170 UNSEGMENTED "WORMS." 



a commissure round the proboscis, and two lateral nerve- 

 cords ; in connection with the brain there is a pair of ciliated 

 pits. The gut terminates in a posterior anus, and is furnished 

 with lateral pockets. There is no body cavity in the adult, 

 but the closed vascular system is probably of ccelomic origin. 

 The excretory system is apparently of the Platyhelminth type. 

 The sexes are usually separate and the organs simple. The 

 development is in some cases direct, while in others there is a 

 peculiar pelagic larva. 



General Account of Nemertea. 



In appearance Nemertines are ribbon- or thread-like, and 

 the cross-section is generally a flattened cylinder. They 

 vary in size, from a Lineus, 1 2 or more feet in length, to the 

 pelagic Pelagonemertes, which is under an inch. The 

 colours are often bright, and tend to resemble those of the 

 surroundings. The ectoderm .is covered with numerous 

 short cilia, and many of its cells are also glandular, secreting 

 the mucus which often forms a tube around the animal, or 

 is exuded in movement. Beneath the epidermis there is a 

 dermis, consisting in part of connective tissue, and often in 

 part gelatinous. The body is remarkably contractile, and 

 in some cases the spasms result in breakage. The muscles 

 are circular and longitudinal, and often also diagonal. The 

 fibres are striped. In the adult there is no distinct ccelom, 

 the space between the gut and the body-wall being filled up 

 with connective tissue. In the larva?, however, a body 

 cavity may be seen, either as an archiccele, i.e. the persistent 

 segmentation cavity (Lineus obscurus), or as a schizoccele, i.e. a 

 space formed by the cleavage of the mesoderm into two 

 layers (Pilidium-\a.rvx). In the adult only the blood spaces 

 and the cavity of the proboscis sheath are ccelomic. The 

 nervous system consists of a brain generally four-lobed. 

 From the dorsal lobes a commissural ring rises and sur- 

 rounds the proboscis sheath ; from the lower lobes two 

 longitudinal nerve-stems run along the sides, and are some- 

 times united posteriorly above the anus (Fig. 76, /.».). 



It is interesting to find that in Drepanophorus the lateral nerve-stems 

 are approximated ventrally, and in Langia, dorsally ; for these two 

 approximations tend towards positions characteristic of the neivous 



