POLYZOA AND TUNIC AT A. 161 



fibres, which may be distinctly made-out with the aid of Chromic acid in 

 the cylindrical joints of the polyzoary. His views, however, have not 

 been universally accepted; some observers still maintaining that what he 

 regards as nerve-fibres are only connective tissue. 



552. Of all the Polyzoa of our own coasts, the Flustrm 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 with even a low magnifying power, a most 

 beautiful network, which at once indicates their real character. The 

 cells are arranged on both sides; and it was 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 polypides. 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 favorable subjects for microscopic examination 

 than are those of the Bowerbankia, a Polyzoon with a trailing stem and 

 separated cells like those of Laguncula, which is very commonly found 

 clustering around the base of masses of Flustrae. It was in this that 

 many of the details of the organization 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 Poly- 

 zoon had previously received; 1 and it is one of the best adapted of all 

 the marine forms yet known, for the display of the beauties and wonders 

 of this type of organization. The Halodactylus (formerly called Alcyo- 

 nidium), however, is one of 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 presenting 

 a spectacle of the greatest beauty when viewed under a sufficient magni- 

 fying power. The polyzoary of this genus has a spongy aspect and 

 texture, very much resembling that of certain Alcyonian Zoophytes 

 ( 529), 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 animals are all withdrawn into their cells; when these are expanded, 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 Alcyo- 

 nium. The opacity of the polyzoary of the Halodactylus renders it 

 quite unsuitable for the examination of anything more than the tentacu- 

 lar crown and the oesophagus which it surmounts; the stomach and the 

 remainder of the visceral apparatus being always retained within the 

 cell. It furnishes, however, a most beautiful object for the Binocular 

 Microscope, when mounted with all its polypides expanded, in the man- 

 ner described in 521. Several of the fresh-water Polyzoa are pecu- 

 liarly interesting subjects for Microscopic examination; alike on account 

 of the remarkable distinctness with which the various parts of their 

 organization may be seen, and the very beautiful manner in which their 

 ciliated tentacula are arranged upon a deeply-crescentic or horseshoe- 

 shaped lophophore. By this peculiarity the fresh-water Polyzoa are 

 separated as a distinct sub-class from the marine; the former being 

 designated as Hippocrepia (horseshoe-like), while the latter are termed 



1 See his Memoir ' On the Minute Structure of some of the higher forms of 

 Polypi, 1 in the " Philosophical Transactions" for 1837, p. 387. 

 11 



