496 



ANNELIDA. 



The muscles are constructed upon the nematode type (i.e., the contractile 



substance partially surrounds 

 the unmodified nucleated pro- 

 toplasm (Fig. 406, A, , C). 

 In the earthworms, however, 

 the longitudinal muscular layer 

 presents an appearance, which 

 it is difficult to interpret. 

 In transverse section they 

 appear to consist of a num- 

 ber of lamellae placed like 



the barbs of a feather upon a 



central axis, the whole structure 



, . 1 



being placed in a connective 

 substance with nuclei (Fig. 

 406, D). The explanation 

 appears to be that we have to 

 do in such cases with a folded 

 layer of longitudinal muscles ; 

 each lamella consisting of a 

 FIG. 405. Section of the body-wall of Alloloboptwra muscle - cell, from which the 

 (Dendrobaena) rubida in the neighbourhood of a seta. granular unmodified protoplasm 



with pigment p; b seta; bf setigerous sac; m its 

 muscles. (From Perrier, after Vejdovsky.) 



( m " e Hesse, Z. f. w. Z., 58, 



1894, p. 394). 



The central nervous system is on 



consisting of two ventral cords closely 

 in a common sheath. They are swollen 

 in each segment where the main nerves 

 are given off, and in front they separate 

 from one another and embrace the 

 anterior end of the alimentary canal, 

 to be continuous with one another 

 dorsally. At this dorsal point there is 

 generally a bilobed swelling which con- 

 stitutes the cerebral or suprapharyngeal 

 ganglia, from which the nerves to the 

 prestomium are given off. The first 

 ventral swelling is where the circum- 

 pharyngeal commissures join to form 

 the ventral cord, and constitutes the 

 sub-pharyngeal ganglion. It gives off 

 a large number of nerves to the 

 peristomium. In some forms a few 

 large, longitudinal fibres resembling 



the usual Annelidan type, 

 approximated and enclosed 



FIG. 406. A-C, sections through the 

 circular muscle-cells of Allolobo- 

 pliora chlorotica ; D, section through 

 the longitudinal muscular layer of 

 a young earthworm ; p coelomic 

 epithelium. (After Hesse.) 



