56 ILUNOIS BIOLOGICAL MONOGRAPHS [250 



and back and down on each side nearly to the level of the eyes. This central 

 broad part as it runs posteriorly along the middle line toward the narrow neck- 

 portion of the body becomes itself very slender, like the handle of a fan, and 

 thus continues back between the eyes for a distance. Nearly the posterior 

 third of the whole group becomes wider again to about one-half the width of 

 the anterior fan, and bends outward away from the middle Hne, reaching almost 

 to the sides of the body. The outer posterior edge is truncate and straight, 

 parallel with the side of the body Another group of rhabdites much smaller 

 than, and lying directly ventral to the dorsal division, runs backward at some 

 distance ventral to the brain. It starts anteriorly near the tip of the head 

 as a part of the dorsal fan-shaped group, and, bending ventrally, becomes very 

 narrow just beneath the brain but broadens out again posterior to it. It ends 

 abruptly under the extreme front end of the intestine. The rhabdites which 

 make up these groups are arranged in strands of very regularly placed cells, 

 each containing a cluster of rather large rods. In the anterior part there are 

 from twelve to eighteen strands which narrow down to two or three, lying very 

 close to one another thruout the middle division, then in the remainder of the 

 group there are from five to six strands broadening out a little. Each strand 

 is composed of from ten to thirty clusters of rods placed end to end so that the 

 whole appears as a series of long lines. 



The rhabdites or rods are surprisingly alike in size, shape, and color. The 

 average length in adult specimens is twelve micra, while the width or diameter 

 is about two micra. When released from pressure of the body, these measure- 

 ments remained constant. The shape is a rather bluntly pointed spindle. 

 Thruout most of the length the diameter of the rod is the same, giving it 

 the appearance of a rather long cyUndrical pencil. The two ends are about 

 equally pointed and generally slightly curved or bent to fit the contour of the 

 bundle. The color is almost a steel white, not at all transparent, but bright 

 and clear, v/ith the edge very distinct and black. They seem like bits of 

 metallic rods sharp and hard, embedded in the most delicate tissue imaginable. 

 There is always a rather constant number in each cell, eight, nine, or ten, lying 

 very closely packed In a soHd ovoidal mass. The cell, itself, is quite large so 

 that the rods occupy only the center and are surrounded by a large space filled 

 with very finely granular protoplasm. These rod-bearing cells are situated 

 either in the epitheHum itself or in the parenchyma just beneath, as tho they 

 had been crowded there. The rods develop one at a time, right in the cell, 

 being differentiated, as it were, from the protoplasm. In many instances the 

 rods lay with their points toward the surface and under very sUght pressure were 

 discharged thru the outside wall, seemingly by internal force, literally punc- 

 turing the cell. If, by chance, any entirely escape from the epidermis into 

 the surrounding water, so that all tension is removed, they immediately curl, 

 sometimes tying a knot, and then spin aroimd as they are swept away by the 

 ciUa-made currents of water. Even under slight disturbance in the body they 



