STRUCTURE OF SALPA. . 399 
system, which is situated ventrally between the mouth and 
vent. We thus have in these Tunicates a front and hind 
end of the body, a dorsal and ventral, as well as a distinct 
bilateral symmetry of the body. This is seen in Appendi- 
cularia as well as in Doliolum and Salpa, however much 
this symmetry may be obscured in the more typical Ascidi- 
ans, such as Ascidia, Molgula, Boltenia, etc. 
The oral aperture leading into the respiratory sac is large, 
being as wide as the body ; the respiratory sac is more com- 
plicated than in other Ascidians, and more so than in Doli- 
olum, where it is a wide, deep passage, the cesophagus at the 
hinder end, the sac itself perforated by two rows of bran- 
chial slits, four or five slits in each row. In Salpa, how- 
ever, the respiratory sac, as described by Brooks, is attached 
to the outer tunic, around the edges of the mouth, as in 
other Tunicates. There are only two branchial slits, one on 
each side; these are very large, and cover almost the whole 
surface of the branchial sac, except the median dorsal and 
heemal lines. On the neural side the branchial slit opens 
directly into the atrium, the ciliated line where the two 
tunics ynite being marked by the so-called “gill” (Brooks). 
In Salpa, according to Brooks, the branchial sac, though 
ciliated within, is not so directly concerned in the respiratory 
act asin other Tunicates, since respiration is effected largely 
by the action of the muscles, which also assist deglutition, 
and are the organs of locomotion. These contract rythmi- 
cally, with great regularity, and at each contraction the 
water is expelled from the branchial sac through the atrial 
aperture ; and when the muscles are relaxed, the elasticity 
of the test distends the chamber, and a fresh supply is drawn 
in through the branchial aperture, the lips of which readily 
admit its passage in this direction, while a similar set of 
valves allows its passage out of the atrial aperture, but pre- 
vents its return.” Thus a chain of individuals move with a 
uniform motion, while the solitary individuals and those 
which have been set free by the breaking up of a chain, move 
by jerks. 
The digestive canal is small, curved on itself, the cesopha- 
gus leading from the bottom of the pharyngeal or respiratory 
