580 POLYZOA : FLUSTRA ; HALODACTYLUS. 



who has discovered what he terms a Colonial -Nervous system in 

 a Serialaria having a branching Polyzoary that spreads itself on 

 Sea-weeds over a space of three or four inches. A Nervous Ganglion 

 may be distinguished at the origin of each branch, and another 

 ganglion at the origin of each polypide-bud ; and all these ganglia 

 are connected together, not merely by principal trunks, but also by 

 plexuses of Nerve-Fibres, which may be distinctly made-out with 

 the aid of Chromic Acid in the cylindrical joints of the Polyzoary. 

 449. Of all the Polyzoa of our own coasts, the Flustrce or Sea- 

 Mats are the most common ; these present flat expanded surfaces, 

 resembling in form those of many Sea-Weeds (for which they are 

 often mistaken), but exhibiting, when viewed even with a low 

 magnifying power, a most beautiful network, which at once indi- 

 cates their real character. The cells are arranged on both sides ; 

 and it has been calculated by Dr. Grant, that as a single square 

 inch of an ordinary Flustra contains 1800 such cells, and as an 

 average specimen presents about 10 square inches of surface, it 

 will consist of no fewer than 18,000 Zooids. The want of trans- 

 parence in the cell-wall, however, and the infrequency with which 

 the animal projects its body far beyond the mouth of the cell, 

 render the Polyzoa of this genus less favourable subjects for micro- 

 scopic examination than are those of the Boiverbankia, a Polyzoon 

 with a trailing stem and separated cells like those of Laguncula, 

 which is very commonly found clustering around the basis of 

 Flustrse. It was in this that many of the details of the organiza- 

 tion of the interesting group we are considering were first studied 

 by Dr. A. Farre, who discovered it in 1837, and subjected it to a 

 far more minute examination than any Polyzoon had previously 

 received;* and it is one of the best-adapted of all the Marine forms 

 yetknown, for the display of the beauties and wonders of this type 

 of organization. — The Halodactylus (formerly called Alcyonidium), 

 however, is among the most remarkable of all the Marine forms for 

 the comparatively large size of the tentacular crowns ; these, when 

 expanded, being very distinctly visible to the naked eye, and pre- 

 senting a spectacle of the greatest beauty when viewed under a 

 sufficient magnifying power. The Polyzoary of this genus has a 

 spongy aspect and texture, very much resembling that of the 

 Alcyonian Zoophytes, for which it might readily be mistaken when 

 its contained animals are all withdrawn into their cells; when these 

 are expanded, however, the aspect of the two is altogether different, 

 as the minute plumose tufts which then issue from the surface of 

 the Halodactylus, making it look as if it were covered with the 

 most delicate downy film, are in striking contrast with the larger, 

 solid-looking polypes of the Alcyonium. The opacity of the poly- 

 zoary of the Halodactylus renders it quite unsuitable for the 



* See his Memoir ' On the Minute Structure of some of the higher 

 forms of Polypi' in the " Philosophical Transactions " for 1837. 



