62 ILLINOIS BIOLOGICAL MONOGRAPHS [256 



clear head region. This color varies somewhat in different individuals but 

 for the most part is heavy, dark, and very opaque. The outside surface of 

 this dark part is exceedingly rough, covered with coarsely granular knobs. 

 The shape, too, is irregular, very jagged, almost spiny, and often with flecks 

 of pigment completely isolated from the rest. The position of the pigmented 

 portion is quite constant. The pointed edge, which narrows out to a sharp, 

 bent tip, always is placed toward the middle line, at right angles with it, and 

 directly opposite the eye of the other side. The remainder of the pigment is 

 arranged like a cone-shaped cap with the much serrated external edge clasping 

 a very transparent lens-like cell. This lens is all but invisible so that the inner 

 surface of the dark cap can be distinctly seen much Ughter than the outside, 

 and also much smoother. The crystalhne part is a soUd bean-like structure, 

 not ever exactly aUke in the two eyes, and varying enormously in both size 

 and shape. The nerve supply comes directly from the ganglion on whose 

 surface the eye rests. It can hardly be said that optic nerves exist, since there 

 are only clusters of cells which send branches or divisions of protoplasm up 

 into the pigmented layer. It seems hardly possible that such an irregularly 

 shaped mass could be capable of being more than a light-detecting organ, 

 but the nerve supply would seem to indicate some true seeing abiUty. 



EXCRETORY SYSTEM 



The excretory tubules are of the typical primitive type. They are pro- 

 tonephridia, rather large and well-developed, but exceedingly thin-walled 

 and dehcate, so that they are invisible except under very favorable conditions. 

 The two long tubules have their beginnings somewhere posterior to the middle 

 part of the body and the extent and size are variable, so that in some individuals 

 the tip may be very near the tail. The tubule, not at aU constant in its position, 

 wanders forward thru the parenchyma, quite deep below the surface, until 

 it reaches the neck-region, where it bends dorsaUy around the anterior end of 

 the intestine. In the head it lies only a httle way beneath the skin, being 

 dorsal to the nerve and muscular systems. A little in front of the eyes and 

 between them, the two tubules, one for each side, approach each other and 

 then bend around ventrally, forming two loops. They then run back still 

 in a ventral position till they reach the edge of the pharyngeal rosette, where 

 they empty. Thruout their whole length these tiny canals are never taut, 

 but are irregularly looped and folded so as to allow for a large amount of 

 stretching without any strain to the dehcate walls. The flame cells are diflScult 

 to find, owing to their diminutive size and the deeply embedded position of the 

 tubule. In some instances, there are a niunber of these waving cells scattered 

 along the length of the tubule, while often only one or two can be found. As 

 a whole there is so Httle variation from the common type that a general de- 

 scription will apply directly to most details of this form. 



