A most important monograph by Bürger[143] on Nemertines has just been published, but unfortunately it appeared too late to be adequately considered here. He gives an elaborate account, illustrated by admirable figures, of the present state of our knowledge of this group, and his work will be indispensable to future students of the subject. The older systems of classification are criticised, and the following scheme is adopted in their place:— ORDER I. PROTONEMERTINI (= part of the Palaeonemertea, e.g. _Carinella_).—The brain and lateral nerve-cords lie outside the muscle layers in the epithelium or below the basement membrane. The body-wall consists of the following layers: epidermis, basement membrane, circular muscles, and longitudinal muscles. The mouth lies behind the brain. The proboscis is unarmed. ORDER II. MESONEMERTINI (= part of the Palaeonemertea, e.g. _Cephalothrix_).—The characters of this Order are similar to those of the Protonemertini except that the brain and lateral nerve-cords lie in the muscle layers. ORDER III. METANEMERTINI (= Hoplonemertea).—The brain and lateral nerve-cords lie in the parenchyma of the body internal to the muscle layers. The layers of the body-wall are {113}similar to those of the Protonemertini. The mouth lies in front of the brain. The proboscis is armed. At the junction of the fore- and mid-gut a diverticulum is given off which projects forwards beneath the fore-gut and ends blindly in front. ORDER IV. HETERONEMERTINI (= Schizonemertea, and the genera _Eupolia_ and _Valencinia_, placed provisionally by Hubrecht in the Palaeonemertea).—The body-wall consists of the following layers: epidermis, thick cutis, and an outer and an inner longitudinal muscle layer separated from one another by a circular muscle layer. The brain and lateral nerve-cords lie between the outer longitudinal and the circular muscle layers. The mouth lies behind the brain. The proboscis is unarmed. DEVELOPMENT OF THE NEMERTINEA.—The development of the Palaeonemertea is at present not known: in the Schizonemertea a larval stage occurs; while in the Hoplonemertea the egg develops directly without undergoing any metamorphosis. There are two forms of larva characteristic of the Schizonemertea, known respectively as Pilidium and the Type of Desor. The Pilidium is hatched early and leads a free-swimming existence, whereas the Type of Desor, though in many respects resembling it, never passes through the free-swimming phase. [Illustration: FIG. 60.—Diagram of a Pilidium larva. (After Salensky.) _c_, Tuft of cilia; _m_, muscle-fibres; _mo_, mouth, seen through one of the lateral lobes; _n_, nerve-fibres; _n.r_, nerve-ring; _n.g_, nerve ganglion; _oes_, oesophagus; _st_, stomach.]