This family resembles the Gigantorhynchidae in the length and curvature of its lemnisci, and the Neorhynchidae in the persistence of the embryonic condition of the nuclei in the sub-cuticle and the lemnisci; but in the shape of the body, its division into three well-marked regions, the absence of eversible proboscis, proboscis sheath, and hooks it stands alone, though it is nearer to the Neorhynchidae than to either of the other families. The single species _Arhynchus hemignathi_ was found attached to the skin around the anus of a Sandwich Island bird, _Hemignathus proceros_. The bird is a member of a family Drepanididae, which is entirely confined to the Sandwich Island group. Professor Newton tells me that it is probable that the "food of _Hemignathus_ consists entirely of insects which it finds in or under the bark of trees," hence it is probable that the second host of this parasite, if such exists, must be looked for amongst the Insecta. {186}CHAPTER VII CHAETOGNATHA STRUCTURE—REPRODUCTION—HABITS—FOOD—CLASSIFICATION TABLE OF IDENTIFICATION At certain seasons and at certain times of the day the naturalist who is investigating the fauna of the surface of the sea is apt to find his tow-net crammed with innumerable transparent spindle-shaped animals, which by their number and the way in which they become entangled with rarer objects, often render useless the result of his labours. These animals belong to the class Chaetognatha, which includes three genera, _Sagitta_, _Spadella_, and _Krohnia_. Amongst them are divided about twenty species, some of which, however, are of doubtful value. ANATOMY.—The body of these animals is as transparent as crystal; it is elongated, and bears a resemblance to certain torpedos, except that the head forms a somewhat blunt termination to the spindle-shaped body. The tail bears a caudal fin, and _Spadella_ and _Krohnia_ have a single pair, and _Sagitta_ two pairs, of lateral fins; all of which are flattened horizontally. The body is externally divisible into three regions—head, trunk, and tail—and these correspond with the arrangement of the internal organs. [Illustration: FIG. 101.—_Sagitta bipunctata._ _a_, Vesicula seminalis. × 4. (After Hertwig.)] The head is surrounded by a fold of skin, forming a hood, {187}which is most prominent at the sides (Fig. 102, _g_); within the hood the head bears from two to four rows of short spines, and outside these a right and left row of sickle-shaped hooks, the free ends of which in a state of rest converge round the mouth, but when disturbed these hooks can be widely divaricated.