The proboscis is unarmed. The epidermis and connective tissue form one layer, below which is the basement membrane. The muscular layers are three in number, two circular separated by a longitudinal layer. The nerve-cords lie altogether external to the muscular layers, and are connected together throughout by a plexus. No nerve branches are given off. The brain is not divided into lobes. The cephalic slits are only represented by a shallow depression on each side of the head, and no canals have been observed leading from them. The intestine is straight, and the pouches are usually absent or rudimentary. The circulatory system is largely made up of lacunar spaces, the closed system being but little developed.
Principal British genera and species:—
Carinella annulata Mont., C. linearis (Mont., MS.) M‘Int., C. macintoshi Bürger (Fig. 59), C. polymorpha Ren.
Cephalothrix bioculata Oerst., C. linearis Rathke.
Valencinia lineformis M‘Int.
Fig. 59.—Carinella macintoshi Bürger, drawn from the living specimen, slightly contracted. Plymouth. Considerably magnified. a, Anterior end; b, posterior end.
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.