450 PROCEEDINGS OF THE AMERICAN ACADEMY. 



13. The photoreceptors in amphioxus are anatomically distinct from 

 the receptors for thermal, mechanical, and chemical stimuli, and these 

 three are at least physiologically distinct one from another. 



14. To all stimuli that induce locomotion amphioxus responds by 

 forward movements when the stimuli are applied to the tail, and by 

 backward movements when they are applied to the middle or to the 



'anterior end. 



15. Amphioxus generally buries itself tail first, and in all probability 

 usually swims tail first, though it may reverse both processes. 



16. When amphioxus is cut in two, both halves lose much in sensi- 

 tiveness, the posterior proportionally much more than the anterior. 

 The anterior part of the nerve-tube is brain-like, the posterior part 

 cord-like. 



17. The skin of amphioxus contains tactile organs, but amphioxus 

 possesses no derived organs such as lateral-line organs and ears. 



18. The photoreceptors of amphioxus are the eye-cups of the nerve- 

 tube, and these probably represent the elements from which the rod- 

 and cone-cells of the lateral eyes of vertebrates have been derived. 



19. The rod- and cone-cells of the vertebrate retina are inverted, 

 not because they have retained a morphological position dependent 

 upon an external origin, but because of their orientation acquired as 

 effective eye-cups in the nerve-tube of a primitive vertebrate. 



20. The chemical sense organs of amphioxus are located in the 

 skin and are chiefly important as organs for testing the character of 

 the chemical environment rather than for the selection of food. From 

 these undifferentiated chemical sense organs have probably been de- 

 rived the organs of taste and smell, of which the former are appar- 

 rently not present in amphioxus and the latter may be represented by 

 the so-called olfactory pit. 



10. Bibliography. 

 Andrews, E. A. 



'93. An Undescribed Acraniate : Asymmetron lucayanum. Studies Biol. 

 Lab., Johns Hopkins Univ., Vol. 5, No. 4, pp. 213-247, pis. 13-14. 



Ayers, H. 



'90*. Contribution to the Morphology of the Vertebrate Head. Zool. Anz., 

 Jahrg. 13, No. 344, pp. 504-507. 



Ayers, H. 



'90". Concerning Vertebrate Cephalogenesis. Jour. Morph., Vol. 4, No. 2, 

 pp. 221-245. 



Ayers, H. 



'92. Vertebrate Cephalogenesis. II. A Contribution to the Morphology 

 of the Vertebrate Ear, with a Reconsideration of its Functions. Jour. 

 Morph., Vol. 6, No. 1, pp. 1-360, pis. 1-12. 



