232 



UNSEGMENTED WORMS 



organs simple. The development is in some cases direct, 

 while in others there is a peculiar pelagic larva. 



General Account of Nemertines 



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

 cross-section is generally a flattened cylinder. They show a greater 

 diversity of size than any other " worms " — from a Linens, 12 or more 



d.n 



P 



Fig. 127. — Transverse section of the Nemertine Drepanophorus latus. 



— After Biirger. 



d.n., Dorsal or proboscis nerve ; P.s., proboscis sheath ; P.c, proboscis 

 cavity ; P.s'., sac of proboscis cavity ; d.v.m., dorso-ventral muscles ; 

 cm., circular muscles ; l.m., longitudinal muscles ; l.n., lateral nerve 

 with branches ; P., parenchyma ; g., gut ; l.v., lateral blood vessel, 

 beside which lies an excretory vessel ; E.p., excretory pore ; d.v'., dorsal 

 blood vessel ; Ep., epidermis. 



feet in length (25 metres has been recorded for an extended Lineus 

 longissitnus), to the pelagic Pelagonemertes, which is under an inch. 

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

 roundings. 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 parenchyma, consisting in part of connective 

 tissue, and often in part gelatinous. The body is remarkably con- 

 tractile, 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 coelom, the space between 



