150 THE CRAYFISH 



8. Olfactory setae, On the ventral surface of each joint of 

 the exopodite of the antennule are two tufts of fine 

 sensory setae. Under a low power of the microscope 

 each is seen to consist of two joints, 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. 



Inject from the ventricle with water coloured with carmine. 



Kill the animal by dropping it into boiling water. Dissect 

 from the left side, removing the left half or more of the cara- 

 pace, and such parts of the liver, genital gland and duct, and 

 of the side-wall and muscles of the abdomen as may be neces- 

 sary to expose the following arteries. 



1. The ophthalmic artery is a median artery arising from 



the anterior end of the heart and running for- 

 wards over the stomach, then curving downwards in 

 front of the stomach and dividing to supply the 

 eyes. 



2. The antennary arteries arise from the anterior end of 



the heart, one on each side of the ophthalmic artery. 

 Each runs forwards, outwards, and downwards over 

 the side of the stomach, to which it sends a gastric 

 artery ; and then forwards to the anterior end of the 

 head, supplying the kidney, antenna, and antennule, 

 and sending a branch to the rostrum. On its way 

 it gives off numerous small arteries to the muscles 

 of the regions which it traverses. 



3. The left hepatic artery arises from the ventral surface 



of the heart near its anterior end, and supplies the 

 liver, in which it is buried. The right one is like it. 



4. The sternal artery is single, and arises from the hinder 



end of the heart, whence it runs directly downwards, 

 passing to the right or left of the intestine, and 

 between the connectives uniting the fourth and fifth 

 ganglion-pairs of the thoracic nerve-chain. It then 



