With only a few exceptions Nemertines are oviparous. _Prosorhochmus claparedii_, _Tetrastemma obscurum_, and _Monopora vivipara_ have been observed to contain embryos at certain times of the year. In other forms the eggs are laid when ripe, and development takes place subsequently to their deposition. GEOGRAPHICAL DISTRIBUTION.—Nemertines have been found in all seas from the arctic to the equatorial regions. Many forms are found in the British Isles both between tide-marks and also at greater depths around our coasts. Some genera seem to be confined to warm climates and others to cold; while others appear to be indifferent to climate, and to subsist equally well under very various degrees of temperature. So far as is known, the land forms are all indigenous to warm countries. LAND FORMS.—Land forms, which occur on or in moist earth under stones or decaying vegetable matter, have been discovered and described by Semper,[146] Willemoes-Suhm,[146] and von Graff.[146] The species found by Semper, and called by him _Geonemertes palaensis_, lives under damp leaves and the roots of trees on Pelew Island in the North Pacific. It is about 2 inches long, of a reddish-white colour, with narrow, brownish-black, longitudinal stripes on its dorsal surface. It possesses six eyes and very small cephalic slits and cerebral organs. The proboscis is armed, and opens by the mouth instead of by a special pore. The same peculiarity as to the opening of the proboscis is found in _Geonemertes chalicophora_, discovered by von Graff in pots of _Corypha australis_ in the palm-house at Frankfurt-on-Main. He found specimens on and beneath the surface of the earth. As it was only found in pots in which this Australian plant was growing, von Graff thought it almost certain that it was a native of Australia. Those found below the surface of the earth were surrounded by a transparent tube in which particles of earth were embedded. The animal is small, only about two-fifths of an inch in length. The colour is milk-white, with a small quantity of red pigment anteriorly: there are four eyes, and the cephalic slits are absent. {118}The species which was discovered by Willemoes-Suhm, and named by him _Tetrastemma agricola_, lives under stones in damp earth in the Bermudas. It differs from the other two in that the proboscis opens by a special terminal aperture. It measures nearly an inch and a half in length, and, like _G. chalicophora_, is milk-white in colour. It resembles it also in possessing four eyes, and in the absence of cerebral organs and cephalic slits. FRESH-WATER FORMS.—In most cases the descriptions of fresh-water forms are so vague and incomplete that it is difficult to determine whether or not they are different species. They are probably more numerous than is at present known, and are certainly scattered widely over the face of the earth, since they have been found in Nicaragua, at Tashkend in Turkestan, and at Philadelphia and Monroe in the United States.