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SCIENCE 



[Vol. LVI, Ko. 1453 



attacks upon problems of this sort in the field 

 of sense physiology, it is needless to say that 

 his survey of the ehemieai senses is accurate, 

 authoritative and judicious. So brief a sum- 

 mary naturally can make no claim to complete- 

 ness, but it is a well balanced selection of 

 topics of significance to students of physiology, 

 psychology and allied sciences. 



The more theoretical discussions, especially 

 those centering about the geneftic relationships 

 of the receptors, naturally enter debatable ter- 

 ritory. In summarizing his well-known ob- 

 servations on the organization of sponges, 

 which "possess muscles but are devoid of ner- 

 vous tissue," Dr. Parker reiterates (p. 21) his 

 belief that in phytogeny differentiated muscles 

 probably preceded nervous tissue. "So far as 

 can be judged these [nervous] elements orig- 

 inated in connection with the previously dif- 

 ferentiated muscle and as a special means of 

 exciting it 'to contraction." This conclusion 

 seems both unphysiological and unsupported 

 by the facts. 



That muscle should be differentiated in ad- 

 vance of the receptive apparatus through which 

 it is activated seems a priori as improbable as 

 that receptors should be developed in advance 

 of the appropriate effectors. And Parker's 

 own experiments strongly suggest that the so- 

 called muscles of sponges are really excito- 

 motor organs with lowered excitation threshold 

 and that the excitation mechanism is elaborated 

 within them parallel with the contractile 

 mechanism rather than apart and subsequently. 

 Ordinary protoplasm is, of course, both excita- 

 ble and contractile, and in some unicellar 

 forms (e. g., Diplodinium, Euplotes, Parame- 

 cium and others recently described at the Uni- 

 versity of California) there are exci'to-motor 

 masses of protoplasm in which these two func- 

 tions are both highly developed and in various 

 stages of separation physiologically and struc- 

 turally. In sponges, as in other lowly multi- 

 cellular forms, the excitation factor can not be 

 regarded as lagging behind the contractile 

 factor in the differentiation of the process and 

 apparatus of reaction, even though the tissue 

 involved may look more like muscle than like 

 nerve. 



In the concluding discussion (chapter 8) all 

 receptors are arranged in three groups: (1) 

 mechanicoreceptors (organs of touch, hearing, 

 equilibration, and probably organs of muscle, 

 tendon and joint sensitivity, lateral line organs 

 of fishes and some others) ; (2) radioreceptors 

 (organs of vision and temperature) ; (3) 

 chemoreeeptors (organs of smell, taste, general 

 chemical sensitivity, and probably some others). 

 This classification is natural and so far as it 

 goes very satisfactory. But when the author 

 adds (p. 180), "To ascertain into which of 

 these three groups a receptor falls it is neces- 

 sary to know how it is stimulated after which 

 its classification is simple and immediate," one 

 begins to question how far the simplicity and 

 immediacy of the procedure reaUy takes us. 



Rays of the solar spectrum with wave length 

 of say .0008 mm. falling upon the retina and 

 upon the skin produce very different excita- 

 tions. Both organs are by definition radio- 

 receptors, but just "how it is stimulated" we 

 do not know in either case, nor do we know 

 how it comes about that, if sensation follows 

 the excitation, it is of red in one ease and 

 warmth in the other. 



Similarly, it has been shown by Parker that 

 ethyl alcohol is an excitant of the organ of 

 smell, the organ of taste and general mucous 

 surfaces and by Carlson of the mucous lining 

 of the stomach also. The threshold is different 

 in each of these cases, the typical reactions are 

 very characteristic in each case, and the sensa- 

 tions (if any) are likewise distinct. Now the 

 fact that these four organs are all chemo- 

 reeeptors, while important, is less significant 

 biologically than any one of the other three 

 criteria mentioned. In fact, the classification 

 of receptors in terms of the effective stimulus 

 has a verjr limited range of usefulness and the 

 ultimate goal of our endeavors should be to add 

 to the simple determination of the adequate 

 stimulus of a sense organ the physiologically 

 far more significant knowledge of the real na- 

 ture of the excitation (that is, of the immediate 

 protoplasmic response to the stimulus) and 

 also a codified statement of the typical or 

 physiologically "normal" more remote effects 

 (reaction, sensation). We are at present very 



