9O8 POLYZOA AND TUNICATA 



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 poly- 

 zoary. His views, however, are not now accepted, observers of 

 great histological experience maintaining that what he regards as 

 nerve-fibres are only connective tissue. 



Of all the Polyzoa of our own coasts the Membraniporidce, or 

 ' sea-mats ' (Flustra, Membranipora), 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 generally 

 arranged on both sides, and it was calculated by Dr. Grant that as 

 a single square inch of an ordinary Flustra contains 1,800 such cells, 

 and as an average specimen presents about ten square inches of 

 surface, it will consist of no fewer than 18,000 polypides. The want 

 of transparence in the cell-wall, however, and the infrequency with 

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

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

 scopic 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 

 Mustrse. It was in this that many of the details of the organisation 

 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 ; l 

 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 organisa- 

 tion. The Alcyonidium, 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 magnifying power. The polyzoary of this 

 genus has a spongy aspect and texture, very much resembling that of 

 certain 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 when its contained animals are all withdrawn into their cells ; when 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 Alcyonidium, 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 polyzoary 

 of the Alcyonidium renders it quite unsuitable for the examination of 

 anything more than the tentacular crown and the oesophagus which 

 it surmounts, the stomach and the remainder of the visceral appa- 

 ratus being always retained within the cell. It furnishes, however, 

 a most beautiful object for the binocular microscope, when mounted 

 with all its polypides expanded. 2 Several of the fresh- water Polyzoa 

 are peculiarly interesting subjects for microscopic examination, alike 



1 See his memoir ' On the Minute Structure of some of the Higher Forms of 

 Polypi,' in the Phil. Trans, for 1837, p. 387. 



2 Mr. J. Lomas has detected calcareous spicules in Alcyonidium gelatinosum, 

 and finds that they are more abundant in older than in younger colonies. See Proceed- 

 ings of the Liverpool Geological Society, v. p. 241. 



