^QQ THE LANCELET. PHYLUM CHORDATA 



mouth and its associated feeding apparatus are on the ventral 

 surface a Httle way back from the anterior end, and the anus, also 

 ventral, and a little to the left, is a slightly greater distance forward 

 from the posterior end. The exterior of the body beyond the anus, 

 containing no part of the gut, is called the tail. A low dorsal fin 

 runs along the middle of the back from end to end, becoming 

 deeper at the hinder end as the upper lobe of a caudal fin, which 

 passes round the end of the tail. The under lobe of this is con- 

 tinuous with a low, median ventral fin which extends along the 

 hinder third of the body. In front of the ventral fin the belly is 

 flattened and bears at each side a continuous lateral fin or meta- 

 pleural fold. At the narrow front end (rostrum) the dorsal fin 

 passes round the tip of the body and runs back below to become 

 continuous with the right side of the oral hood (see below). The 

 sides of the body are marked by a series of about sixty V-shaped 

 lines, with their apices forwards, which are septa of connective 

 tissue known as myocommata, which divide the muscles of the 

 body-wall into segments called myomeres. Certain of the internal 

 organs are repeated in correspondence with these, so that the 

 body is segmented, though not so completely as that of the earth- 

 worm. The segmentation is peculiar in that the myomeres of 

 opposite sides alternate. About seven myomeres lie in front of the 

 mouth. At the end of the fiat region of the belly is a mid- ventral 

 opening known as the atriopore, through which a current of 

 water escapes. 



ATRIUM 



The atriopore leads from a large cavity (best seen in a transverse 

 section. Fig. 226) which lies below and at the sides of the middle 

 part of the body and is known as the atrium. This cavity is not 

 really within the body, but is enclosed between the body and 

 two longitudinal folds of the body-wall, the metapleural folds, 

 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. The atrium is prolonged backwards on the right side 

 behind the atriopore almost as far as the anus. 



