614 



IN VERTEBRA TE MORPHOL OOT. 



exterior by the atrial pore. Along the dorsal and ventral 

 mid-lines of the branchial region is a distinct ciliated groove, 

 the ventral one having projecting from its floor a longitudinal 

 ridge, while ventral to it is a chitinous skeletal plate composed 

 of paired moieties having a metameric arrangement. This 

 ventral or hypopharyngeal groove (Fig. 279, e) is termed the 

 eudostyle, and from its anterior end a band of ciliated cells 

 passes dorsally on each lateral wall of the pharynx to unite 

 dorsally with the epithelium of the dorsal or hyperpharyngeal 

 groove. 



From the digestive tract behind the branchial region a 

 diverticulum, termed the liver (Fig. 278, I) arises, and pro- 

 jects forwards, covered of course by the body-wall, into the 

 atrial cavity (Fig. 279, I), and behind this the intestine passes 

 straight back to open at the anus (Fig. 278, a), situated, as 

 already indicated, upon the left side of the body, some dis- 

 tance from the posterior end. 



The nervous system consists of a thick-walled tube (Figs. 

 278, and 279, r) which lies immediately above the notochord 

 and is enclosed by the connective tissue lamellae which arise 

 from the uotochord-sheath. It extends throughout the entire 

 length of the body, tapering rather suddenly at either ex- 

 tremity. Throughout the greater part of its course the lumen 



FIG. 281. DIAGRAM OF THE ANTERIOR PORTION OF THE .NERVOUS SYSTEM 



OF Ampliioxus (after HATSCHEKI. 



c7t = notochord. N= hypophysis. 



1, 2, 3 = are phiced over the three veutricles. 



is very small, forming the central canal from which a well- 

 marked cleft, the dorsal fissure, extends to the dorsal surface. 

 At the anterior end of the tube, however, the lumen enlarges 

 to form an anterior ventricle (Fig. 281, 1) which has been 

 compared with the anterior of the three primary vesicles of 

 the Vertebrate brain, and behind this the lumen contracts 



