CIRCULATORY OR BLOOD-VASCULAR SYSTEM.—The circulatory system consists of three main longitudinal vessels, a median dorsal and a pair of lateral ones. These are connected together posteriorly by a transverse trunk, and also throughout the whole length of their course by branches, which are given off at regular intervals. Anteriorly the three longitudinal vessels {108}either all unite and form a collar (Fig. 55, _v.s_) round the oesophagus, or they break up into a number of lacunar or open spaces in free communication with one another. The blood is usually colourless, but in some cases the corpuscles are coloured red by haemoglobin. [Illustration: FIG. 56.—Excretory system of Nemertines. A, _Drepanophorus spectabilis_ Qtrf., part of one of the lateral vessels encircled by branches of the excretory organ, × 585; _e_, main canal of the excretory system: B, _D. crassus_ Qtrf., a terminal branch of the excretory system, × 585; _f_, ciliated flame: C, _Malacobdella grossa_ O. F. Müll., entire animal, slightly magnified, showing the excretory system (black) and the vascular system; _e.a_, excretory aperture; _d.v_, dorsal vessel; _l.v_, lateral vessel. (From Bürger.)] EXCRETORY SYSTEM.—Max Schultze[133] found in _Tetrastemma obscurum_, on the outer side of, but near to the lateral blood-vessels, a pair of canals. He observed ciliary movements in the canals, but could not discover flame cells. Further contributions to our knowledge of the excretory system were made by Semper,[134] von Kennel,[135] Hubrecht,[136] and Oudemans.[137] The latter states that the excretory system consists of a pair of canals situated laterally near the anterior end of the body. Each canal communicates with the exterior by one or more ducts having lateral regularly-arranged apertures. In some cases he was unable to make out any communication with the vascular system, but in others {109}a direct communication, by means of open connexions with the lacunar blood spaces, is said to occur. Silliman[138] in _Tetrastemma aquarum dulcium_ describes the excretory vessels as ending in numerous capillary branches, at the blind terminations of which cilia are present. He states that there is no important difference between the excretory systems of Rhabdocoeles and Nemertines. Bürger,[139] as the result of recent investigations on the excretory system in Nemertines, finds that the minute branches end in flame-cells (Fig. 56, B) lying on and among the blood-vessels, but having no open connexion with them. GENERATIVE SYSTEM.—The Nemertines are for the most part dioecious, only a few certainly hermaphrodite species having been described, e.g. _Tetrastemma_ ("_Borlasia_") _kefersteinii_ Mar.[140]