148 SEA-SQUIRTS. 
rigging of vessels, as if by magic. Bebra, the naturalist, wrote a de- 
scription of one in his cabin by its own light, and read by their phos- 
phorescence from his cabin-window. Professor Moseley wrote his 
name upon one with his finger as it lay upon deck; each letter 
seemed to ignite, increasing in brilliancy, spreading over the entire 
animal until it seemed at a white heat. They are found in nearly all 
the southern seas, and are important factors in the illumination of 
the submarine world. 
Salpz.—These are minute pelagic, free-swimming as- 
cidians. There are solitary and chain See ; the former 
(Fig. 187) are barrel-shaped, even the hoops being repre- 
Fic. 187.—Dolio/um, an ascidian allied to the salpa. a, nerve; ¢, mouth; 
£, esophagus; z, stomach; /, intestine; 7, heart; ¢ ¢, muscles. 
sented by circular bands or muscles. In both, the open- 
ings are at each end, and locomotion is produced by a 
regular contraction and expansion of the mantle. The 
salpa in its development affords another example of alter- 
nate generations. The solitary asexual salpa, by bud- 
ding, produces a family of chained salpz; these in turn 
produce, not buds, but fertilized eggs, that, after pass- 
ing through several changes, assume the solitary asexual 
salpa form. 
Notr.—They are brilliantly luminous, the chains stretching away 
for great distances, resembling luminous snakes winding their way over 
the sea. Their light in the darkest night rivals that of the moon upon 
the water. 
