In the suborder Luciæ, including the family Pyrosomidæ, the colonies are thimble-shaped and hollow, the incurrent openings being on the outer surface of the thimble, the outgoing stream opening within. Pyrosoma is highly phosphorescent. In the tropical seas some colonies reach a length of two or three feet. It is said that a description of a colony was once written by a naturalist on a page illumined by the colony's own light. "Each of the individuals has a number of cells near the mouth the function of which is to produce the light."
Thaliacea.—In the order Thaliacea the Tunicates have the two orifices at opposite ends of the body. All are free-swimming and perfectly transparent. The principal family is that of Salpidæ. The gill-cavity in Salpa is much altered, the gills projecting into it dividing it into two chambers.
In these forms we have the phenomena of alternation of generations. A sexual female produces eggs, and from each hatches a tadpole larva which is without sex. This gives rise to buds, some at least of the individuals arising which in turn produce eggs.
Fig. 285.—Botryllus magnus Ritter. Part of colony. (After Ritter.)
In the family Salpidæ two kinds of individuals occur, the solitary salpa, or female, and the chain salpa, or bisexual males. The latter are united together in long bands, each individual forming a link in the chain held together by spurs extending from one to the next. From each solitary individual a long process or cord grows out, this dividing to form the chain. Each chain salpa produces male reproductive organs and each develops as well a single egg. The egg is developed within the body attached by a sort of placenta, while the spermatozoa are cast into the sea to fertilize other eggs. From each egg develops the solitary salpa and from her buds the chain of bisexual creatures. Dr. W. K. Brooks regards these as nursing males, the real source of the egg being perhaps the solitary female. Of this extraordinary arrangement the naturalist-poet Chamisso, who first described it, said: "A salpa mother is not like its daughter or its own mother, but resembles its sister, its granddaughter, and its grandmother." But it is misleading to apply such terms taken from the individualized human relationship to the singular communal system developed by these ultra-degenerate and strangely specialized Chordates.
Fig. 286.—Botryllus magnus Ritter, a single Zooid. Shumagin Islands, Alaska. (After Ritter.)