DIGESTIVE SYSTEM 173 



8. 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. 



C. The Digestive and Respiratory Systems. 



The alimentary canal is a nearly straight tube, the anterior 

 part of which is modified for respiration, 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 flagella. 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 denned sinuous 

 line. /\ *i- 



. 2. Thejvejum 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 hinder 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 slit -like apertures the gill-slits which 



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