DIGESTIVE SYSTEM 173 



Smaller muscle-bundles are found in relation with the 

 mouth and its tentacles, with the gill-apparatus, and 

 with the anus and atrial pore. Nearly all the muscles 

 are striated. 



G. The Digestive and Bespiratory Systems. 



The alimentary canal is a nearly straight tube, the anterior 

 part of which is modified for respiratioii, as in aquatic verte- 

 brates generally. 



1. The buccal cavity is bounded laterally by folds, which 



are anterior prolongations of the atrial folds : within 

 these and near their free margins are the curved bars 

 which support the tentacles. There are no jaws. 



The anterior part of the buccal cavity is lined by 

 a single layer of short columnar epithelial cells, some 

 of which bear short flageUa. In the hinder part of 

 the cavity the epithelium is altogether' different, 

 the cells being very long and slender, and provided 

 with long flagella. The boundary between these 

 two regions is marked by a sharply defined sinuous 

 line. 



2. The velum is a muscular diaphragm between the buccal 



cavity and the pharynx, opposite the anterior angle 

 of the seventh myotome. It is perforated below its 

 middle by an aperture which leads upwards and 

 backwards into the pharynx, and the hinde;r border 

 of which is fringed with cirri. Below this oral aper- 

 ture are a pair of small ' hyoidean apertures ' which 

 also connect the buccal cavity with the pharynx. 



3. The pharynx is a wide sac, forming about half the length 



of the alimentary canal. It is attached along its 

 mid-dorsal line to the under surface of the sheath of 

 the notochord, from which it hangs down freely' into 

 the atrial cavity. Its sides are perforated by a large 

 number of sht-like apertures — the gill-slits — which 



