THE PHYLA ARTHROPODA AND ONYCHOPHORA 



\ Tubule 

 Excretory pore 



''Cortex 



Fig. 15.6. Green gland of the crayfish; semidiagrammatic. A, the gland extended, showing 

 relationships of the various portions. B, the gland in vertical section, as it lies in the 

 hemocoel. (Redrawn from P. Marchal, 1892, Archives de zoologie expenmentale et generate, 

 vol. 10.) 



passed into the bladder, which empties periodically to the exterior. It is thus 

 apparent that through the selective activities of the secretory tubular cells, 

 the green glands are important not only in excretion but also in salt and 

 water balance. 



The Nervous System and Responsiveness. The general plan of the nervous 

 system in the crayfish is similar to that of annelids (Fig. 15.7). There is a 

 dorsal ganglionic mass, the supraesophageal ganglion, or "brain," from which 

 nerves extend to the eyes and antennae. The brain also gives rise to a pair 

 of circumesophageal connectives which pass around the esophagus and join with 

 the subesophageal ganglion at the anterior end of the ventral nerve cord. 

 Nerves arising from the subesophageal ganglion innervate the six pairs of oral 

 appendages, the green glands, the esophagus, and the muscles of the anterior 

 region of the thorax. In the course of embryonic development the subeso- 

 phageal ganglion is formed by the coalescence of six pairs of ganglia, 

 corresponding to the six pairs of oral appendages. The ventral nerve cord is 

 a double structure, composed of fused, paired ganglia, joined together in a 

 linear series by connectives. Posterior to the subesophageal ganglion there 

 is a pair of ganglia for each somite; the metameric nature of the animal is thus 

 revealed by the segmental organization of the nervous system, as well as by 

 external divisions and the distribution of appendages. 



The antennules and antennae bear very numerous receptors, for both tac- 

 tile and chemical stimuli. In addition to these receptors, the hair-like proc- 

 esses upon the appendages and other parts of the body, such as the edge 

 of the carapace, are tactile in function. Related to these tactile receptors 

 are two statocysts, which are organs of equilibration located in the basal 

 segment of each antennule. The statocysts are sac-like invaginations from 

 the outer surface of the appendage; they are thus lined with an exoskeletal 



433 



