376 J. M. D. OLMSTED 



markedly from that of sense organs in other regions. But 

 neither of these conditions have, as yet, been demonstrated for 

 synaptids t 



In Miss Langdon's ('95, p. 215) account of the sense organs 

 of Lumbricus, she estimates that in a worm 19 cm. long 

 with 152 metameres, there are 150,000 sense organs present, 

 about 1000 to a single metamere. The magnitude of the task 

 of stimulating separate sense organs in the earthworm can thus 

 be readily appreciated. In regard to the histological structure 

 of these organs she says (p. 226), " Since these sense organs form 

 the only known sensory apparatus of Lumbricus, and since their 

 structure is not visibly different in different parts of the body, 

 it is likely that they are sense organs of a general nature cap- 

 able of reacting to mechanical, chemical, thermal or luminous 

 stimuli." She also finds (p. 218) that "the sense organs are 

 distributed over the entire surface of the body, but are most 

 numerous and largest at each end." Since these features of the 

 worm are exactly paralleled by those of Synaptula, it would not 

 be unreasonable to draw the same conclusion here, i.e., that in 

 Synaptula also there are present "universal sense organs." 



From the foregoing account it is possible to defend the fol- 

 lowing statements regarding Synaptula: (1) it has a well de- 

 veloped chemical sense, since it responds to many of the cate- 

 gories of substances which are stimulating to man, viz., sour, 

 bitter, salt, sweet (?), and alkaline; (2) the order of sensitivity 

 in different parts of the body to chemical agents is the same as 

 to light, and to touch; (3) except for the eyes and otocysts, 

 only one type of sense organ has, as yet, been described for it; 

 (4) the relative abundance of the sense organs in the three* or 

 four chief regions of the animal is the same as the order of sen- 

 sitivity in these regions to these three kinds of stimuli; chemical, 

 photic and tactile. 



In connection with the idea of 'universal sense organs', it 

 is interesting to note, and not without significance, that, as the 

 studies of Herrick and Coghill ('15) have shown, "in the de- 

 velopment of the nervous system of Amphibia, the first reflex 

 circuits to come to maturity are made up of rather complex 



