THE NEMERTIN1 163 



able distance along the body, and containing a corpusculated 

 fluid. 



The nervous system consists of a pair of lateral nerve cords run- 

 ning the entire length of the animal, and connected with one another 

 au the hinder end, above the intestine (Fig. I.). At the anterior 

 end each cord is enlarged to form a more or less complex cerebral 

 ganglion, which is connected to its fellow by a dorsal and a ventral 

 commissure embracing the proboscis. There is usually in addition 

 a median dorsal nerve. The peripheral system is formed of a 

 diffuse plexus, or of a more regular series of commissural nerves 

 connecting the longitudinal nerves. On each side of the head 

 there is, with few exceptions, a ciliated pit of simple or complex 

 character the cerebral organ in close relation to the hinder part 

 of the brain. 



The excretory system (Fig. II.) consists of a longitudinal canal 

 on each side of the stomodaeum, opening to the exterior by one or 

 more short ducts, and at its inner end gives rise to a greater or 

 less number of short, simple, or slightly branched tubules, each of 

 which terminates in a multicellular swelling containing a bunch of 

 cilia similar to the " flame " of the terminal cell of the Platy helminth 

 system (Fig. XXVIIL). 



There is in the Nemertines another system of tubes entirely 

 shut off from every other cavity, and containing a corpusculated 

 fluid, sometimes red in colour. This " vascular system " (Fig. III.) 

 which is not present in the Platyhelmia, consists fundamentally of a 

 pair of contractile lateral vessels extending the whole length of the 

 animal, and connected with each other by a preoral and a supra- 

 anal anastomosis. To this system there is usually added a median 

 dorsal vessel, lying between the intestine and the wall of the 

 rhynchocoel. In the majority of Nemertines these three longi- 

 tudinal vessels are connected by transverse vessels more or less 

 regularly arranged. 



Between the muscular coat of the body wall and the wall of 

 the gut is a loose connective tissue or " parenchyma," in which the 

 excretory canals and the lateral blood-vessels are embedded. In 

 it too the genital organs are developed. The sexes of the 

 Nemertines are, as a rule, separate, and each genital organ is a 

 simple sac, surrounded by a thin cellular membrane, which, at the 

 breeding season, is prolonged outwards as a duct, to open 

 externally above the lateral nerve. These genital sacs appear to 

 be the only representatives of a coelom ; and are repeated, more or 

 less regularly, usually alternating with the intestinal pouches, 

 throughout the body (Fig. II. m). 



The development of the Nemertine is either direct or in- 

 direct ; the larva is of a characteristic form, known as a Pilidium 

 (or in a modified condition, as Desor's larva). In this larva 



