_Malacobdella grossa_ O. F. Müll. {111}ORDER II. SCHIZONEMERTEA. The proboscis is unarmed. The epidermis is separated from the layer of connective tissue by a thin basement membrane, hence the glands in the connective tissue are more deeply situated and have long ducts. The muscular layers are three in number, an outer and an inner longitudinal layer between which lies a layer of circular muscles. The lateral nerve-cords lie between the outer longitudinal and the circular muscle layers. They are connected throughout the body by a nerve plexus, the only definite nerve branches given off being those to the brain, oesophagus, and proboscis. The cephalic slits are a pair of deep longitudinal grooves at the sides of the head. From each groove a canal passes inwards into a posterior brain-lobe. The mouth opens behind the brain, and is an elongated slit bounded by corrugated lips. Auditory organs have not been observed. The longitudinal vascular trunks are connected anteriorly by lacunar spaces, and not by closed vessels. [Illustration: FIG. 58.—Head end of _Cerebratulus marginatus_ Ren., from the ventral surface. Drawn from a spirit specimen. Naples. × 1. _c.s_, Cephalic slit; _m_, mouth; _p.p_, proboscis pore.] Principal British genera and species:— _Lineus bilineatus_ Ren., _L. lacteus_ Mont., _L. marinus_ Mont. (= _L. longissimus_ Gunnerus), _L. gesserensis_ O. F. Müll. (= _L. obscurus_ Desor and _L. sanguineus_ M‘Int.). _Borlasia elizabethae_ M‘Int. _Cerebratulus angulatus_ O. F. Müll., _C. fuscus_ M‘Int., _C. pantherinus_ Hubr. _Micrura aurantiaca_ Grube, _M. candida_ Bürger, _M. fasciolata_ Ehrenb., _M. purpurea_ J. Müll. _Meckelia asulcata_ M‘Int. ORDER III. PALAEONEMERTEA. 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 {112}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. [Illustration: FIG. 59.—_Carinella macintoshi_ Bürger, drawn from the living specimen, slightly contracted. Plymouth. Considerably magnified. _a_, Anterior end; _b_, posterior end.]