176 UNSEGMENTED " WORMS." 



in size, from a Lineus, 12 feet long, to the small pelagic 

 Pelagonemertes, which is under an inch. There are no 

 appendages. The colours are often bright. 



Skin. 



The ectoderm is covered with numerous short cilia, and 

 many of its cells are also glandular, secreting mucus which 

 often forms a tube around the animal, or is exuded in move- 

 ment. Some of the glandular cells extend into the subjacent 

 cutis, which consists in part of connective tissue. 



Muscular System. 



The Nemertines are remarkably contractile, and in some 

 cases the spasms result in the breakage of the body. The 

 muscles are circular and longitudinal, but their arrangement 

 is variable even in individuals. 



Body Cavity. 



In the adult there is no distinct coelome, the space 

 between the gut and the body wall being filled up with con- 

 nective tissue. In the larvae, however, a body cavity may 

 be seen, either as an archicoele, i.e., persistent segmentation 

 cavity (Lineus obscurus\ or as a schizocoale, i.e., a space 

 formed by the cleavage of the mesoderm into two layers 

 (Pilidium-laiv^). In the adult, however, only the blood 

 spaces and the cavity of the proboscis sheath are coelomic. 



Nervous System. 



In the head there is a brain, generally four lobed, with a 

 commissural ring surrounding the proboscis and its sheath ; 

 from the lower brain lobes two longitudinal nerve stems run 

 along the sides, and are sometimes united posteriorly above 

 the anus (Fig. 57, In). 



Hubrecht suggests that the nerve stems and the brain may "be looked 

 upon as local accumulations of nervous tissue in what was in more primi- 

 tive ancestors a less highly differentiated nervous plexus, situated in the 

 body wall," as in many Ccelentera. In some cases (Schizonemertea) 

 this nerve plexus persists, and then the longitudinal stems do not give 

 off regular peripheral branches as is the case in another sub-class (Hoplo- 

 nemertea) where there is no definite plexus. 



It is interesting to find that in Drepanophorus the lateral nerve steins 

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



