52 



ORIGIN OF CHORDATES 



and shortening. It is possible that the diverticulum and plate found 

 in the collar represent the remains of a notochord, serving as a 

 fixed point by which the body is drawn forward on to the proboscis. 

 However, many prefer to call it a 'stomochord' to avoid too close a 



card. 



Fig. 24. Diagrammatic section of front end of Balanoglossus. 

 c. collar coelom ; card.s. sac around heart ; div. pharyngeal diverticulum ('stomochord') ; dn 

 dorsal nerve-root ; dv. dorsal vessel ; gl. glomerulus ; £s. gill-slit ; Im. longitudinal muscles of 

 proboscis ; n.c. nerve-cord ; p.p. proboscis pore ; sk. skeletal plate. (Modified after Spengel.) 



comparison with the notochord. The external cilia probably play a 

 considerable part in locomotion; possibly they are the chief burrowing 

 organs, the muscles serving mainly to perform escape movements. 



The mouth lies in a groove between the proboscis and collar (Fig. 

 25). The proboscis contains many mucus-secreting cells and the food 

 particles are captured on its surface and conveyed by ciliary currents 

 to the mouth. In the anterior part of the trunk there is a wide pharynx, 

 opening by a series of gill-slits (Figs. 24, 26). These resemble the 

 gills of amphioxus in the presence of a supporting skeleton in the gill 

 bars; there are also tongue bars dividing the slits from above, and 



