UI STRUCTURE 67 
ganglion. No neural gland is found. The branchial aperture or 
mouth leads into the simple branchial sac or pharynx (Fig. 30, 
br.s). There are no tentacles. The endostyle is short, is a closed 
tube both anteriorly and posteriorly (Fig. 29),and has about four 
longitudinal rows of gland-cells. There is no dorsal lamina, 
and the peripharyngeal bands run dorsally and posteriorly to 
unite close in front of the oesophageal opening. The wall of the 
branchial sac does not show the complex structure usual in Tuni- 
cata, and has only two ciliated apertures (Figs. 30, 31, 32, sq). 
These are homologous with the primary stigmata of the typical 
Ascidians, and with a pair of the gill-clefts of Vertebrates. 
They are placed far 
back on the ventral 
surface, one on each 
side of the middle 
line, and lead into 
short funnel-shaped 
tubes which open on 
the surface of the 
body behind the 
anus (Fic 30, az). 
These tubes corre- 
spond to the right 
and left atrial in- 
Fie. 29.—Transverse section through anterior part of Oiko- 
pleura to show ganglion, sense-organs, endostyle, etc. 
volutions, which in x 300. br.s, Branchial sac; ¢,f, ciliated funnel ; ec, 
an ordinary Asci- dorsal SORES 6 end, closed anterior end of endo- 
3 5 style ; hy, hypobranchial groove in floor of branchial 
dian fuse to form sac ; 7.g, nerve-ganglion ; o7.g/, oral gland ; of, otocyst ; 
the peribranchial x, opening of ciliated funnel into pharynx. 
cavity. The remainder of the alimentary canal consists of 
oesophagus, stomach (which may have a glandular diverticulum), 
intestine and rectum (Fig. 30). The heart, surrounded ventrally 
by a delicate pericardial membrane, lies below and in front of the 
stomach, and is formed by the differentiation of the outer ends 
of epithelial cells into muscular fibrillae. Two specially large 
glandular cells are placed at the opposite ends of the heart. 
There are no blood-vessels except the remains of the primary 
body-cavity (blastocoel). No heart can be seen in some of the 
smaller species of Oikopleura. Nearly all the species are herma- 
phrodite, and the large ovary and testis are placed at the posterior 
end of the body. There is no proper oviduct, the genital pro- 
