56 Prof. M'lutosh's Notes from the 



EXPLANATION OF THE PLATES *. 



Plate I. 



Fig. 1. Transverse section through the region of the cephalic ganglia, 

 eg., of a large Bispira volutacornis, Montagu. The chordoid 

 skeleton, eh., is at this level divided into lateral halves, whereas 

 a little in front it forms a continuous arch from side to side. 

 cm., ganglionic commissure ; d., oesophagus ; cd., dorsal pro- 

 cesses; ct., thoracic collar or jacket; m., anterior single mass 

 of longitudinal muscles ; tgo., median or common duct of 

 thoracic glands. Enlarged. 

 Fig. 2. Similar section anterior to the former, the chordoid arch being 

 now complete, bv., branchial blood-vessel, which is dividing 

 into branches; ch., chordoid skeleton; vc, ventral region of 

 the collar ; n., nerve. Enlarged. 

 Fig. 3. Transverse section of the cephalic region of a young example 

 (partly macerated) at the origin of the branchial filaments 

 indicating the tentacles, t. From its macerated condition the 

 margins and posterior region are only diagrammatic. Slightly 

 reduced from Zeiss oc. 4, obj. A. 



Transverse section of the distal region of a macerated branchial 

 filament. X oc. 4, obj. A. 



Longitudinal section of a branchial filament in a similar con- 

 dition, to show the arrangement of the chordoid skeleton. 

 X oc. 2, obj. D. 



Longitudinal section of another filament, indicating the appear- 

 ance of the cellular hypoderm covering the chordoid skeleton. 

 Young example. X oc. 4, obj. D, with 2 inches of draw-tube. 



Transverse section of a tentacle, with its peculiarly curved 

 lamellae and its central skeleton and vessel. 



Plate II. 



Fig. 8. Transverse section of the anterior region of Bispira volutacornis, 

 Mont. The dorsal muscles are proportionally small and some- 

 what rounded, the bristles are still at the dorsal edge, and the 

 ventral longitudinal muscles are somewhat pointed externally, 

 though little weight is to be placed on this feature. A complex 

 series of muscular fibres passes from the dorsal longitudinal 

 muscles downward to the inner border of the nerve-area, and 

 above the point of meeting is the ventral blood-vessel, vv. 

 s., blood-sinus around the cesophagus. Enlarged. 



Fig. 9. Transverse section a little behind the former. The dorsal and 

 ventral longitudinal muscles are larger, whilst the absence of 

 the sheets of muscle passing from the dorsal to the ventral 

 aspect permits the oblique muscles, otn., to be seen passing to 

 the edge of the nerve-cords. The commissure between the 

 ganglia is marked, the ventral vessel being above it. The 

 hypoderm in the mid-ventral line remains massive. Enlarged. 



Fig. 10. Portion of the chordoid skeleton. The passage of processes 



* I am indebted to the Carnegie Trust for the artists' aid with these 

 Plates. 



