The sexually-mature form lives in the carp, Cyprinus carpio; the larval form is found, according to Villot,[[221]] encysted in the fat bodies of the larva of Sialis lutaria, one of the Neuroptera, and in the alimentary canal of the leech Nephelis octocula, and successful experiments have been made in infecting some species of the water snail Limnaea. N. agilis occurs in Mugil auratus and M. cephalus.
Family IV. Arhynchidae.—Short forms with the body divided into three well-marked regions—head, collar, and trunk. The head is pitted, the collar smooth, and the trunk wrinkled, not annulated, in spirit specimens. There is no eversible introvert, and no introvert sheath and no hooks. The sub-cuticle and the lemnisci have a few giant nuclei, and the lemnisci are long and coiled.[[222]]
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.
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.