1 86 



UNSEGMENTED WORMS. " 



often forms a tube around the animal, or is exuded in movement. Beneath 

 the epidermis there is a parenchyma, consisting in part of connective 

 I issue, 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 gelatinous connective 

 tissue. In the larvae, however, a body cavity may be seen, either as an 

 archicoele, i.e. the persistent segmentation cavity {Linens obscurus\ or 

 as a schizocoele, i.e. a space formed by the cleavage of the mesoderm 



d. v. m. 



FIG. 90. Transverse section of the Nemertean Drepanophorus latus. 



After Burger. 



if.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; 

 c.m., circular muscles; /.;;/., longitudinal muscles; /.., 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. t epidermis. 



into two layers (Pilidium-\3irv?e.) In the adult only the blood spaces 

 and the cavity of the proboscis sheath are crelomic. The 

 nervous system consists of a brain generally four-lobed, the two lobes 

 of each side being closely united and connected with those on the other 

 side by a commissure above and by another below the proboscis cavity. 

 From the lower lobes two longitudinal nerve-stems run along the sides, 

 and are sometimes united posteriorly above the anus (Fig. 90, /.;/.). 

 In some forms there is in addition a dorse-median nerve, and some- 

 times a ventro-median nerve. 



On each side of the head there is a ciliated pit communicating 



