Sagitta, or ARROW-WORM, a genus of small pelagic worms, included along with one other genus—Spadella—in the class Chaetognatha. The arrow-worms occur in all seas; they often measure about an inch in length, and are quite translucent. The animal is divided by two partitions into three regions, head, body, and tail. In the mouth there are sickle-shaped bristles or 'jaws,' to which the term Chaetognatha refers. There are two pairs of thin lateral 'fins,' and the tail is similarly fringed. The animals are hermaphrodite, the paired ovaries and testes developing from two cells which appear at a very early stage in the history of the embryo. Each cell divides into two—an ovarian and a testicular rudiment. The development is very regular; the anatomy of the adult is in many ways unique and of much zoological interest.
See Chaetognathi (1883), by Grassi, in the monographs on the Fauna of the Gulf of Naples; 'Die Chaetognathen' by P. Hertwig, in Senaische Zeitschrift. f. Naturwiss. xiv. (1880).