130 WORMS 



and position. In the center is the intestine. Notice a large 

 fold of the intesine hanging from the dorsal side. This 

 is the typhlosole. Of what use is such a structure? Just 

 dorsal to the ventral bundle of longitudinal muscles may 

 be seen the nerve cord. Do you see nerves extending to 

 right and left of it? Just dorsal to the nerve cord and 

 ventral to the intestine is the ventral blood vessel. Make 

 a sketch of the cross section, showing all the parts studied. 

 • (id) Cut an earthworm in two, place the pieces in some 

 moist earth in a box. Keep moist, and see what becomes 

 of them. 



The earthworm belongs to the class Annelida. 



Economic Value. — Every student should read Darwin's 

 " Vegetable Mould " in connection with the study of the 

 earthworm. If we consider the vast work of mixing the 

 soil with subsoil, the opening of passages for water and 

 air, and the working over of minerals unfit for absorption 

 by plants, then we may have some slight idea of the inesti- 

 mable value of the earthworm to man. 



THE NEREIS 



The Nereis lives in burrows in sand on the seacoast near 

 low-water mark. In the breeding season it leaves its bur- 

 row and swims about. 



(i) Note its color. Its length. Its shape. Are the 

 anterior and posterior ends alike? Has it a dorsal and 

 a ventral side? Note the appendages along each side. Is 

 there a pair to each segment? They are called parapodia. 

 Cut off a parapodium close to the body and examine with 



