ii. 8 FEEDING OF AMPHIOXUS 33 



same regions, the latter having an optimum action at about pH 8-o, 

 being, that is to say, a tryptic type of enzyme. There is no sign of 

 any protease with an acid optimum, similar to the pepsin of higher 

 forms. 



Behind the ileo-colon ring the intestine runs as a straight hind-gut 

 to the anus. Absorption of food takes place here, and perhaps also 

 in the mid-gut, apparently partly by intracellular digestion, since 

 ingested carmine particles are taken into the cells. 



The feeding current is regulated by the rate of beat of the cilia and 

 the degree of contraction of the inhalent and exhalent apertures. The 

 walls of the atrium contain an elaborate system of afferent and 

 efferent nerve-fibres. The receptors include a set of large peripheral 

 nerve-cell bodies, lying beneath the atrial epithelium and sending 

 axons in by way of the dorsal roots. The motor fibres also pass through 

 the dorsal roots and run without synapse to the cross-striated fibres 

 of the pterygial muscle, which forms the floor of the atrium. The 

 stream flowing into the pharynx is tested by the receptors of the velum 

 and atrium, and if noxious material is present, the water is expelled by 

 closing the atriopore and contracting the pterygial muscle, producing 

 a 'cough'. The system can distinguish between suspensions of food 

 material and inorganic particles. When sufficient food has been taken, 

 collection is suspended until it has been digested (Bone, i960). 



The atrial nervous system probably regulates spawning as well as 

 feeding. It has often been compared with the sympathetic system of 

 craniates but there are almost no close similarities. The nerve cells in 

 it are receptors and there is no sign of the peripheral synapse on 

 the efferent pathway that is so characteristic of the true autonomic 

 system. The atrial system is developed in relation to filter feeding 

 and has perhaps been completely lost in higher forms that feed by 

 other methods and have developed new methods to control them 

 (p. 117). 



8. Circulation 



The blood-vessels of amphioxus show in diagrammatic form the 

 fundamental plan on which the circulation of all chordates is based 

 (Fig. 11). Slow waves of contraction occur in various separate parts 

 in such a way as to drive the blood forwards in the ventral vessels, 

 backwards in the dorsal ones. Below the hind end of the pharynx 

 there is a large sac, the sinus venosus, into which blood from 

 all parts of the body is collected. From this there proceeds for- 

 wards a large endostylar artery (truncus arteriosus or ventral aorta) 



