ORGANS OF DIGESTION. 315 



into its posterior portion. This small coecum, the rudiment 

 of a liver, presents a continued revolution of the particles 

 contained in it, and is sometimes seen to pulsate like a 

 heart. It is smaller in .the flustra foliacea where the lower 

 curved part of the polypus is dilated into a wide gastric 

 cavity. These polypi have also distinct bands passing 

 from their body to the aperture and to the base of the cells, 

 apparently to assist in their rising and retreating ; the polypi 

 appear to be capable of subsisting in an isolated condition, 

 when detached from the cells to afford space for the develop- 

 ment of the gemmules, as I have often found them 

 swimming free in the water by the rapid contraction and 

 extension of their tentacula. It is chiefly in the lowest zoo- 

 phytes and in the smallest and simplest forms of polypi that 

 the tentacula are furnished with vibratile cilia, as in sertula- 

 ria, plumularia, cellaria, flustra, alcyonium, alcyonella ; in 

 some, as campanularia and tubularia, the cilia are disposed 

 round the extensile lips, and the tentacula are simple ; and 

 in many higher forms of polypi, as in madrepora, gorgonia, 

 and lobularia, where the cilia have generally a similar dispo- 

 sition around the mouth, the tentacula are furnished with 

 lateral appendices which are not vibratile, and the stomach 

 open at both ends, forms a separate internal sac, as in 

 actinia, allowing the gemmules of each separate polypus to 

 escape through this open passage. The large polypi are 

 more nearly isolated in their condition in many of the 

 massive lithophytes, as in the large deep-green polypi of the 

 astrea viridis (Fig. 109. A. B. C.), where they are more 

 than six lines in length, and protected in deep laminated poly- 

 gonal cells (109. A. g. h.) two lines in diameter, they are 

 striated with longitudinal (109. A. b.} and transverse (109. B. 

 C. d. d.) bands, and are connected only by a thin fleshy layer 

 (109. A. /.), covering the dark brown coral, and scarcely 

 perceptible when the polypi are retracted into their cells 

 (109. A. e.f.}. The numerous bright-green tentacula (C. c.), 

 alternately large and small, disposed around the very pro- 

 minent blue mouth (C. a.) of the polypus, appear to 

 constitute a double row of simple arms as in the tubularice, 

 and the surface of the polypi in their contracted state is 

 marked with regular vertical rows of prominent tubercles 

 (6. d.). The polypi of lobularia are provided externally 



