14 PHYLUM CEPHALOOHORDA. 



swelling, immediately over the hind end of the notochord ; in 

 front it tapers very slightly, and possesses a somewhat blunt 

 termination placed some little distance behind the front end of 

 the notochord. On the ventral side of the notochord is the 

 alimentary canal, which has the form of a straight tube extending 

 between the mouth and the anus. The central nervous system 

 therefore, lies entirely dorsal to and the alimentary canal 

 entirely ventral to the notochord. 



The alimentary canal consists of three parts : — 



1. The buccal cavity. This is a short chamber opening to 

 the exterior by the mouth and behind by a somewhat constricted 

 opening into the pharynx. 



2. The pharynx is the widest and longest portion of the 

 alimentary canal, extending nearly half the length of the body. 

 Its walls are perforated on each side by a number of obliquely 

 directed slits (from above and in front ventralwards and back- 

 wards) which place its cavity in communication with a space 

 lying immediately outside it, and called the atrial or peripharyn- 

 geal chamber. The atrial chamber entirely surrounds the 

 pharynx except along the dorsal middle line (vide Fig. 10 and 

 explanation). It opens to the exterior by the atrial pore — 

 already mentioned — which is found at the hind end of the ventral 

 groove (Fig. 5). The pharynx is mainly a respiratory organ, 

 inasmuch as the blood which circulates in its walls and in the 

 walls of the atrial cavity is aerated by the water which is con- 

 tinually being taken in by the mouth and driven by the action 

 of cilia through the pharyngeal slits — or gill slits as they 

 may be called — into the atrial cavity. 



3. The intestine which extends as a straight tube from the 

 hind end of the pharynx to the anus. The anterior part of the 

 intestine is slightly dilated and receives ventrally a simple 

 caecalsac, which, pushmg the body wall before it, extends for- 

 wards in the atrial cavity on the right side of the pharynx and 

 is called the liver. 



Detailed Description of the Organs. 



The ectoderm consists of a single layer of columnar or in some 



places cubical cells, which cover the whole external surface of 



the animal, are prolonged for a short distance into the buccal 



cavity and line the whole of the atrial cavity (Fig. 10). They 



