192 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 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 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 Respiratory 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 

 are 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. This epithelium forms 

 the * wheel-organ,' by which water and floating 

 food-material are wafted into the pharynx. 



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 

 projecting tentacles. 



