THE WORM-LIKE ANIMALS 



349 



be used to demonstrate the presence of digestive, nervous, excretory 

 (kidney), and reproductive systems. The general plan of these 

 organs may be learned from examining an earthworm in a later 

 lesson. 



Nereis and other marine segmented worms have an inter- 

 esting, free-swimming larva in their development. Figure 111 

 shows the main stages in another segmented worm which 

 has such a stage. The fertilized egg divides into two, then 

 four, eight, sixteen, etc., cells and 

 forms a hollow sphere of cells. Then 

 one pole of this sphere turns inward as 

 one might push in one hemisphere of 

 a hollow rubber ball. This forms two 

 layers, as in Fig. Ill, 5, the outer 

 being ectoderm and the inner endo- 

 derm. Such a two-layered embryo is 

 called a gastrula. The primitive ali- 

 mentary canal (e in Fig. Ill, 5-8) 

 soon forms mouth and anus, and the 

 embryo grows into a top-like form. 

 In this condition it begins to lead an 

 independent existence and is called a 

 larva. Gradually the lower end of 

 the larva becomes elongated and seg- 

 mented (Fig. Ill, 10), the head be- 

 comes relatively smaller, and the larva is metamorphosed 

 into the adult worm (11), which begins to live in sand. In 

 all cases like this where the young animal which hatches 

 from the egg is quite different from the adult, the young 

 animal is called a larva, and the change to the adult is 

 metamorphosis. 



302. The Earthworm. One other segmented worm de- 

 serves more than the brief attention which our time will 

 allow, and that is the land worm, commonly known as earth- 

 worm or " fishing worm." It lives in moist soil which con- 



FIG. 112. An annelid 

 worm which has an un- 

 usual habit of multiply- 

 ing asexually by dividing 

 off young worms in the 

 order 1 to 5. a is the 

 original worm. (After 

 Milne-Edwards.) 



