PLATE VI. 



Fig. 1. — Hydractinia echinata, Flem. (recent) ; tangential section of a thick 

 crust, enlarged 90 times, p p, Radial pillars, transversely divided, c c, Hori- 

 zontal connecting processes or " arms " given out by the pillars at successive 

 levels. 



Fig. 2. — Portion of a young colony of the same, consisting of a single lamina, 

 viewed as a transparent object, and enlarged 90 times. Letters as before. 



Fig. 3. — Portion of the surface of H. echinata, greatly enlarged. On the left 

 half of the figure the spines (s) and the astrorhizal grooves (g) are alone shown ; 

 while on the right half of the figure are shown the small tubercles (b) which 

 represent the free ends of the radial pillars and also the openings of the zooidal 

 tubes (t t). 



Fig. 3 a. — Part of the surface of the same, free from the large spines, still 

 more highly enlarged. The tubercles representing the free ends of the radial 

 pillars (p p) and their horizontal connecting-processes are shown, as well as a few 

 of the openings of the zooidal tubes. 



Fig. 4. — Vertical section of the skeleton of Hydractinia echinata, enlarged, 

 showing three laminae (I, I', I") with their interlaminar spaces (ii), and a single 

 spine (s). [Copied from Carter.] 



Fig. 5. — Part of a vertical section of H. echinata, enlarged 90 times. Showing 

 the radial pillars (p p) and the connecting-processes or concentric laminse (c c), 

 with the intervening interlaminar spaces. 



Fig. 6. — Spine of Hydractinia echinata, greatly enlarged. 



Fig. 7. —Hydractinia circumvestiens, S. V. Wood, Red Crag, Suffolk. Vertical 

 fracture of the skeleton, enlarged 3 times, showing the zooidal tubes (t t) and the 

 rows of chambers representing the interlaminar spaces. 



Fig. 8. — Part of the surface of a worn example of the same, enlarged, showing 

 the large perforated radial pillars (p) and the mouths of the zooidal tubes. 



Fig. 9. — Surface of an unworn example of the same, enlarged 24 times, 

 showing the large perforated pillars (p), the surface-tubercles representing the 

 small radial pillars (b b), the astrorhizal grooves (g), and the zooidal apertures (t). 



Fig. 10. — Portion of the same, further enlarged. These two figures are from 

 a beautiful specimen of H. circumvestiens in the British Museum, and were kindly 

 drawn for me by Mr. Arthur H. Foord. 



Fig. 11. — Vertical section of H. circumvestiens, enlarged 12 times, showing 

 the zooidal tubes {t t), the irregular interlaminar chambers (i i), and the large 

 radial pillars (p p). These last have their axes traversed by irregular canals, 

 giving them a cribriform structure. 



Fig. 12. — Tangential section of the same, similarly enlarged, showing the 

 transversely divided radial pillars and zooidal tubes. 



Fig. 13. — Part of the last section, enlarged 24 times, showing the apparent 

 composition of the skeleton out of irregular calcareous granules. 



