226 COLLEGE ZOOLOGY 



to activity is the effector. Within the ventral nerve-cord are 

 association neurons whose fibers serve to connect structures 

 within one ganglion or two succeeding ganglia. These short 

 neurons overlap one another, and are doubtless responsible for 

 the muscular waves which pass from the anterior to the posterior 

 end of the worm during locomotion. The three giant fibers, 

 which lie in the dorsal part of the ventral nerve-cord throughout 

 almost its entire length, are connected by means of fibrils with 

 nerve-cells in the ganglia, and probably distribute the impulse 

 that causes a worm to contract its entire body when strongly 

 stimulated. 



SENSE-ORGANS. The sensitiveness of Lumbricus to light 

 and other stimuli is due to the presence of a great number of 

 epidermal sense-organs. These are groups of sense-cells con- 

 nected with the central nervous system by means of nerve- 

 fibers and communicating with the outside world through sense- 

 hairs which penetrate the cuticle. More of these sense-organs 

 occur at the anterior and posterior ends than in any other region 

 of the body. 



REPRODUCTION. Both male and female sexual organs occur 

 in a single earthworm. Figure 158 shows diagrammatically 

 the position and shape of the various structures. The female 

 system consists of: (i) a pair of ovaries (o) in segment XIII; (2) 

 a pair of oviducts (od) which open by a ciliated funnel in seg- 

 ment XIII, enlarge into an egg sac (R) in segment XIV, and 

 then open to the exterior; and (3) two pairs of seminal receptacles 

 or spermathecce (s), in somites IX and X. The male organs are 



(1) two pairs of glove-shaped testes (T) in segments X and XI, 



(2) two vasa defer entia (vd) which lead frorrxdliatcd-frmnels (SF) 

 to the exterior in segment XV, and (3) three pairs of seminal 

 vesicles in segments IX (A), XI (C), and XII, and two central 

 reservoirs (B). 



Self-fertilization does not take place, but spermatozoa are 

 transferred from one worm to another during a process called 

 copulation. Two worms come together, as shown in Figure 159, 



