As an example of this division of Latreille we may give the Nebalia tripes, which includes the two species N. ylabra and N. ciliate of Lamarck. This creature has an ovate body of a pale yellow colour, with a darker longitudinal line along each aide ; anternue long, the Inferior pair as long as the body, and aetiferous ; beak of carapace sharp-pointed and moveable ; four pairs of natatorial feet of moderato length, and setiferoua ; caudal appendages rather long, and furnished at the extremity with one long slender seta and three or four short nettle, not plumose. This species in a native of the sea, and has been found on the coast of Devonshire, on the coast of Ireland, and the Shetland Isles.
ff. One Eye.
Thorax divided into many segments. The anterior, and much the largest segment, presents a single eye only, placed in the middle of the front between the superior antennae. Cyclops (Nitillerl, which has been so well illustrated by the acute observations of the elder Jurino and of ltamdohr, is the only genus of this subdivision.
The body of the species of Cyclops is more or loss approaching to oval, soft or rather gelatinous, and is divided into two portions, the one anterior, consisting of the head and thorax, the other posterior, forming what is commonly called the tail. The segment immediately preceding the sexual organs, and which in the females carries two supporting appendages in the form of little feet (fulcra, Jurine), may be considered as the first segment of the tail, which is not always very clearly defined or strongly distinguished from the thorax, and consista of six segments or joints), the second of which in the males is provided on its lower side with two articulated appendages of varied form, sometimes simple, sometimes having a small division at the internal edge, and constituting entirely or in part the organs of generation. In the other sex the female organ is placed upon the same joint. The last segment terminates in two points forming a fork, and more or less bordered with delicate beards or penniform fringes. The anterior portion of the body is divided into four segments, of which the first and by far the largest includes the head and a portion of the thorax, which are thus covered by one scale common to both. Here aro situated the eye, four antennae, two mandibles (internal mandibles of Jurine) furnished with a feeler (which is either simple or divided into two articulated branches), two jaws (the external mandibles, or lip with little beards, of Jurine), and four feet, divided each into two cylindrical stems, fringed with hairs or bearded. The
anterior pair representing the second pair of jaws differ a little from the succeeding pair, and are compared by JurMe to a kind of hand. Each of the three succeeding segments nerves as the point of attach ment to a pair of feet The two superior antennae are longest, setaceoua, simple, and formed of a great number of small articulations. They facilitate by their action the motion of the body, and perform very nearly the office of feet. The lower antennae (antennulea of Jurine) are filiform, consisting most frequently of not more than four joints, and are sometimes simple, sometimes forked. By their rapid motion they produce a small eddy in the water. In the males the upper antennae, or one of them only, as in Cyclops Castor, are con tracted in parts, and exhibit a swelling portion which is followed by a hinge joint. By means of these organs, or of one of them, the males seize either the hind feet or the end of the tail of their females during the season of fecundation, and are thus often found attached. On each side of the tail of the females is an oval bag filled with eggs (external ovary of Jurine), adhering by a very fine pediele to the second segment, near ita junction with the third, and where the orifice of the deferent egg-canal may be seen. The pellicle which forme these bags is only a continua tion of that of the internal ovary. The number of contained eggs increases with age. They are at first brown or obscure, but afterwards present a reddish tinge and become nearly transparent, without how ever increasing in size when the young are about to come forth. When isolated or detached, up to a certain period at least, the germ perishes. A single fecundation suffices for successive generations, and the same female can lay eggs ten times in the course of three months, so that the number of births amounts to something enormous. Thus, taking eight ovipositione and allowing forty eggs for each, it has been calcu lated that one female Cyclops may be the progenitress of four thousand five hundred millions. The time the foetus remains in the ovary varies from two to ten days, the variation depending on the temperature of the seasons and on other circumstances.