446 SUB-PHYLUM UROCHORDA OR TUNICATA, 
single layer of ectoderm cells, which secrete the test. 
Beneath these there lies a gelatinous matrix containing 
numerous connective tissue cells, blood-carrying spaces, 
muscle cells forming slender fibres, and so on. 
A true coelom has been described in some embryos, but 
it is afterwards almost suppressed, being represented at 
most by the pericardium and small lacunar spaces. The 
apparent body cavity of the Ascidian—the space between 
gut and body wall—is, as we shall see, lined throughout by 
ectoderm. 
The muscular system is not well developed. The muscle 
cells are much elongated and unstriped ; they are aggregated 
into fibres of varying thickness, which form an irregular net- 
work on the right side of the body, while they are virtually 
absent on the left. Special sets of fibres form sphincters 
round the apertures. 
Alimentary and respiratory systems.—The mouth opens 
into a short stomodzum, separated from the branchial 
sac itself by a sphincter muscle, whose posterior border 
is furnished with numerous simple elongated tentacles. 
Behind this lies a ciliated peripharyngeal groove. In the 
living animal the tentacles form a sort of sieve over the 
opening of the branchial sac. This sac is morphologically 
the pharynx, and extends almost to the posterior end of the 
body. It is separated from the mantle by a space whose 
dimensions vary greatly in the different regions of the body. 
This space is the peribranchial chamber, which is formed 
from the ectoderm, and communicates with the exterior by 
the atrial opening, and with the branchial sac by innumer- 
able slits). The remainder of the alimentary canal lies on 
the left side of the body, between pharynx and mantle, and 
consists of a short cesophagus leading from the pharynx to 
the fusiform stomach, and of an intestine which describes 
an S-shaped curve, and then crosses the atrial chamber, to 
end in an anus iying beneath the exhalant opening. The 
absorbing surface of the intestine is increased by a marked 
infolding, corresponding to the typhlosole of the earthworm. 
A mass of tubules connected by a duct with the cavity of 
the stomach is possibly a digestive gland. 
The structure of the pharynx is exceedingly complex, for 
it has a double function—respiratory and nutritive. More- 
