NERVOUS SYSTEM 



105 



proboscis, and will there be in a position for offence or defence 

 (Fig. 57,s). 



Nervous System. 1 The brain is composed of two ganglionic 

 masses (Fig. 53, n.rj] lying at the anterior end of the body, one 

 on each side of the proboscis, and united by commissures passing 

 round it (Fig. 55, d.c and v.c}. Each ganglionic mass is often 

 partially divided into a dorsal and ventral lobe (n.g.d and 

 n.g.v}. From the brain a pair of cords pass off backwards 

 along the sides of the body (n.c) ; these cords, which have 



FIG. 54. Diagrammatic representation of the proboscis, (A) in the retracted condition, 

 (B) in the everted condition, g.p, Glandular portion of the proboscis ; m, muscle 

 attaching the proboscis to its sheath ; m.p, muscular portion of the proboscis : 

 p.p in A, proboscis pore ; p.p in B represents the position of the proboscis pore in 

 the retracted condition of the proboscis ; p.s, proboscis sheath. 



no ganglionic swellings, in some forms unite with one another 

 above the anus. Anteriorly nerves are given off from the brain 

 to the eyes and front part of the head (a.ii). A nerve to the 

 proboscis is given off from the commissure which unites the two 

 halves of the brain dorsal to the" proboscis (</.?i). 



In two out of the three groups into which the Nemertines are 

 divided, the lateral nerve-cords are in connexion with a network 

 or plexus of nerves lying between the muscular layers of the body- 

 wall (Fig. 52, n.l\ and in some forms constituting a compara- 

 tively thick layer. In these two groups there are no definite 



1 See Hubrecht, in Verli. Ak. Amsterdam, vol. xx. 1880 ; and in Quart. J. Mia: 

 Sci. vol. xx. 1880, p. 431. 



