§11 THE FLUSTRA. 605 



1 1 . The Flustra. — From this digression we return to 

 the consideration of those zoophytes which possess a calca- 

 reous framework or support, and are termed Corals ; and 

 I will select one of the most simple and common forms in 

 illustration of the subject. Most persons in their rambles 

 along the sea-shore, must have noticed on the fuci, shells, 

 and pebbles, patches of a white calcareous substance, which 

 when closely examined appear like delicate lace-work : 

 these apparently mere earthy particles, are clusters of 

 the compound zoophyte termed Flustra^ or Sea-mat (See 

 Plate V. figs, o, 6, 7, 8). 



The Flustra, when taken alive out of the water, presents 

 to the naked eye the appearance of fine net-work, coated 

 over with a glossy varnish (PL V.fig. 5) ; with a glass of 

 moderate power, the surface is discovered to be full of 

 pores, disposed with much regularity (PL Y.fig. 8). If a 

 magnifying glass be employed, while the Flustra is im- 

 mersed in sea-water, very diiferent phenomena appear ; the 

 surface is found to be invested with a fleshy or gelatinous 

 substance, and every pore to be the opening of a cell, whence 

 issues a tube with several long feelers or arms (Lign. 133) ; 

 these expand, then suddenly close, withdraw into the cells, 

 and again issue forth ; the whole mass being studded with 

 these hydra-like forms. The Flustra thus constitutes, as it 

 were, a family of polypes, each individual of which is per- 

 manently fixed in a calcareous cell, and the whole united 

 by a common integument, by which the calcareous frame- 

 work was secreted. Under a powerful microscope the form 

 and structure of the animal of the Flustra may be easily 

 distinguished. PL V. fig. 6, represents a single polype 

 with its tentacula extended ; and fig. 7, the same animal- 

 cule, withdrawn into its cell. The figure, Lign. 133, shows 

 a polype protruded, under a much higher power.* 



* This exquisite figure is from a memoir by Mr. Lister (Pkilos. 

 Trans. 1834), on the structure and functions of the Tubular and 



