404 ESSENTIALS OF ZOOLOGi' 



The excretory system consists principally of a pair of large bodies 

 (green glands) located in the ventrolateral portion of the head. These 

 are richly supplied with blood and draw the nitrogenous wastes and 

 excess water from the blood to deliver them externally through excre- 

 tory pores located in the coxopodites of the antennae. 



The nervous system is of the same structural plan as that of the 

 earthworm, which is a modified ''ladder type." The two longi- 

 tudinal cords have come together in the ventral line and run the entire 

 length of the body to form a ventral nerve cord with ganglia. This 

 arrangement constitutes the central nervous system. The ganglia 

 of the anterior three segments are fused into the ''brain" or supra- 

 esophageal ganglion which is located anterior to the esophagus and is 

 joined to the cord by two circumesophageal commissures or connec- 

 tives, one passing on each side of the esophagus. From this dorsal 

 ganglionic mass, nerves pass to the eyes, antennae, and antennules. 

 The most anterior portion of the ventral cord receives these com- 

 missures. This portion, which consists of the fused ganglia from 

 segments three to seven, is known as the suhesophageal ganglia. 

 Nerves go from it to the mouth parts, first and second maxillipeds, 

 green glands, esophagus, and muscles of the thorax. Each segment 

 posterior to the suhesophageal ganglia possesses a segmental ganglion 

 with branches to its respective appendages and muscles. The sense 

 organs include antennae, antennules, sensory hairs, statocysts, and 

 eyes. The antennae are tactile organs (sensitive to touch), the end- 

 podite of which is a relatively long jointed filament. The exopodite 

 is much shorter and fan-shaped. The basipodite and coxopodite are 

 closely fused to the ventral side of the cephalic region. An excretory 

 pore opens to the exterior through the coxopodite of each antenna. 

 The hairlike processes along the edge of the carapace, on the legs, 

 and other parts of the body are also sensitive to touch. The anten- 

 nules are tactile and each has two slender filamentous processes, the 

 exopodite and endopodite. In addition to these slender jointed proc- 

 esses each antennule has a saclike statocyst in its coxopodite. This 

 structure is an infolding from the outside and is lined with exo- 

 skeleton and sensory hairs. Inside of each are small particles of 

 solid material, such as grains of sand, which are called statoliths. 

 As the animal changes its position the statoliths move about inside 

 of the statocyst and stimulate the sensory hairs. From these stimu- 

 lations the crayfish is able to determine its orientation in space 



