DIGESTIVE SYSTEM 175 



on opposite sides of the body, aided by the elasticity 

 of the notoehord, that the lateral undulatory move- 

 ments of the animal when swimming are produced. 



2. The ventral muscles form a sheet covering the ventral 



surface of the body from the mouth to the atrial pore. 

 The muscle-fibres run transversely from side to side, 

 and by their contraction aid in expelling the water 

 frora the atrial cavity. 



3. 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 buccgljjavity 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 defined sinuous 

 line (fig. 40, p. 172). 



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 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. 



