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 (ygochord) (and in its allies a pair of 
pharyngeal diverticula, called plewrochords) are of this nature. ence 
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, 7.¢., 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 7wzdcata. 
The Chordata fall very naturally into sub-phyla, Azriozoa 
and Vertebrata. 
SUB-PHYLUM I.—ATRIOZOA. 
The AZviozoa 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 a¢rium 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 chorduda larva. 
The sub-phylum is entirely marine and mainly pelagic 
or sedentary. 
It contains two classes—r1. Tunicata (UROCHORDA) ; 
2, CEPHALOCHORDA. 
Cuiass I.—TuNIcATA. 
Ascidia was the type of this class and is representative of 
the simple sedentary Zunicata. 
