256 Elementary Biology. 



aortae. Posteriorly to the pharynx the two aortse unite into 

 one dorsal aorta, which gives off secondary branches or 

 arteries as it goes, each of these in turn breaking up into a 

 series of lacunae in the several organs of the body. 



In the intestinal wall there are a number of vessels, 

 known as portal veins, carrying impure blood and food 

 material from the intestine to the saccular dilatation from 

 the wide portion of the intestine the so-called liver. In 

 the liver the portal veins form a capillary network, and from 

 it the blood is again collected and carried to the cardiac 

 aorta to be distributed to the branchial arches. The cardiac 

 aorta also receives impure blood from the muscles and other 

 organs of the body. 



Circulation. We have seen that there is no distinct 

 heart in Amphioxus, its place being taken by the cardiac 

 aorta and the small branchial hearts at the origin of the 

 branchial vessels. 



The cardiac aorta contains impure blood that is, blood 

 laden with gaseous and liquid excreta from the various organs 

 of the body. The impure blood passes into the branchial 

 arches, there, as we have seen, coming in contact with fresh 

 oxygen dissolved in sea-water, whilst at the same time the 

 gaseous waste matters are got rid of. The blood collects in 

 the two dorsal aortae, which may be said now to contain so 

 far pure blood, and is distributed to the body generally. 

 The blood is again collected by veins from the intestine 

 and is carried to the liver, and from the liver again and body 

 generally to the cardiac aorta, thus comoleting the circuit. 



The excretory system. It is doubtful whether Amphi- 

 oxus possesses any organs comparable to the nephridia of 

 the earthworm. Two short secretory tubules, with pig- 

 mented walls which open into the atrial cavity, opposite the 

 end of the pharynx, are looked upon by many as renal 

 organs. 



It will be remembered that in the earthworm the ali- 

 mentary canal, nephridia, and reproductive organs lay in 



