PARKER. — THE SENSORY REACTIONS OF AMPHIOXUS. 443 



impulses pass are separate from those which transmit sensory impulses 

 from the integumentary terminals. Since they show, further, that the 

 anterior half of the nerve-tube is different in function from the pos- 

 terior half, they are opposed to Steiner's view of a metameric nervous 

 system with equivalent segments, and favor the opinion advanced by 

 Ayers ('90b, p. 223) and supported by Danilewsky ('92), that the 

 anterior end of the nerve-tube of amphioxus is already a primitive 

 brain and the posterior portion a spinal cord. 



8. Sensory Mechanisms in Amphioxus and their Relations 

 to Vertebrate Sense Organs. 



The conditions presented by the sensory mechanisms in amphioxus 

 give some clue to what was probably a step in the differentiation of the 

 sense organs in primitive vertebrates. In these forms tactile organs 

 doubtless covered the whole exterior, as they now do the body of amphi- 

 oxus and that of the higher vertebrates, but these primitive ancestors, 

 like amphioxus, probably possessed nothing by way of differentiations 

 of these organs. Such differentiations are represented by the lateral- 

 line organs and the ears, both of which occur in the cyclostomes and the 

 higher vetebrates, but are wholly unrepresented in amphioxus, for the 

 ear supposed by Peters ('77, p. 854) to have been seen in this animal is 

 well known not to occur there. From the embryology of these organs it 

 seems probable, as Ayers ('92) has pointed out, that specialized 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 physi- 

 ology of these parts, for the lateral-line organs are stimulated by 

 material vibrations of low rate (Parker, : 05^ ; : 03* ; : 03t>), possibly also 

 effective as tactile stimuli, and the ear is stimulated by material vibra- 

 tions of a higher rate, such as we recognize as sound. In my opinion 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. Although the primitive 

 functions of these parts were doubtless (1) touch, (2) reception of slow 

 vibrations, and (3) hearing, all these parts, but especially the ear, 

 became involved more or less in the reflexes of equilibrium. This 

 relation, however, I believe to have been entirely a secondary one, and 

 not in any way to represent the original function of these organs as 

 intimated by Lee ('98) ; hence I have avoided any such expression as 

 equilibration sense. Amphioxus thus represents an ancestral verte- 

 brate with tactile organs, but without lateral-line organs or ears, and 



