POLYPIFERA. 



51 



seen radiating, and in their contracted state. These 

 filaments are about ' inch diameter in this state. 



c, An immature animal. The tentacula and ali- 

 mentary canal rudely formed; the cavity in the 

 latter very distinct. The tentacular and opercular 

 retractors also shown ; 1, the gizzard. 



d, one of the gemmae in its earliest state. The 

 cavity just defined, but no animal distinguishable. 

 (After Farre.) 



scribed by Dr. Arthur Farre, is cylindrical, and 

 closely embraces the body of the animal ; it is 

 of a firm unyielding consistence in the lower 

 two -thirds of its extent, but terminates above 

 by a flexible portion, which serves to protect 

 the upper part of the body when the whole is 

 expanded, in which state it is of the same 

 diameter as the rest of the cell ; but when 

 the animal retracts, this portion is folded up 

 and drawn in after it, so as to close its mouth. 

 The flexible part consists of two portions, the 

 lower half being a simple continuation of the 

 rest of the cell, the upper consisting of a row 

 of delicate bristle-shaped processes, or setae, 

 which are arranged parallel with each other 

 round the top of the cell, and are prevented 

 from separating beyond a certain distance by 

 a membrane of excessive tenuity which sur- 

 rounds and connects the whole. This arrange- 

 ment is common to all the species possessing 

 a cylindrical cell ; but the length of the setae 

 is very variable ; indeed they are sometimes so 

 stunted in their development that their pre- 

 sence is hardly recognisable. 



The cells of the Flmtrce and Eschar<s are 

 disposed side by side upon the same plane, so 

 as to form a broad leaf-like polypary, which is 

 in the former genus of a coriaceous or horny 

 texture, but in the latter so completely calci- 

 ned as to resemble the skeletons of the Litho- 

 phytous Polypes. The individual cells (fig. 

 59), which are so extremely minute that they 



Fig. 57. 



Eschara cervicornis, natural size. (After Milne 

 Edwards.) 



require a microscope for their examination, 

 vary in shape in different species, and gene- 

 rally have their orifices defended by project- 

 ing spines, or sometimes by a movable oper- 

 culum or lid, which answers the same purpose 



as the setae of Bowerbankia, by closing the 

 entrance during the retracted ' state of the 

 animal. The growth of these polyparies, 

 which are thus densely populated, is effected 

 by the progressive addition of new cells 

 around the circumference, those occupying 

 the margin being of course the most recently 

 formed, and, indeed, the latter are not unfre- 

 quently found inhabited by the living animals, 

 whilst in the older or central ones the original 

 occupants have perished. 



Fig. 58. 



A polype of Eschara cervicornis highly magnified. 



a, tentacula ; 6, first digestive cavity, which seems 

 to be analogous to the respiratory cavity of the com- 

 pound ascidians ; c, filaments arising from the part 

 of the alimentary canal immediately below this ca- 

 vity ; d, stomach ; e, intestine ; /, anus ; g, retractor 

 muscles. (After Milne Edwards.) 



The facts observed by Dr. Milne Edwards * 

 relative to the mode of formation of these 

 cells possess a high degree of interest, and 

 materially support the views already given 

 concerning the organised nature of the skele- 

 tons of zoophytes in general ; proving that 

 the calcareous matter to which their hardness 

 is owing is not a mere exudation from the 

 surface of the animal, but is deposited in the 

 meshes of an organised tegumentary mem- 

 brane, from which it can be removed with 

 facility by means of extremely dilute muriatic 

 acid. When so treated a brisk effervescence 

 is produced, the cells become flexible, and are 

 easily separated from each other; but they 

 are not altered in form, and evidently consist 

 of a dense and thick membrane, forming a 

 sac, in which the digestive organs of the ani- 

 mal are contained. In this state the opening 

 of the cell has no longer a defined margin, as 

 it seemed to have before ; but, as in the case 

 of the Tubipora musica, described in a pre- 

 ceding page, the membranous cell is found to 

 be continuous with the tentacular sheath. 

 We see, therefore, that in these creatures the 

 shell is an integrant portion of the animal 

 itself, not a mere calcareous crust moulded 

 upon the surface of its body, being, in fact, a 



* Re'cherches anatomiques," zoologiques, et phy- 

 siologiques sur les Eschares ; An. des Sc. Nat. for 

 1836. 



