ix CHAETAE OF NEREIS 247 



extremity (Fig. 123). The shape of the cup varies; it is in 

 some cases of equal height all round, or it is higher on one side 

 than on the other. Further, the appendix may be short and 

 curved, or more elongate and spear-like ; it is generally notched 

 or finely toothed oil one side. 



In addition to these projecting locomotor chaetae, there is 

 embedded in each of the two chaetigerous lobes a much stouter 



nrp'.lig 



&, N ( c ^^^T \^>-^ cir 



v. lony Y y e obi 



FK;. 124. Nereis. Somewhat diagrammatic transverse section through the body. On the 

 left the chief constituents of the vascular system are represented ; on the right side 

 the chaetae and their muscles, as w'ell as the distribution of the lateral nerve, etc., 

 are shown, ac, aciculum ; bv, network of blood-vessels ; ch, chaetae (only a few 

 are shown) in two bundles; ch.l, lips of the chaetigerous sac; cil.org, dorsal ciliated 

 organ ; cir, circular muscular coat ; coe, coelom ; d.cir, dorsal cirrus ; d.long, 

 dorsal bundle of longitudinal muscles ; d.ve, dorsal blood trunk ; ep, epidermis ; 

 INT, intestine; int. cap, blood capillaries in its wall; m.ch, muscles which move 

 the chaetae ; N.c, ventral nerve cord ; ner, lateral sensory nerve, dividing into a 

 ventral branch entering the ventral cirrus, and a dorsal branch (n.eir) for the 

 dorsal cirrus ; neph, nephridium, seen through the oblique muscle through which 

 its funnel passes ; nrp, neuropodium ; nrp.lig, neuropodial lobe or ligule ; ntp, noto- 

 podium ; ntp.liy, notopodial ligule ; obi, oblique transverse muscle (muscle of the 

 parapodium); pv, peripheral blood-vessel; v.cir, ventral cirrus; v.long, ventral 

 bundle of longitudinal muscles ; v.ve, ventral blood trunk. 



and dark-coloured, needle-shaped bristle known as an " aciculum," 

 whose point only just projects beyond the surface. This aciculum 

 extends into the interior of the body much farther than do the 

 locomotor chaetae, and it is to it that the muscles serving to 

 move the whole bundle of chaetae are attached. The acicula 

 thus serve as an internal skeleton to the parapodium. The shape 

 of the parapodium, the relative lengths of cirri and lobes, the 

 shape and arrangement of the chaetae, are all employed as specific 

 characters. 



