304 THE EARTHWORM AND OTHER ANNULATA 



It might be expected that special sense-organs would exist for 

 the reception of the tactile stimuli, for the chemical stimuli that 

 must act upon the senses of taste and smell, and for stimulation by 

 light. It has been known for many years that sense-organs exist 

 upon the surface of the earthworm's body, particularly at the 

 anterior end, but all of these seemed to be of one sort (Fig. 145). 

 Recently, however, sensory cells that appear to be those respon- 

 sive to hght have been discovered (Fig. 146), and some progress 

 has been made in differentiating the common sense-organs previ- 

 ously mentioned into more than one type. It is, therefore, 

 probable that the worms possess special groups of sensory cells 

 for the senses above mentioned, after the manner of higher animals. 

 Certain other senses may exist in the absence of special sense- 

 organs that can be recognized as such. For instance, earthworms 

 are sensitive to temperature, at least to the extent that they are 

 sluggish at low temperatures, as may be shown by their not coming 

 out of their burrows during a frost, and as demonstrated by labora- 

 tory experimentation in which they can be shown to react to opti- 

 mum temperatures. They also react to mechanical vibrations, as 

 may be demonstrated by striking the receptacles in which they are 

 kept for observation, or by stamping upon the ground where they 

 are being collected at night by the dim light of a lantern. Since 

 even the protozoa respond to these and other stimuli in the absence 

 of special sense-organs, it is not necessary to postulate such struc- 

 tures for the earthworm, even though sense-organs adapted for 

 receiving thermal and mechanical stimuH do exist in some of the 

 higher animals. 



If the foregoing very general account of the behavior of the 

 earthworm be stated in the terms of laboratory experimentation 

 upon animal behavior, one may say that the worms exhibit 

 responses to gravity, geotropism, as shown by their behavior in 

 relation to their burrows; to light, phototropism ; to temperature, 

 thermotropism; to contact, stereotropism; to chemicals, chemo- 

 iropism; to moisture, hydrotropism. As with other animals, it is 

 in general true that earthworms respond to stimuH such as nor- 

 mally affect them in nature, in a manner that is related to the con- 

 ditions under which their lives are conducted. In other words, 

 their behavior is adaptive to the extent that it fits the necessities 

 of their environment fairly well. If this were not the case, earth- 

 worms would soon cease to exist. 



