TUNICATA. 101 
which occur in abundance at the ocean surface. They are so 
transparent that they are rarely seen, except in calm weather 
from the side of small boats ; yet they frequently swarm in 
countless multitudes. From five to ten bands of muscles partially 
or entirely surround the body, like hoops. The branchial and 
atrial openings are at or near the opposite ends of the body. 
The branchial sac has almost disappeared, the dorsal lamina and 
ventral gutter (or endostyle) alone remaining, the interval be- 
tween the two on each side representing an enormous stigma ; the 
dorsal lamina, or " gill " is the transversely striated band passing 
obliquely across the body and forming the only barrier between the 
branchial and atrial cavities. Water enters at the mouth, and, by 
the contraction of the muscle-hoops, is driven out through the atrial 
aperture at the opposite end, which is then closed by a sphinctei 
muscle. The elastic walls of the body expand, and water again 
enters through the mouth, the valve-like lips of which prevent its 
being driven out that way. The Salpa swims along in jerks, and 
along with each gulp of water takes in Radiolaria, Foraminifera, etc., 
which are retained by the mucus of the endostyle and carried to the 
gullet. The Salpa, in fact, lives, as Professor Brooks observes, in a 
" living broth," so abundant is the food supply. 
The intestines usually form an oval mass termed the " nucleus," 
which is a conspicuous object at the posterior end. 
The solitary Salpa above described is asexual. In the ventral 
Fig. 21. 
~gen\ 
Posterior part of solitary form of Salpa democratica-mucronata, showing a chain 
of embryos nearly ready to be set free. 
gem, young chain of Salpse ; st, stolon ; t, test ; vise, visceral mass. 
region of its body it forms a stolon which becomes segmented into a 
series of buds (Fig. 21). As the stolon grows the end series of buds 
