328 MANUAL OF ELEMENTARY ZOOLOGY 



mouth is below the pointed front end in a vestibule sur- 

 rounded by an oral hood, the edge of which is beset with 

 slender, ciliated tentacles or cirri. It opens through a 

 muscular partition at the hinder side of the vestibule, 

 known as the velum. The opening is bordered with about 

 a dozen velar tentacles, which filter out coarse particles from 

 the food. On the inside of the hood a lobed tract of 

 epithelium, which bears long cilia and is known as the wheel 

 organ, encircles the vestibule just in front of the velum. 



The atriopore leads from a large space which lies below 

 and at the sides of the middle part of the body and 



-* inch. 

 ;»•<•■• , ijmyc. A , nym ,d.f.r. 



Fig. 236. — A view from the left side of the region around the atriopore 

 of a specimen of Amphioxus with the atrial floor expanded. 

 Lettering as in Figs. 233-235. 



is known as the atrium. This space is not really within 

 the body, but is enclosed between the body 

 and two longitudinal folds of the body-wall, like 

 those which form the branchiostegites of the crayfish, 

 save that they meet in the middle line below, leaving at 

 their hinder end an opening which is the atriopore. The 

 atrium communicates with the pharynx by a number of 

 slits at each side, known as the gill slits, separated by narrow 

 gill bars, and the current of water which is passed into the 

 mouth by the cilia around it is caused by cilia on the gill 

 bars to flow through the slits into the atrium, and so out 

 at the atriopore. The atrium is prolonged backwards on the 

 right sideibehind the atriopore almost as far as the anus. 



