PAL1NURUS OR THE SPINY LOBSTER OF BOMBAY. 381 



a number of longitudinal ridges set with small papillae. The hind- 

 gut opens on the ventral surface of the telson by a longitudinal 

 anus. 



The Excretory Organs. 



The nitrogenous waste of the body is got rid of from the blood by 

 the nephridia (Gr. nephros, kidney), a pair of " green-glands" whose 

 apertures we have already seen on the ventral side of the basal joints 

 of the antennae. 



Insert a bristle into this opening. You will find it enters a delicate 

 thin walled sac or bladder, into which the secreting portion, a coiled 

 tube lined with epithelium, discharges. The glands are of a light 

 greenish-yellow color and lie in the extreme anterior part of the 

 head. The stomach should be pushed back a little to see them. 



The Nervous System. 



After examining the heart, alimentary canal and reproductive sys- 

 tem, the gut should be cut across about its middle and each end 

 turned aside. The abdominal muscles should be carefully removed, 

 when the nerve cord will be exposed lying in the middle line close 

 to the abdominal sterna. The thoracic portion is enclosed in the 

 oentral canal or tunnel formed by the endophragmal skeleton which 

 should be removed by forceps and scissors. The brain is seen in 

 front of the oesophagus close to the origin of the ophthalmites. 



When compared with other arthropods the nervous system of 

 Palinurus presents a very interesting study with many useful lessons 

 on the subject of development and the origin of species. 



In the Earthworm you have seen a long nerve cord stretching 

 throughout the length of the body with a small thickening in each 

 somite, which from its likeness to a knot on the cord is called a 

 ganglion — Greek for a knot. You learned that this cord and its knots 

 were originally a pair placed side by side, but have become enclosed 

 in a common sheath so as to appear single. The microscope shows 

 that in the worm the structure of cord and knot is much alike, both 

 containing nerve-cells and nerve fibres, but in different proportions. 



In the Arthropods we have a specialisation of structure in 

 accordance with a differentiation of function. We have a series of 

 ganglia which alone contain nerve cells, the function of which is to 

 originate or translate nervous impulses, while in the connecting cords 

 we have only nerve fibres whose function is simply to transmit 



