THE EARTHWORM 299 



of sensory cells (Fig. 145) the inner ends of which extend as 

 axons which enter the cord by way of the nerves. Following 

 the terminology used in describing the nervous system of hydra 

 (c/. p. 269) , these sensory cells are the receptors. Within the ventral 

 cord the end processes of the axons have contacts, or synapses, 

 with processes of the neurones of the ganglion. Certain of these 

 neurones, the adjustors, have their processes confined to the 

 nerve cord, and are not represented in a nervous system like 

 that of hydra (cf. p. 317). Others of the nerve cells, the motor 

 or efferent neurones, have axons that pass out from the cord to 

 the muscles. Comparisons should be made with the two similar 

 types of cells found in the spinal cord of a vertebrate animal 

 (Fig. 61, p. 109). The mechanism of response to stimulation 

 within a single segment will now be apparent. The stimulus is 

 received by the sensory cell, or receptor, and a nervous impulse 

 is transmitted along its axon, either to adjustor neurones or 

 directly to the efferent or motor neurones which have axons 

 leading to the muscle cells which arc effectors as in hydra. In this 

 manner a stimulus applied to sensory cells brings about muscular 

 contraction. As in the vertebrate, coordination is due to the many 

 interconnections between the nerve cells of the central system. 

 The coordination between segments may be effected by exten- 

 sions of the sensory axons somewhat beyond the hmits of the 

 segment, but it seems to be principally effected by the extension 

 from segment to segment of the processes of adjustor neurones. 

 However, the longitudinal extent of this inter-segmental coordina- 

 tion is not so great as might be supposed, since many of the 

 worm's responses give evidence of spreading from one segment 

 to the next even when they seem to be transmitted with compara- 

 tive rapidity from one end of the body to the other. Compari- 

 sons between the earthworm's nervous system and that of other 

 animals are made in Chapter 13. 



Microscopic Organization. — The cellular structure of the 

 various organs has been described in the preceding paragraphs 

 only so far as was necessary in explaining the general structure 

 and functions of the several systems. We may now examine the 

 microscopic organization of the earthworm as an illustration of 

 the Cell Doctrine and as an introduction to a study of the physio- 

 logical processes. In a transverse section of the intestinal region 

 {cf. Figs. 140, 141, and 145), the cuticle is seen as a delicate non- 



