STRUCTURE OF AMPHIOXUS 52I 



Tunicates, connected both with respiration and nutrition. 

 The food particles, entangled in mucus, are said to pass 

 backwards along the hyperpharyngeal groove ; the water 

 passes down the pharynx, through its numerous gill-slits 

 to the atrial chamber, and so to the exterior by the single 

 atriopore. In the larva the gill-slits are few in number, and 

 open directly to the exterior ; in the adult they are con- 

 cealed by the atrial chamber, and have greatly increased in 

 number ; there may be more than 100 pairs. The water- 

 currents are kept up by the cilia, probably assisted by the 

 transverse muscles. 



The first sign of the atrial chamber is the appearance of two lateral 

 folds on the body wall, which form the metapleural folds of the adult. 

 On their inner apposed, but not united, surfaces, two ridges appear 

 (Fig. 297, I.). These grow towards one another and unite, leaving 

 only the atriopore open. Thus the floor of the atrial chamber (Fig. 297, 

 II.) is produced. The chamber, as first formed, is a tube with a very 

 small lumen, but, secondarily, it becomes enlarged, and extending 

 upwards and inwards, constricts the coelom, until it comes almost to 

 siuTound the gut. The atrium eventually becomes a cavity, crescent- 

 shaped in cross-section, surrounding the pharynx and extending back- 

 wards as a blind pouch on the right side of the intestine. At the same 

 time, the metapleural folds increase in size until they assume the adult 

 appearance (Fig. 297, III.). During these processes the originally few 

 gill-shts have been increasing in number, both by the addition of new 

 slits and by the division of those first formed. The division is effected 

 by the downward growth of a secondary bar or tongue-bar in the 

 middle of each slit. The primary bars differ from these tongue-bars 

 in being split at their lower ends, in enclosing a coelomic space, and 

 in some other respects. 



The pharynx opens into the intestinal region of the gut, 

 which is straight and simple. Near its commencement a 

 pouch-like " liver " or caecum (Fig. 296, C) arises, and 

 extends forwards on the right side of the pharynx. The 

 anus is some distance from the end of the body (cf . Fishes) ; 

 in the larva it is close to the caudal fin. 



Body cavity. — This can only be understood when its development 

 is studied (see Fig. 297). It is a fine example of what is called the 

 enteroccelic mode of origin. From the archenteron of the embryo a 

 hollow ridge grows out on each side, and becomes almost at once 

 segmented into a series of small sacs. These lie one behind the other, 

 and lose all connection with the gut. Each ultimately divides into two 

 — a dorsal thick-walled portion, and a ventral thin-walled portion. 

 The dorsal portions form the body musculature, and retain their 

 segmentation. Their cavity, the myocoel, persists to some extent in 



