PHYLUM ARTHROPOD A 115 



e) The excretory system: In the anterior end of the cephalic cavity, inside 

 of the base of the antenna, locate a circular greenish mass, liberally supplied by 

 branches of the antennary artery. This is the excretory organ, commonly 

 called " green gland." It is in reality a modified nephridium, which has lost 

 its funnel-shaped opening into the coelome (since there is no coelome). By 

 carefully spreading it out with a forceps, determine that it is a blind sac, curved 

 upon itself into a circle. The renal opening in the basal joint of the antenna 

 has already been observed. The student should note that while in the annelids 

 there is a pair of such nephridia in practically every segment, in the lobster they 

 are present in one segment only, the antennary segment. This is another 

 example of the loss of segmental structures, which characterizes the arthropods 

 as compared with the annelids. Draw in the green gland on your side view. 



/) The nervous system: Remove all soft parts from the interior, leaving the 

 esophagus and sternal artery in place. First begin in the abdomen and care- 

 fully remove all of the ventral abdominal muscles, noting their segmental arrange- 

 ment and the complex manner in which they loop over one another. In the 

 median ventral line of the abdomen, next to the inner surface of the shell, is a 

 white cord, the ventral nerve cord. In the thoracic region this will be found 

 to disappear into a cavity which is roofed over by hard plates. Clean out the 

 muscles and other soft parts in the thorax until you have exposed these plates. 

 They form an internal skeleton, called the endophragmal skeleton; this is really 

 produced by ingrowth from the exoskeleton. 



The cavity underneath the endophragmal skeleton is the sternal sinus, which 

 was mentioned in connection with the circulatory system as the large sinus in 

 which all of the venous blood collects before going to the gills. In this sinus 

 are also located the nerve cord and the branches of the sternal artery. Remove 

 the endophragmal skeleton by cutting along each side and taking out the middle 

 piece, and trace the nerve cord forward in the sternal sinus to the esophagus. 

 Here note that it forks, passing on each side of the esophagus and uniting again 

 into a bilobed mass just within the region occupied by the eyes. 



The bilobed mass beneath the eyes is the so-called brain, better designated 

 as the supra-esophageal ganglia. It consists of two ganglionic masses fused 

 medially, and sends nerves to the eyes (where they expand into the optic ganglia, 

 situated in the eye stalks), the antennules, and the antennae. The brain, there- 

 fore, represents at least three pairs of ganglia fused together. From the brain 

 arise the two circum-esophageal commissures. Trace these around the esophagus 

 to the large sub-esophageal ganglion, just behind the esophagus. Note branches 

 arising from the circum-esophageal commissures and sub-esophageal ganglion. 

 Trace the nerve cord posteriorly along the floor of the thorax, noting the enlarge- 

 ments or ganglia. Behind the sub-esophageal ganglion there are three ganglia; 

 then the nerve cord forks to allow the sternal artery to pass through; behind the 

 sternal artery are two more thoracic ganglia. Since in a segmented animal 



