STRUCTURE OF THE ADULT ANIMAL. 41 



of the pharynx, the stream being kept up by the action of 

 columnar flagellate cells which clothe the gill-arches, and the 

 water serving to aerate the blood in the vessels of the arches 

 as it swills over them. 



The water that has passed through the gill-slits escapes into 

 a large space, the atrial or epipleural cavity (Fig. 12, H) : this 

 lies between the pharynx and the body wall, and into it the 

 pharynx hangs freely, slung up to the body walls by suspensory 

 folds (Fig. 12, y). The atrial cavity extends back some distance 

 behind the pharynx, and along it the water passes, escaping 

 finally by the atrial pore (Fig. 11, i), an aperture on the ventral 

 surface of the body, bordered by prominent lips, and about one- 

 third the length of the animal from its hinder end. The atrial 

 cavity of Amphioxus is a very characteristic feature in its ana- 

 tomy, and is apparently unrepresented in the higher Verte- 

 brates. 



The ccelom or body cavity is quite distinct from the atrial 

 cavity, though its boundaries are not easy to follow. In the 

 posterior part of the body, behind the atrial pore, the ccelom is a 

 cavity of some width, surrounding the intestine and separating 

 this from the body wall ; in front of the atrial pore it becomes 

 greatly reduced owing to the increased size of the atrial cavity ; 

 it is, however, readily recognisable as a narrow space im- 

 mediately surrounding the intestine and the liver. Further 

 forwards, in the region of the pharynx, the ccelom becomes much 

 subdivided, and more difficult to trace; its principal divisions 

 are a pair of dorsal ccelomic canals (Figs. 12 and 13, m), lying 

 at the sides of the dorsal part of the pharynx, between the body 

 walls and the suspensory folds of the pharynx. From the 

 dorsal ccelomic canals a series of tubular diverticula extend down 

 the outer sides of the primary gill-arches, as far as their ventral 

 ends. A series of spaces surrounding the reproductive organs 

 (Fig. 13, ov) are also parts of the ccelom. 



The large spaces, P, in the metapleural folds do not belong 

 to the ccelom, but are apparently lymphatic in nature. 



In the circulatory system the more important features are 

 the following. There is no heart, but the general course of the 

 circulation is the same as in a fish. A median longitudinal vessel, 

 the cardiac aorta or endostylar artery (Figs. 12 and 13, s), receives 

 venous blood from the body at its hinder end, and carries it 



