THE PHYLUM ANNELIDA 



urine during its passage through the convoluted, glandular j)ortions of the 

 nephridium. On the basis of earlier research it has been assumed that 

 metabolic wastes may be accumulated and stored as granular deposits in the 

 chloragogue cells, modified peritoneal cells covering the digestive tract and 

 particularly concentrated about the dorsal and dorsointestinal blood vessels. 

 The assumption has been that upon the disintegration of these cells their 

 granules were released into the coelomic fluid, whence they were removed by 

 the nephridia. More recent investigations, involving isolation and analvsis 

 of the intracellular granules, have established the fact that they do not con- 

 tain nitrogenous wastes but may represent stored protein reserves. The role 

 of the chloragogue cells in excretion thus appears to be questionable. 



Relatively little is known of the specific metabolic processes of the earth- 

 worm and similar annelids. Analysis of what is known and comparison of 

 the facts with the well-known functions of higher animals, however, indicate 

 that the processes are entirely comparable. 



Structures and Function Related to Responsiveness. The nervous system, 

 with its cerebral ganglia, circumpharyngeal commissures, ventral nerve cord, sub- 

 epidermal plexus, and nerves, is essentially like that of Nereis. The notable 

 difference in the sensory-neural system appears in the anterior region. Nereis 

 is an active, predatory animal, whereas the earthworm is less active and does 

 not seek or capture prey in feeding. In correlation with these diflferent 

 ways of life. Nereis is well equipped with sense organs and has a well- 

 diflferentiated head region; the earthworm has nothing that can properly be 

 called a head and lacks tentacles, palps, and eyes, although the anterior end 

 of the body is the region most sensitive to stimulation. 



Earthworms react to mechanical stimuli, withdrawing or extending the 

 body on contact with foreign objects, or retreating into their burrows in 

 response to vibrations transmitted through the ground. Evidences of response 

 to chemical stimuli appear in their reactions to food, which is to some extent 

 selected as though by a sense of taste. Earthworms also detect food a short 

 distance away, as though by some form of olfaction. Sensitivity to light is 

 revealed by the nocturnal habits of earthworms, as well as by their withdrawal 

 reactions when light is flashed upon the surface of the body. Sense organs 

 have long been recognized in the surface layers of the body wall, although 

 only comparatively recently has much progress been made in identifying 

 the specific receptors involved in reactions to different kinds of stimuli. 

 Sense organs which resemble taste buds are present, as well as specialized 

 cells which appear to function in the perception of light. 



The cellular basis of the reaction to stimuli is shown diagrammaticallv 

 in Figure 14.10. Receptor cells in the epidermis respond to specific stimuli by 

 conducting impulses into the subepidermal nerve plexus, and along nerve 

 fibers which pass into the ventral nerve cord. Within the cord these afferent 

 fibers make synaptic connections with adjustor or association neurons. The 

 adjustors, in turn, conduct impulses to other association neurons which dis- 

 tribute the excitation to other regions of the central nervous svstem. From 



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