184 AMPHIOXUS. 



There have been found to be 61 myotomes on 

 each side of the body in a considerable number of 

 specimens. 



2. The ventral muscles form a sheet covering Ihe 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 expel the water from the 

 atrial cavity. 



3. Smaller muscle-bundles are found in relation with the. 



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

 the anus and atrial pore. 



Nearly all the muscles are striated. 



D. The Digestive and Eespiratory Systems. 



The alimentary canal is a nearly straight tube, the anterior 

 part of which is so modified as to allow of the escape of water 

 taken in at the mouth with the food, and to utilise this water 

 for respiration, as is the case in aquatic vertebrates generally. 



1. The buccal cavity or oral vestibule is bounded laterally 



by folds, within which and near their free margins 

 ar« the curved bars which support the cirri. 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. 180). 



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 a circle of twelve backwardly 



