Chap. 28 



ANNELIDS PIONEERS IN SEGMENTATION 



563 



Circumpharyngeol 

 Cerebral ganglion connecfive 



Buccal cavity 

 Prosfomiom 



Segmenfol nerve 

 iVI 



Mourh 



Motor fiber ending in 

 longitudinal muscle 



Body wall 



Sobphoryngea! ganglion 



Ventral nerve cord 

 at ganglion 



\ I 



Septal nerve 



Motor neuron cell body 

 Sensory fibers 



_ — Body cavity 



Longitudinal 

 muscle 



Circular 

 muscle 



Epithelium 

 Sensory cells 



Fig. 28.10. Upper, forepart of an earthworm showing the nervous system with 

 the ganglia repeated in each segment. Lower, diagram of the nerve cells involved 

 in a simple reflex movement of the earthworm. {A, after Hess. B. after Parker. 

 Courtesy, Mavor: General Biology, ed. 3. New York, The Macmillan Co., 1947.) 



the worm, each organ in a particular segment (Fig. 28.11). The male cells 

 originate in two pairs of minute testes. These are surrounded by conspicuous 

 seminal vesicles, sacs in which the sperm cells mature. They finally pass into 

 the ruffled sperm funnels and through slender sperm ducts to the two external 

 openings on the ventral side of segment 15. Two pairs of small sacs, the semi- 

 nal receptacles, open through pores, on the ventral surface of segments 9 and 

 10. During mating these receive sperm from the sperm ducts of the part- 

 ner worm. The microscopic eggs are formed in a pair of translucent ovaries in 

 segment 13. As the eggs mature they are shed into the funnels of the oviducts 

 almost in touch with the ovaries. At the side of each oviduct is a minute pouch 

 in which they collect. Behind the sex organs is the conspicuous clitellum 

 (saddle) of gland cells. These secrete the mucous belt and cocoon that later 

 protects the developing embryos. 



The seminal vesicles very often contain large numbers of the parasitic proto- 

 zoan, Monocystis agilis, in various stages of development. In one stage they 



