STEUCTUEE OF THE ADULT ANDIAL. 41 



of the pharynx, the ^ stream being kept up by the ac tion of 

 columnar flagellate cells w hich clothe the gill-arches, and the 

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

 as it swills over them. 



T he water that has passed through the gill-slits escapes into 

 a l arge spac e, th e atrial or epipleuial cavity (Fig. 12, h) : this 

 lies between the pharynx and the body w all, and into it the 

 p harynx hangs freely , slung up to the body walls by suspensory 

 folds .( JJ'ig. i."^, 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. 



T he ccelom o r body cavity is quite distinct from the atrial 

 cavity, tti ouga its boundaries are not easy to follow . Jn th e 

 p osterior p art of the body, behind the atrial por e, the coelom is a 

 c avity of some width, surrounding the intestine an d separating 

 t his from th e l)ody wall ; in front of the atri al pore it becomes 

 g reatly reduc ed owing to the increased size of the atrial cavity ; 

 i t is. however, readily recognisable as a narrow space im- 

 m ediately surroundin g the intestine and the livei:. Further 

 forwards, in the regi on of the TJJiarynx. the cceJ om becomes muc h 

 su bdivided, and m ore ditlicult to tr ace ; i ts principal division s 

 a re a pair of dors al coelomiS'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 coelomic canals a series of tubular diverticula extend down 

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

 ends. A s eries of spaces surrounding the reproductive orga ns 

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



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

 to the coelom, but are apparently lymphatic in nature. 



In the circulatory svstem the more important features are 

 the following. JLhere 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 



