THE ABTERIES. 159 



Examine the sac in water or glycerine on a slide under a 

 low power of the microscope. 



a. The ' auditory ' sac is a transparent ovoid chitinous 



sac, continuous at its opening with the cuticle of 

 the antennule. 



b. The sensory setae are nearly straight simple pointed 



setae varying greatly in size, the outer ones being 

 the largest. The larger ones are arranged in 

 parallel curved lines along the floor and inner 

 wall of the sac ; of these there are two rows 

 behind, the number increasing to five or more in 

 front. Parallel to these runs a row of very minute 

 setse on the inner side. 



c. The sensory nerve supplying these rows of setse may 



have been removed with the muscles, but should 

 be looked for. 



3. Olfactory setse. On the ventral surface of each joint of 

 the exopodite of the antennule are two tufts of fine 

 sensory setse. Under a low power of the microscope 

 each is seen to consist of two segments, of which the 

 distal one is much the longer, and is flattened. They 

 are probably olfactory in function. 



III. THE AETEEIES AS SEEN IN AN INJECTED SPECIMEN. 



Bore a small hole in the carapace immediately above the 

 heart : inject with Prussian blue, using a syringe just large 

 enough to jill the hole. Dissect from the left side, removing 

 the left half or more of the carapace, and such parts of the 

 liver, genital gland and duct, and of the side-wall and muscles 

 of the abdomen as may be necessary to expose the following 

 arteries. 



1. The ophthalmic artery is a median artery arising from 

 the anterior end of the heart and running forwards 

 over the stomach, then curving downwards to supply 

 the front of the stomach and the oesophagus. 



