GENERAL ZOOLOGY 



Nerve 



Subepidermal 

 nerve net 



Receptor 



Epidermis 



Muscle cell 

 (effector) 



Fig. 14.10. Diagram of 

 the sensory-neuro-muscular 

 mechanism of the earthworm. 

 Arrows indicate direction of 

 conduction of nerve impulses. 



Nerve cord 



the adjusters impulses are also conducted to appropriate eflPerent neurons 

 whose fibers conduct the impulses out of the nerve cord to muscular or gland- 

 ular cells, the effector organs. Alternatively, the pathway may omit the 

 adjustor neurons and pass directly from the afferent fiber to an efferent 

 neuron, the conduction occurring within the ventral nerve cord. A com- 

 parison between this system and the reflex mechanism in the nervous system of 

 vertebrates (p. 96) shows that the relationships between cells are essentially 

 the same in the two systems. The receptor cells of the annelid may be com- 

 pared with such receptors as the olfactory cells of vertebrates, which are 

 similarly located in the epidermis and send nerve fibers into the central 

 nervous system (pp. 109-110). The relationships of the dorsal root cells 

 (afferent neurons) in the vertebrate are more complex, but the differences 

 involve chiefly the interpolation of an additional neuron between the receptor 

 cell and the adjustor neuron. The adjustor and efferent neurons have identi- 

 cal relationships in the annelid and the vertebrate, although in correlation 

 with the greater structural complexity of the vertebrate the adjustor or 

 association mechanism is more elaborate. 



The Reproductive System, Reproduction, and Development. Unlike 

 .Vereis, earthworms are monoecious, and the reproductive systems are perma- 

 nently developed, involving a complex group of accessory structures (Fig. 14. 1 1 ). 

 There are two ovaries, attached to the posterior face of the septum, near the 

 ventral body wall, in somite 13. The oviducts are short tubes with expanded 

 funnel-like openings, located just posterior to the ovaries. These funnels 

 receive the ova emerging from the ovaries and conduct them into a small 

 ovisac, an expanded chamber in each oviduct. Here the eggs may be retained 

 for some time before they are "laid." Like nephridia, the oviducts pass 

 through a septum before piercing the body wall to reach their external 

 openings. The testes are four in number and are located in segments 10 

 and 11, being attached to the posterior surfaces of septa, as the ovaries are. 

 In the adult worm the testes are enclosed within a cavity formed by the 

 fusion and growth of three pairs of seminal vesicles, which partition off a part 

 of the coelom. Within the spacious chamber thus formed the male gametes, 

 detached from the testes in an immature state, complete their maturation. 



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