SERTULARIA. 161 



lonnf clavate summit. Others had two buds under a common involucrum. 

 Two days later one had a hydra with 15 tentacula, and two buds preparing 

 above it. — Fig. 13. 



In these young specimens, although the pith seems confined within 

 its own peculiar channel, a wider cortical covering apparently forms the 

 stem around it. 



This product usually occurs as a parasite on other zoophytes ; and it 

 is often involved by the Spongia coalita, spreading rapid destruction around 

 the precincts of its vegetation. 



Plate XXVI. 



Fig. 1. Sertularia Fosacea. 



2. Prolific specimen. 



3. Branch with hydrse. 



4. Hydra and cell. 



5. Branch with hydrse and vesicles. 



6. Section with hydrse and vesicles. 

 7-10. Vesicles full and empty. 



11. Planulse from the vesicles. 



12. Planulse from the vesicles. 



13. Nascent Sertularia bred from a planula. 



All the preceding figures, unless figs. 1, 2, enlarged. 



^ 5. Sertularia pumila, Sea Oak Coralline. — Plate XXVI. 

 Figs. 14-21. — This diminutive product occurs in considerable abundance as 

 a parasite on some of the marine fuci ; likewise it grows on rocks and stones. 

 The Fucus serratus is most profusely invested by such quantities as almost 

 to weigh down the leaves, when nearest to ebb tide. 



Larger specimens rise an inch high, by an erect stem, from which a 

 few boughs and branches, in the same plane, issue almost at right angles to 

 their principal part. The whole is of greenish colour, and generally of 

 rather a meagre aspect. — PI. XXVI. figs. 14, 19. All the parts are bor- 

 dered by a row of cells, which are arranged in pairs. Each pair forms a 

 division or articulation, somewhat heart-shaped. — Fig. 16, enlarged. The 



VOL. I. X 



