EXPLANxVTION OF PLATE WXIN .—Continued. 



into upper and lower compartments, h. Cliitinous support along the 

 lower border of the plate, r. Branch of the lower blood-channel (w) 

 in the plate, t. Fibres found along the upper border of the plate, u. 

 Pibres that are given off from the stem, slightly above the blood- 

 channel (;•), and that approach and finally touch the latter near its 

 lower end. ad and b d. Characteristic columnar epithelium cells 

 found along the lower borders of the plates, having longer cilia than 

 those on the other parts. 



Fig. 6. — Cross-section of a plate cut rather below the middle, so that 

 the attachment of the plate to the stem is not seen at all. Enlarged about 

 270 diameters. Letters same as in Fig 5, except t.f., which stands for 

 columnar cells having longer cilia than those on the other parts (Peck's 

 latero-frontal cells). 



Fig. 7. — Diagrammatic representation of the cliitinous framework. 

 The trougii is seen as the middle piece from which a pair of branches {h) 

 are sent into each plate. c.p. Cross piece that divides the trough into 

 upper and lower compartments. o v. Oval openings in the cross piece. 

 op. Openings in one side of the trough that connect the space between the 

 closely-applied pairs of branches {h) with the lower compartments of the 

 trough, o' p. The same of the other side of the trough. 



EiG. 8. — A part of the chitinous framework seen from below. Letters 

 same as in Fig. 7. aaaa are placed opposite each plate. 



YOLDIA. 

 EiG. 9. — The gill of one side dissected out by itself. Seen from below 

 and one side. Corresponds to Fig. 4 of the Nucula gill. xdp. Median 

 ventral line. xcp. Near the line of suspension of the gill. 



Fig. 10. — Diagram of the parts around the front part of the gill. /. 

 Labial palpi. I. a. Tentacular labial appendage, g. Gill. v.m. Visceral 

 mass. X, Membranous ridge-like portion of the gill in front. 



EiG. 11. — An opposed pair of the plates composing the gill. Enlarged 

 about 75 diameters. Remarks about Fig. 5 apply to this figure as well. 

 Letters same as in fig. 5, except s, which is here a bundle of longitudinal 

 fibres, and not the chitinous trough. In addition,/. Longitudinal bundle 

 of a fibrous tissue running the whole length of the gill. ch. A second 

 system of chitinous support, spreading out like the frame of a fan. 



EiG. Wa. — The outline of a plate as seen from one side. 



FiG. 12. — Lacunar tissue seen in a cross-section of a plate. Tlie 

 peculiar shape is due to the distortion of the plate. Enlarged 400 dia- 

 meters. 



