282 AN INTRODUCTION TO THE STUDY OF FOSSILS 



Internally they are bounded by the wall of the thorax, since they 

 are outpushings of the body wall. They are freely open below 

 for the entrance of water. The renewal of fresh water is brought 

 about by the vibratory movements of the exopodite of the max- 

 illa, causing a current to set in over the gills from below and out 

 in front. 



The gills thus excrete carbon dioxid. Another excretory 

 organ is situated at the base of each posterior antenna ; this is 

 the green gland which collects the uric acid, urea, etc., in a 

 urinary sac and discharges it through a duct opening on the 

 proximal segment of the antenna. 



The nervous system consists primarily of a brain in the dorsal 

 region of the head united by nerve cords around the oesophagus 

 to the anterior end of a ventral nerve cord. The former supplies 

 the eye, antennae, etc. The latter is composed primarily of a 

 double, ladder-like chain of ganglia united by connectives and 

 extending to the posterior end of the body. Most segments 

 have their own ganglia. 



The sense organs include those of touch, sight and possibly 

 also smell and hearing. 



The setae on the two pairs of antennae are tactile as are those 

 in many other parts of the body. These are hollow outgrowths 

 of the epidermis and its chitinous covering and contain the end 

 of a nerve fiber prolonged outward from the dermis. There are 

 two compound eyes at the front of the head, and since the latter 

 is firmly fixed to the thorax the eyes are raised on movable stalks. 



Each eye is covered with the protecting, transparent cornea, 

 the surface chitin ; this is divided into very many four-sided 

 facets. Beneath each facet is the eye proper, or ommatideum, 

 optically separated from its neighbor by a black pigment. The 

 outer portion of each ommatideum is the refractive vitreous 

 body, which bends the light rays falling upon it downwards to 

 the inner portion, the small retina. The retina is composed of 

 very sensitive cells, the ends of the optic nerve fibers, and 

 through these is connected with the brain. The pigment sur- 

 rounding the ommatideum absorbs all light not reaching the 

 nerve fibers, thus preventing a distortion of the imag 3. Each sep- 



