214 W. J. CROZIER 



Examination of the surface of many individuals of Ptycho- 

 dera with a small intense beam of light (cf. Patten, '15) showed 

 that the only part which is conspicuously sensitive is the very 

 tip of the proboscis. In the Tornaria larva there are found, in 

 the apical region, which becomes the tip of the proboscis, two 

 well defined groups of sensory cells, which have been termed 

 eyes (cf. Bourne, '89; Morgan, '94; etc.). Ritter and Davis 

 ('04, p. 195) state, however, that the Tornaria examined by 

 them is not reactive to light. Certain annelid larvae are, as is 

 well known, markedly phototactic, and posses organs corre- 

 sponding to the 'eyes' of Tornaria (cf. Gerould, '06, p. 105). 

 The behavior of Tornaria might, therefore, bear reexamination 

 on this point. 



The 'eyes' of the larva disappear, however, as the adult 

 form is gradually assumed. No trace of them has yet been 

 located in the proboscis of later post-larval stages. Never- 

 theless this region of the animal remains markedly sensitive 

 to light. The whole tip of the proboscis, nearly one-fifth its 

 length, seems about equally sensitive and I have been quite 

 unable to establish any bilateral distribution of sensitive parts. 

 The course of phototropic orientation in Ptychodera agrees 

 with this condition, for the proboscis is slowly pushed from 

 side to side as the animal moves away from the light. The 

 extent of the sideward excursions is never considerable, unless 

 the incident light be strictly horizontal, in which case the 

 creature's proboscis may largely be shaded by its own body. 

 When the light is made to fall at a slight angle, so that this 

 shading is avoided, orientation is much more precise. Ptycho- 

 dera illustrates the principle that an animal having an axial 

 photosensitive spot may yet by suitable movements convert this 

 terminal area into a bilateral sense organ. 



Photic sensitivity in Ptychodera may easily be separated 

 from that to tactile or chemical excitation If the animals 

 be strongly illuminated for some time, they cease to respond 

 to light after a brief intermission in darkness. Cocaine hydro- 

 cloride or chloretone, added to the water, likewise abolish photic 

 irritability before tactile and chemical responses are notably 



