68 W. H. TALIAFERRO 
The use of such aquaria is essential, as it is practically impossible 
to control the path of the rays of light after being refracted by 
the irregular curved sides of the ordinary aquarium. All obser- 
vations were made in a dark room and, as far as possible, all 
apparatus was painted dull black so as not to reflect light. 
The apparatus used for measuring the rate of movement in 
specimens was essentially like that of Walter (07, p.55). The 
animals were placed in an aquarium which was held a few inches 
above a table by means of a burette stand and clamp. A panto- 
graph was then so arranged that the style on the tracing arm 
was directed upward and could be moved beneath the aquarium. 
Care was always taken not to allow the tracing style to come in 
contact with the aquarium, as the consequent jarring causes a 
disturbance in the reactions of the specimens. <A pencil in the 
remaining arm of the pantograph was directed downward and 
placed in contact with a sheet of paper on the table. By means 
of the style, a given worm could be followed and the path recorded. 
All such paths were made at a magnification of two diameters 
to facilitate measuring. After the path of a specimen was re- 
corded for a definite number of minutes it was measured with a 
chartometer. In this way the character as well as the length of 
the path was ascertained. 
STRUCTURE OF THE EYE 
According to all of the recent descriptions of the structure of 
the turbellarian eye, it contains two regions—a pigment-cup 
formed by the pigment or accessory cells and a sensory region 
composed of visual cells or retinulae. As has been previously 
stated, a careful study of the retinula has shown in at least one 
form, Prorhynchus applanatus, that it is composed of three re- 
gions—the nucleus-bearing portion, a central highly refractive 
lens-shaped portion, and the true sensory portion or rhabdome. 
These three regions bear a striking resemblance to the three 
regions of the rods and cones in the vertebrate eye, i.e., the myoid, . 
ellipsoid, and rhabdome, respectively. The comparison is of 
especial interest in that the retinulae in both groups of animals 
