SENSORY REACTIONS 469 



The whole outer surface of the Lancelet's body is sensitive 

 to chemical reagents of varied kinds, such as nitric acid, alcohol, 

 turpentine. " This sense," Professor Parker says, " is doubtless 

 serviceable chiefly as a means towards escape from unfavour- 

 able chemical surroundings and probably has little or nothing 

 to do with the direct feeding habits of the animal. As 

 is well-known, Amphioxus does not seek its food, but takes 

 what is brought to it in water currents, selecting from this 

 supply only in the crudest fashion, if in fact it can be said to 

 select at all." It is probable that the undifferentiated chemical 

 sense-organs or sense-cells in the skin of the Lancelet re- 

 present a state of affairs antecedent to the specialisation of the 

 senses of taste and smell, although it is possible that a minute pit 

 at the very front end deserves the name " olfactory " which it 

 usually gets. Similarly, it is probable that the tactile sense-or- 

 gans or sense-cells in the skin of Amphioxus represent the 

 elements from which the lateral-line organs of fishes and 

 the ears of Vertebrates in general have been derived. On 

 embryological grounds it is likely that specialised tactile 

 organs gave rise to lateral-line organs, and that from certain 

 of these lateral-line organs the ear was differentiated. " This 

 history, based upon morphological considerations, is parallel to 

 what is known of the physiology of these parts, for the late- 

 ral-line organs are stimulated by material vibrations of low 

 rate, possibly also effective as tactile stimuli, and the ear is 

 stimulated by material vibrations of a higher rate, such as 

 we recognise as sound. In my opinion, Prof. Parker con- 

 tinues, the stimuli for these three sets of sense-organs may 

 often overlap and the three sets of organs constitute a 

 genetic series, in which the tactile organs are the oldest 

 members and the ear the newest." 



" Amphioxus may, therefore, be said to be an animal that 

 possesses in potentia at least the sense-organs of the verte- 

 brates. Its outer surface is provided with tactile organs, 

 but it does not possess the derivatives of these, the lateral- 

 line organs and the ear. Its outer surface also contains un- 

 differentiated chemical sense-organs, but it cannot be said to 

 have a sense of taste, and the only evidence of a sense of smell 

 is morphological. Its outer surface, like that of the higher 

 vertebrates, contains temperature organs. Amphioxus also has 



