DEVELOPMENT OF AMPHIOXUS. 369 



growth of two lateral folds or epipleura, which grow as flaps 

 over the gill-slits, and join one another ventrally. The 

 water-currents are kept up by the action of cilia, and by the 

 movements of the transverse ventral muscles. 



The gill-slits gradually become more numerous as the 

 animal grows older, and in the adult there are more than a 

 hundred. The unsplit or secondary rods supporting the 

 pharynx seem to divide the halves of originally single gill-slits. 



Circulatory System. — The blood is colourless, with a few 

 amoeboid cells. There is no definite heart, but the vessels 

 are said to be at several places contractile. Vessels from 

 the body and from the liver unite in a ventral vein (" cardiac 

 aorta "), lying beneath the pharynx. Thence the blood is 

 perhaps driven along the primary branchial rods between 

 the gill-slits (in " aortic arches "), and may flow into two 

 dorsal vessels ("dorsal aortae") which unite behind the 

 pharynx. The most anterior aortic arch on the right side is 

 much larger than the rest, and sends branches to the head. 

 The blood-vessels, which presumably take blood from the 

 intestine to the liver, may be called portal veins, those 

 which presumably lead blood from the liver and unite to 

 form the ventral vein may be called hepatic veins. The 

 portal vein and the cardiac aorta are said to be specially 

 contractile. But while the distribution of the vessels is well 

 known, we are ignorant in regard to the flow of the blood. 



Excretory System. — In regard to this there is uncertainty. 

 At most there are only traces of nephridia. 



(a) Hatschek discovered in the larva a nephridial tube 

 near the mouth, but this does not persist in the full-grown 

 adult. " It extends on the left side from the margin of the 

 mouth to close behind the velum." 



(V) Ray Lankester discovered a pair of short pigmented 

 funnel-tubes which lie in the twenty-seventh segment, and 

 place the coelomic canals of the epipleural folds in com- 

 munication with the atrial cavity. They may be compared 

 with the pores which open from the collar region of Balano- 

 glossus, with the abdominal pores of higher Vertebrates, and 

 with nephridia. 



Reproductive System. — The sexes are separate and similar 

 to one another. The reproductive organs are very simple 

 and ductless. They form 26 pairs of horse-shoe-shaped 



2 A 



