47 



be found to be composed of light and dark lines, formed by 

 large numbers of irregular cellular bodies arranged longi- 

 tudinally in various degrees of aggregation. When separated 

 from each other, these cellules present one general lengthened 

 form, terminating at either end in a point, at the base of 

 whic'.i is a rim, resembling the neck and pointed stopper of a 

 decanter, Between the two extremities, the body is straight, 

 but has minute globular cells arranged sometimes in pairs, at 

 others irregularly, and occasionally in rings round it. If 

 allowed to undergo still further decomposition, these separate 

 into smaller bodies of the shape of a Florence flask, which 

 under still further decomposition, separate into minute glo- 

 bular cells, which form the ultimate component parts of the 

 whole crust. Their union is of the most intimate kind, and 

 before decomposition has begun, it is impossible to detect the 

 points of union between them. Ellis, in his " Essay" on 

 Corallines, has given a figure of a perfectly organized cell of 

 the G. Placomus, which differs but slightly from those of the 

 G. Verrucosa; but most probably they differ in every species. 

 The polypidom of the Hydroida, we have seen is external, 

 but here a new and remarkable change has taken place, and it 

 has become the internal solid skeleton which gives form 

 and consistence to the whole. It is covered trom the root to 

 all the branches with an investing membrane, similar to the 

 periosteum of bones. This membrane, which is fibrous^ is not 

 equally demonstrable at all seasons of the year, or in all 

 specimens. Though this appears to be independent of 

 seasons, yet I have found it more clearly apparent about 

 September, October, and r^ovember, or at least my notices 

 more frequently refer to these than any of the other months. 

 At those periods it is frequently so very loose that it may 

 be stripped off; in the branches it is sometimes detached 

 from the axis, and elevated into large cavities which are par- 

 tially filled with a white granular matter; a section of these 

 cavities therefore presents, first, the fleshy crust with its proper 

 membrane, then, the investing membrane of the axis raised 

 in contact with the crust, the white granular matter, and the 

 surface of the axis itself. Under the microscope, this mem- 

 brane appears striped and perforated with oval openings; on 

 one or two occasions, in which the membrane was more than 

 usually unattached and fine, the stripes appeared to be com- 

 posed of cellules similar in shape and arrangement to those 

 described in the membrane of the fleshy crust. The oval 

 openings are continuous with the tubes of the polypes and 

 the rugae of the axis, and doubtless serve for the irausmission 

 of matter to the horny stem. This investing sheath is fre- 

 quently so obscure as to bid defiance to a separation from its 

 ultachments ; at this time the ruga^ generally are absent and 



