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.
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.