TUNIOATA. 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 sphincter 

 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 Sa^ja 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 



Fio-. 21. 



Posterior part of solitary form of Salpa democratica-mucronata, showing a chain 



of embryos nearly ready to be set free. 



gem, young chain of Salpx ; 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 



