ATRIOZOA. 403 



also present in another member of the class. In addition, there is a 

 dorsal nervous system, partially tubular, but there is no brain, and the 

 whole nervous system is still in structural continuity with the epiblast (or 

 ectoderm). Lastly, there are certain portions of the endoderm (or 

 hypoblast), the epithelial cells of which undergo a modification into 

 chordoid tissue histologically similar to that of the notochord. 



In Balanoglossus, a pre-oral part called the stomochord (the "noto- 

 chord " of some writers), the whole anterior wall of the pharynx, and 

 an area in the intestine ( pygochord ) (and in its allies a pair of 

 pharyngeal diverticula, called pleiirochords) are of this nature. Hence 

 the Archichorda resemble the true Chordata in having pharyngeal 

 clefts, a dorsally-situated though simpler nervous system, and incipient 

 chordoid structures. 



In the other two features they differ from the Chordata^ i.e., they 

 have no true metameric segmentation and no ventral heart. The 

 circulation is usually forwards dorsally, but one member of the 

 Archichorda has a reversible circulation like the Tunicata. 



The Chordata fall very naturally into sub-phyla, Atriozoa 

 and Vertebrata. * 



SUB-PHYLUM I.— ATRIOZOA. 



The Atriozoa are more lowly organised than the Verte- 

 brata. The pharyngeal clefts are multiplied and the pharynx 

 is specialised into a huge sac (or sieve) for obtaining food, 

 with a complex apparatus of dorsal and ventral grooves and 

 gland-cells. The water separated from the food-particles 

 passes into a spacious atrium which arises from the epiblast. 

 (Hence the name of the group). The notochord is never 

 replaced by any other axial skeleton, and at most is sur- 

 rounded by a membranous sheath. The brain has only a 

 single internal cavity or vesicle, and the eye is single and of 

 simple structure. 



The development is external to the parent, purely larval 

 (except for the very earliest stages), and there is a gastrula 

 larva followed by the chordula larva. 



The sub-phylum is entirely marine and mainly pelagic 

 or sedentary. 



It contains two classes — i. Tunicata (Urochorda); 

 2. Cephalochorda. 



Class I. — Tunicata. 



Ascidia was the type of this class and is representative of 

 the simple sedentary Tunicata. 



