162 



THE NEMERTIN1 



-4U' 



e- 



The General Characters of the 

 Nemertines. These worms are, with 

 a few exceptions, elongated, and sub- 

 cylindrical in form, and marine in 

 habitat. As in the Turbellaria, the 

 epidermis is ciliated and the body 

 is devoid of external segmentation. 

 But unlike the Platyhelmia, the gut 

 is provided with an anus, situated at 

 the posterior end of the body (Figs. 

 L, II., III.). The mouth, always 

 anterior and subterminal, leads into 

 stomodaeum, which is, in the 

 majority, a short, simple, cylindrical 

 tube, leading to the enteron. The 

 latter is a straight canal, usually 

 provided with regularly arranged, 

 paired, lateral diverticula. The most 

 characteristic organ of the Phylum 

 is the proboscis, which is a muscular 

 pleurecbolic introvert, capable of 

 eversion through an anterior terminal 

 pore the rhynchostome. This tubu- 

 lar proboscis is invested in an epi- 

 thelium similar to the epidermis, 

 containing rhabdites and nemato- 

 cysts and in one group calcareous 

 stylets are carried on its wall in 

 such a position that they lie at 

 the apex when the proboscis is 

 everted. In a state of introversion, 

 the proboscis is contained in a closed 

 tubular cavity, with a muscular 

 wall the rhynchocoel lying above 

 the enteron, extending for a vari- 



FlG. I. 



Diagrammatic view of Cerebratulus, as seen from 

 above when the dorsal wall of the body has been re- 

 moved, to show the proboscis and its sheath, the 

 enteric and nervous systems ; in the middle region 

 the gut has been omitted, a, rhynchostome ; ft, 

 rhynchodaeum ; c, proboscis, the tubular, eversible 

 region ; c', the posterior, solid, non-eversible region, 

 which serves as the retractor muscle ; d, rhyncho- 

 coel ; e, dorsal ganglion, or lobe of the brain ; the 

 dorsal commissure has been removed, to show the 

 continuity of proboscis and rhynchodaeum ; /, ventral 

 ganglion, or lobe of the brain ; g, horizontal cephalic 

 cleft, the depth of which is indicated ; h, cerebral 

 organ ; the canal of which opens externally into the 

 hinder end of g ; i, lateral nerve trunk ; j, stomo- 

 daeum ; k', intestinal caeca ; z, anus. 



