Langdon, Sense-organs of Nereis virens. 37 



method of conveying stimuli from without, must be considered 

 a nerve-fiber. 



G. Summary of the Literature. 



Claparede ('70) seems to have been the first to perceive these 

 sense-organs in any species of Nereis. He studied the -'terminaisons 

 nerveuses"in "des tentacules, des palpes, et des rames pedieuses " 

 of N. peritoneahs and N. cultrifera. It is difficult to understand his 

 description, but he probably merely saw the sensory hairs in an optical 

 section. 



Retzius ('92a and '95) studied the peripheral nervous system 

 of various species of Nereis — of which the only one named is Nereis 

 diversicolor — by means of the methylene blue and silver nitrate meth- 

 ods. He found a sensory system^ of isolated, spindle or flask-shaped 

 bipolar cells in the body wall, the parapodia, the tactile appendages 

 and the buccal cavity. His figures and descriptions of these isolated 

 bipolar nerve-cells in N. diversicolor closely resemble the bipolar nerve- 

 cells of the diffuse sense-organs of N. virens. As before stated, with 

 the exception of a few doubtful cases in the parapodial cirri, I have 

 found that all of these bipolar nerve-cells in N. virens are grouped into 

 definite sense-organs. Either two species of the same genus which live 

 practically under the same conditions have the one a sensory system 

 composed of isolated nerve-cells, the other one composed of the same 

 kind of cells grouped into sense-organs or else, as seems to me more 

 probable, the sensory cells described by Retzius are in reality grouped 

 into definite organs. He does not describe any branching of a peri- 

 pheral process or any especially modified area in the cuticula over 

 these sensory cells, yet it seems probable that he has seen both. In 

 one of his articles (see Retzius, '95, Plate II), he figures without re- 

 mark a peripheral process which forks in the cuticula and also, in the 

 cuticula above a nerve-cell, the outline of some structure which sug- 

 gests the ovoid inner cuticular cavity of the sense-organs in the body 

 epidermis. 



Retzius describes the peripheral processes as usually ending just 

 beneath the cuticula, sometimes in a little knob. Occasionally he found 

 one ending in a yet finer part which ran partly through the cuticula in 



* Retzius describes a system of branching nerve-fibers surrounding the 

 setae-fibers which he believes have no connection with the musculature of the 

 setae but probably form a second sensory nerve ending. In some of my prep- 

 arations, these same nerve fibers have been richly stained, but they appear to me 

 to be motor fibers innervating the muscles of the setae. 



