CHLOILEMID^B 



83 



fibres are visible through the translucent walls. In transverse section the body-wall 

 is comparatively thin, having externally the papillse of the cuticle and hypoderm, with 

 the sand-grains and mud, then a tough layer which stains, beneath which is a dense layer 

 which is usually unstained. Though this may only be the inner part of the former, yet 

 it seems to run into or fuse with the circular muscular fibres which form a continuous 

 coat. Both dorsal and ventral longitudinal muscles form continuous bands at their outer 

 borders, but the inner divisions have a tendency to split into separate bundles, which in 

 section are somewhat conical, the base being external. The nerve-cords lie in the space 

 between the ventral longitudinal muscles, the slender oblique muscles passing to their 

 exterior. The cords fuse and form ganglionic enlargements in each segment, and a 

 median mesentery connects the sheath superiorly with the ventral blood-vessel, whilst both 

 in this and the large lateral branch which joins it in each segment, as well as in the dorsal 

 trunk, the contained blood is minutely granular. A branch leaves the ganglion at each 



k 



om 



-b. 



TV'C. 



Fig. 110.— Transverse section of the anterior third of Stylarioides plumosa, O. F. M. b. Muscles of the 



setigerous region, p. Papillae of the skin. 



side to supply the ventral longitudinal muscles and the surface. The nerve-cells in the 

 ganglion stain deeply and are very distinct in the ventral median line, and in a lateral area 

 on each side. Externally to the nerve-area are the circular muscular coat, the basement 

 layers, hypoderm, and cuticle. The blood in spirit-preparations is red. In Stylarioides 

 monilif erics it is pale (green in life). A heart-body is present anteriorly. A feature of 

 moment is the occurrence of a symmetrical thickening of the ventral wall of the alimentary 

 canal in the anterior region, the massive median area on each side of a central groove 

 thinning off laterally. The great size of the perivisceral cavity in such types is noteworthy. 

 The sandy mud in the interior of the gut is rich in diatoms, sponge-spicules, radiolarians, 

 and organic debris. The posterior alimentary apparatus is in marked contrast, from its 

 delicacy, with the condition of the organ in such as Ophelia limacina. The mesentery 

 encloses the intestinal loop in the anterior dorsal region with the ovaries or spermaries. 



In Flabelligera affinis the thin body- wall has the translucent cuticle crowded with the 

 filamentous papillse ending in clavate extremities. Beneath is a hypoderm so attenuate as 

 to be almost indistinguishable. A tough, though thin, basement-layer follows with a few 

 circular muscular fibres. The dorsal and ventral longitudinal muscles are thinly spread 



