38 Journal of Comparative Neurology. 



a little canal and more rarely he found these finer processes raised 

 above the cuticula. From these various positions in which he found 

 the peripheral end of a peripheral process — beneath, in, or above the 

 cuticula — he decided " dass sie vielleicht beweglick sind; man erhalt 

 namlich den Eindruck, dass sie sich moglicherweise ausstiilpen und 

 wieder zuriickziehen konnen." From my own work, I judge that these 

 finer processes are the sensory hairs of the nerve-cells, that the little 

 canal in which one was occasional found must be one of the canals of 

 a perforated membrane, that the end-knob in which a peripheral pro- 

 cess occasionally terminates is purely an artefact — a varicosity which 

 often forms at the tip of a sensory hair, that normally each peripheral 

 process must project above the surface as a sensory hair, and that the 

 retraction of these processes which Retzius regards as normal must be 

 an abnormal processes — a process which takes place when the tissue is 

 dying and which is usually caused by the formation of varicosities in 

 the peripheral processes themselves. 



Retzius states that the greater number of his bipolar nerve-cells 

 lie beneath the epidermis — " die bei weitem grosste Anzahl derselben 

 sich mit den eigentlichen zellenkorper aus der epithelschicht gelost 

 und in das unterliegende Gewebe eingesenkt hat." He considers this 

 true, not only in the tactile appendages in which the position of the 

 cells in or beneath the epidermis depends upon one's definition of the 

 limits of the latter structure, but also in the body itself. He considers 

 that this position indicates an advance in organization of the sensory 

 nervous system over that of Lumbricus (Retzius, '92b) and, therefore, 

 uses Nereis as an illustration of the second stage in the passage of the 

 sensory-cells from the epidermis to the central nervous system during 

 the evolution of the vertebrate sensory system. In the early part of 

 my work, I failed to perceive the true limits of the epidermis in the 

 appendages and therefore fell into the error of supposing the bodies of 

 the sense-cells in the diffuse sense-organs of these structures were "be- 

 neath the epidermis " (Langdon, '97}. In my later work, I have been 

 able to see that the bodies of the sense-cells in the tactile appendages 

 always lie in the epidermis itself because, as previously stated, each of 

 these appendages is purely an epidermal outgrowth and, therefore, all 

 structures within one of them are in the epidermis, and that the bodies 

 of the sense-cells in the body wall not only lie in the epidermis but 

 generally even nearer the cuticula than in Lumbricus. I therefore con- 

 sider that the sensory nervous systems of Nereis, if the position of the 

 bodies of its bipolar nerve-cells can be taken as a criterion, is not 

 higher but lower in organization than that of Lumbricus and that there- 



