64 ORIGIN OF CHORDATES in. 4 



circulate about the heart and hence to help excretion (and respiration ?). 

 In other ascidians the epicardium loses its connexion with the pharynx. 

 The closed sac functions in some cases as an excretory organ, con- 

 taining concretions of uric acid, whereas in other animals it becomes 

 the main source of the cells that make the asexual buds. 



The central nervous system consists of a round, solid ganglion 

 (Fig. 36), lying above the front end of the pharynx. The ganglion has 

 a layer of cells around the outside and a central mass of neuropil and 

 is therefore quite unlike the nerve-cord of a vertebrate. From the 

 ganglion nerves proceed to the siphons, other parts of the mantle, 

 muscles, and viscera. Receptor cells with nerve-fibres ending around 

 the base have been described, especially in the siphons. The gut is 

 said to contain a plexus of cells and fibres, whose relation to the 

 autonomic system of higher forms remains uncertain. 



Movement consists mainly of contraction and closure of the aper- 

 tures. Light touching of either siphon causes closure proportional to 

 the strength of the stimulus. Stronger stimuli cause closure of both 

 siphons and if very strong there is contraction of the whole body and 

 ejection of the water in the pharynx and atrium. Stimulation just 

 inside either siphon produces closure of the other one and also, if 

 strong enough, contraction of the body, ensuring that a jet of water 

 sweeps out the aperture that received the stimulus. These crossed 

 reflexes depend upon the integrity of the ganglion. 



The surface of the body is sensitive to changes in light intensity, 

 and these are followed by local or total contractions, according to their 

 extent. After removal of the ganglion the wider reflexes can no longer 

 be obtained but local responses continue, suggesting the presence of 

 nerve-cells in the body wall. Electrical stimulation also provides evi- 

 dence of this. One shock may produce only a small response but if a 

 second shock follows shortly afterwards there is marked facilitation 

 and a large contraction occurs. These responses are also seen after 

 removal of the ganglion. The various parts of the body are not all 

 equally sensitive to light, the highest sensitivity being in the region 

 of the ganglion. The 'ocelli' are cup-like collections of orange- 

 pigmented cells around the siphons ; according to Hecht they are not 

 photoreceptors. 



The neuromuscular system thus appears to function mainly as a 

 reflex apparatus for producing protective movements in response to 

 certain stimuli. This is the role that might be expected of it in an 

 animal that remains fixed in one place. The 'initiative' for food- 

 gathering activities comes from the continuous action of the cilia of 



