180 



COLLEGE ZOOLOGY 



before we close our eyes when we see an 

 object about to strike us in the face. 



Sense organs. The sensitiveness of the 

 earthworm to hght and other stimuli is due 

 to the presence of a great number of epi- 

 dermal sense organs. The two main types 

 of epidermal receptors are the light-sensitive 

 cells (photoreceptor cells) and the sense 

 organs (Fig. 96), composed of a group of 

 sensory cells surrounded by supporting cells. 

 These sense organs are connected with the 

 central nervous system by means of nerve 

 fibers and communicate with the outside 

 world through sense hairs which penetrate 

 the cuticle. In addition to these sensory or- 

 gans there are also free endings of nerve 

 fibers between the cells of the epithelium. 

 More of these sense organs occur at the 

 anterior and posterior ends than in any 

 other region of the body. 



Reproductive system 



The earthworm is not known to reproduce 

 asexually although it has great powers of 

 regeneration of lost parts. Mating takes 

 place at night and requires two or three 

 liours. Both male and female sexual organs 

 occur in a single earthworm (Fig. 91). The 

 female system consists of: (1) a pair of 

 ovaries in segment 13; (2) a pair of ovi- 

 ducts, which open by a ciliated funnel in 

 segment 13 and pass to the exterior in seg- 

 ment 14; (3) an egg sac, which is a small 

 diverticulum of the septum associated wath 

 the funnel; and (4) two pairs of seminal 

 receptacles, in segments 9 and 10. The male 

 organs are: (1) two pairs of minute glove- 

 shaped testes in segments 10 and 11, and 

 back of each, (2) a ciliated sperm funnel 

 which is connected to (3) a tiny duct, the 

 vas eflEerens. The two ducts on each side 

 connect to (4) a vas deferens, that leads 

 to the outside. The testes and funnels are 

 contained in (5) the seminal vesicles, con- 

 spicuous saclike structures which surround 

 the testes and in which the sperms mature. 

 Self-fertilization does not take place, but 

 sperms are transferred from one worm to 



another during a process called copulation. 

 Two worms come together, as shown in 

 Fig. 98; then spermatozoa from the seminal 

 vesicles of each worm are expelled. They 

 pass along the seminal grooves into the 

 seminal receptacles of the other worm. The 

 worms then separate. When the time for 

 egg laying approaches, the glandular clitel- 

 lum secretes a bandlike mucous tube which 

 is forced forward by movements of the 

 worm. Eggs are discharged through the 

 oviducts, and sperms through the openings 

 of the seminal receptacles into the space 

 between this tube and the body wall. The 

 tube is then forced forward over the an- 

 terior end; its ends become closed, and a 

 cocoon, about the size of an apple seed, is 

 thus fonned containing fertilized eggs which 

 develop within the cocoon into minute 

 worms. The reciprocal fertilization insures 

 cross-fertilization in the earthworm. 



The eggs of the earthworm are holoblas- 

 tic, but cleavage is unequal. A hollow blas- 

 tula is formed, and a gastrula is produced 

 by invagination. The mesoderm develops 

 from tw^o of the blastula cells called meso- 

 blasts. These cells divide, forming two meso- 

 blastic bands which later become the epithe- 

 lial lining of the coelom. There is no 

 swimming stage such as occurs in the 

 marine annelids. The embryo escapes from 

 the cocoon as a small worm in about two 

 to three weeks. 



Regeneration and grafting 



Earthworms have considerable powers of 

 regeneration. No more than 5 new segments 

 will regenerate at the anterior end, and no 

 "head" will regenerate if 15 or more seg- 

 ments have been cut off. A posterior piece 

 may regenerate a "head" of 5 segments 

 (Fig. 98B); or, in certain cases, a tail (Fig. 

 98C). Such a double-tailed worm slowly 

 starves to death. An anterior piece regener- 

 ates a tail. Three pieces from several worms 

 may be united to make a long worm (Fig. 

 98D); two pieces may fuse, forming a worm 



