skies 

 during storms are perhaps founded on the 

 same basis as the stories of frogs and other 

 animals having "rained." This usually fol- 

 lows a tornado where masses of water have 

 been carried into the air, resembling a 

 water spout in the ocean, and these animals 

 are taken along in the water and released 

 many miles away. 



The bristle-like setae are the earthworm's 

 chief organs of locomotion. While the ani- 

 mal is in its burrow the setae in the pos- 

 terior end project out and are imbedded in 

 the wall of the burrow. This is done by 

 contraction of the muscles at the base of the 

 setae. Those at the anterior end then relax 

 and the circular muscles contract, thus 

 elongating the worm. The anterior setae 

 then secure their end of the animal and 

 the longitudinal muscles contract, bringing 

 the posterior end forward. In such fash- 

 ion the animal moves through its burrow. 

 Since the setae are located on the ventral 

 sides as well as the lateral walls, the animal 

 is able to crawl slowly over a surface when 

 removed from the burrow. If the anterior 

 segments containing the brain are removed, 

 the worm seems to move in a normal 

 fashion, which means that the nerve centers 

 for crawling movements are located in the 

 ganglia and not centered in the brain. 



Reproduction. In order to survive in its 

 terrestrial habitat, the earthworm has been 

 forced to undergo some drastic adaptations 

 in its reproductive system. It will be re- 

 called that most annelids discharge their 

 sex products into water where union of the 

 eggs and sperms is purely fortuitous. On 

 land, obviously, some other means must be 

 provided to bring about this union and to 

 insure adequate conditions for the develop- 

 ing embryo. A most unique method has 

 been devised for this purpose. In the first 

 place, the sexes, which are separate in other 

 annelids, are united in the earthworm, that 

 is, it is monoecious, or hermaphroditic. This 



has the advantage of making it unnecessary 

 for worms of different sexes to unite; any 

 two worms can exchange sperms. This is 

 important because the chances of animals 

 meeting are less than would be the case in 

 water where seasonal aggregations occur. 

 The ovaries and testes are located in the 

 anterior end of the worm where ducts pro- 

 vide the proper exit for eggs and sperms 

 (Fig. 10-19). There are two pairs of tiny 

 testes located in the tenth and eleventh seg- 

 ments, surrounded by large sac-like bodies, 

 the seminal vesicles, which are storehouses 

 for the sperm cells. Funnels direct the 

 sperm into the sperm ducts which open to 

 the outside on the fifteenth segment. A pair 

 of ovaries cling to the posterior wall of the 

 septum in segment 13, and small funnels 

 catch the eggs and direct them into a sac 

 where they are temporarily stored. Even- 

 tually the eggs pass to the outside through 

 the oviduct in sesi;ment 14. 



The process of exchanging sperms occurs 

 at night, usually following a rain. Two 

 worms become attached along their ventral 

 sides, as shown in Fig. 10-19; this usually 

 occurs while the posterior ends of the 

 worms remain in the burrows. Some, how- 

 ever, crawl some distance away from the 

 burrow until contact is made with another 

 worm. A slimy material is then secreted 

 mutually by the worms which, as it hard- 

 ens, encases both animals together in a 

 temporary sheath. Small tubular passage- 

 ways form between the sheath and the body 

 walls, thus providing a pathway for the 

 sperms, which are forced from each worm 

 along these channels until they reach the 

 seminal receptacles of the other worm. 

 After this exchange of sperm cells the ani- 

 mals separate. 



At some later period when the eggs are 

 mature, the clitellum secretes a mucous 

 sheath which slips forward like a tight 

 sweater over a man's head. In the vicinity 

 of the fourteenth segment eggs are forced 

 into the mucous ring, and as it slips over 

 the ninth and tenth segments sperms pass 



