108 PROCEEDINGS OF THE ACADEMY OF [Feb., 



they were recognized in fixed and stained material. In life the two 

 larger sense hairs appear as tapering homogeneous threads of proto- 

 plasm, and such structures were sought for in sections. Wlien they 

 were finally identified by their position, they were found to be com- 

 posed of a considerable number of cilia fused together, and borne Ijy a 

 cell essentially similar to those which constitute the adjacent ciliated 

 bands (fig. 12). The smaller sense hairs (fig. 20, t.cil.) differ from the 

 larger in being composed of fewer cilia, and borne by smaller cells. In 

 view of the function of these hairs, a nervous connection was to be 

 looked for. This was demonstrable in a few cases, the clearest of 

 which is shown in fig. 12. Two sensory neurons, similar to the adjacent 

 ganglion cells, in this case at least of the bipolar type, lie between the 

 neuropil and the sense cell. Their peripheral processes are in close 

 contact with the base of the sense cell, while their shorter central 

 processes join with the neuropil. The composite nature of the sense 

 hairs has been already noted by Schimkewitsch (1895) who speaks of 

 the ''Buscheln," and figures the pair of large sense hairs as made up 

 of smaller elements. Miss Moore (1899) says of D. gardneri that "the 

 head bears two tufts of long cilia in front .... these are probably 

 of a sensory nature. ' ' Sense hairs of precisely similar structure, being 

 composed of fused cilia, springing from cells like those which bear the 

 locomotory cilia, are described by Meyer (1901) as found on the 

 ventral surface of the umbrella of the trochophore of Lopadorhynchus. 



Ciliated Grooves. — ^These structures have been noticed under the 

 section on The External Form. Their histological structure offers 

 nothing worthy of comment. The ciliated cells composing these 

 grooves are similar to the other cilia-bearing cells of this form. There 

 is no good evidence of any direct connection with the nervous system, 

 although it is possible that they may be innervated by nerves given 

 off from the preoral transverse commissure. 



Problematical Sense Organs. — These are narrow oval areas, with the 

 long axis of the oval vertical, measuring ca. 10 x 22 //, situated, one on 

 each side, in the constriction between the head and the first trunk seg- 

 ment, and slightly dorsad of the mid-level. A section through one 

 of these organs is represented in fig. 11. The cuticle is seen to be 

 thickened over the whole ciliated surface, and especially at its centre. 

 In the middle of each the cilia are lacking over a space about one-third 

 of the diameter of the whole, their place being supplied by a number of 

 spherical bodies, borne on slender rods, which project barely above 

 the surface of the cuticle. Midway between the outer and inner sur- 

 faces of the body wall the rods stain more intensely and appear to 



