Salpa

Salpa, a remarkable genus of free-swimming Tunicates, included along with Doliolum and Achinia in the order Thaliacea. Several species occur in the warmer seas, transparent pelagic animals, complex in structure and in life-history. The body is somewhat barrel-shaped, open at both ends, ringed round by several distinct but incomplete hoops of muscle, controlled by a complex nervous system, possessed of eyes and other less definite sense-organs, with compressed lateral viscera. In the life-history there is an alternation of generations. There are asexual forms or 'nurses,' from which there grows out a long ventral 'stolon.' This stolon is segmented into a chain of sexual buds, and the whole chain is set free. As the individuals become mature they separate from one another, the chain breaks into its links. Each of these produces an ovum, which after fertilisation develops into an embryo and into the asexual 'nurse' form with which we began. The sexual forms are hermaphrodite, like all Tunicata, but cross-fertilisation seems to occur.