CILIOBRACHIATE POLYPES ; BOWERBANKIA. 527 



singly, but they sometimes present themselves in an almost 

 isolated form ; their cells arising separately from a creeping stalk. 

 In other instances they are closely aggregated, as in the common 

 Flustra, where they are spread out into an expanded surface ; 

 but they never seem to exhibit the same degree of structural 

 connection, as that which exists in the inferior groups. In a 

 large proportion of instances, the cells are of a delicate horny 

 structure ; and in those which are calcareous, the animal mem- 

 brane remains as a much more definite tissue, when the stony 

 matter has been removed by an acid, than in lithophyte corals 

 of the other groups, As in the Tubipora, the wall of the cell 

 is distinctly continuous with the membrane which closes the 

 mouth of it ; as this is with the external integument of the Polype 

 itself. Moreover, the mouth of the cell generally possesses some 

 flexibility, and is drawn inwards, so as to form a kind of oper- 

 culum or lid to the cavity, when the Polype retracts itself within. 

 The cells undergo remarkable modifications in form at different 

 periods of age, even in the calcareous polypidoms. The poly- 

 pidoms of this group never attain any considerable size ; and 

 they are almost always parasitic upon other marine formations, 

 such as corallines, sea-weeds, shells, or even different species of 

 their own tribe. 



1 100. The species, which has been most carefully studied and 

 minutely described, belongs to that group in which the isolation 

 of the Polypes is most complete ; and it will, therefore, be con- 

 venient to enter first upon the consideration of its structure. The 

 Polypes of the Bowerbankia densa are, when fully expanded, 

 about half of an inch in length ; when retracted, they are com- 

 pletely inclosed in delicate horny cells, so transparent as to admit 

 of the whole structure being seen through their walls. These 

 cells, which, when the edges are turned in by the retraction of 

 the animal, are not above half the length of the expanded body, 

 arise separately from creeping stems, which attach themselves 

 to the Flustra in aggregated masses, of from half an inch to one 

 inch in diameter. The animal possesses ten tentacula, arranged 

 in the usual manner around the mouth ; and each of these is 

 thickly set with cilia on both sides. The mouth does not lead 



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