PTEROBRANCHIA OF THE SCOTTISH NATIONAL ANTARCTIC EXPEDITION. 441 



EXPLANATION OF PLATES. 

 PLATK I. 



Figs. 15. Selected pieces of colony of Cephalodiscus agglutinans, n. sp., drawn by Miss G. M. 

 WOODWARD. Natural size. The specimen drawu in fig. 1 is selected as the type-specimen of the species. 



PLATE II. 

 Cephalodiscus agglutinans, n. sp. 



The figures were drawn with the Zeiss objectives severally indicated, and were then reduced to two-fifths 

 linear. The scale for the two objectives used in drawing the figures is given, in hundredths of a millimetre, 

 at the bottom of the plate. 



Fig. 1. A young bud. Obj. A. 



Fig. 2. A slightly older bud. Obj. A. 



Fig. 3. An older bud. Obj. A. 



Fig 4. A still older bud. Obj. A. 



Fig. 5. Obliquely frontal section, showing both testes (t.l., t.r.), parts of both gill-slits (</.?.), and the 

 left collar-canal (c.c.). Obj. C. 



Figs. 6-11. Frontal sections of one zooid. 



Fig. 6. Showing eight of the left arms (as numbered) and seven of the right arms. Obj. C. 



Fig. 7. Showing the same fifteen arms, the anterior dorsal horns of the collar-cavities (b.c.*a.), the peri- 

 cardium (per.) enclosing the pericardial sinus or heart, and the internal openings of the proboscis-pores (p.p.). 

 Obj. C. 



Fig. 8. Showing the anterior end of the notochord (nch.) and of the right lobe (op.r.) of the operculum. 

 Obj. C. 



Fig. 9. Through the mouth (m.). The double appearance of the left lobe (op.l.) of the operculum is 

 due to the fact that the organ in question is folded ventrally. To see the correspondence between the two 

 sides of the section, the part of the left lobe which is nearer the proboscis (p.) must be imagined to have 

 been folded dorsally. It would then not have appeared in the section, and the left lobe of the operculum 

 would have had much the same appearance as the right lobe. Obj. C. 



Fig. 10. The mouth (m.) is still visible. The reflected part of the left lobe (op.l.) of the operculum is 

 almost separate from the rest of the organ. Obj. C. 



Fig. 11. Through the region of the collar-canals (c.c.) and gill-slits (g.s.). The right testis (t.r.) is very 

 small in this zooid. Two of the terminal lobes of the scalloped edge of the right half (op.r.) of the operculum 

 are visible. Obj. C. 



Fig. 12. Stomach (atom,), intestine (int.), rectum (r.), aiid one testis (t.*) of an old zooid. Obj. A. 



Fig. 13. Stalk, in transverse section. Obj. C. 



Fig. 14. Proboscis-stalk, in transverse section, showing both proboscis-pores (p.p-), the anterior dorsal 

 horns (b.c.-a.) of the collar-cavities, the anterior end of the central nervous system (c.n.s.), and the peri- 

 cardium (per.) with the included pericardial sinus. Obj. C. 



Fig. 15. Median sagittal section of the anterior end of a zooid, showing the notochord (nch.). Obj. C. 



Fig. 16. Nearly median sagittal section of another zooid, showing the alimentary canal. Obj. C. 



N.B. In several of the sections, figured parts of the epithelia were somewhat macerated, as the result 

 of imperfect preservation. The epithelium of the stomach in fig. 16, part of the anterior epidermis of the 

 proboscis in figs. 6-9, and part of the epidermis containing the central nervous system in figs. 10 and 11 

 have accordingly been restored where necessary. 



(HOY. soc. EDIN. TRANS., VOL. XLIX., 565). 



