POLYZOA FLUSTRA, BOWERBANKIA. 497 



both sides ; and it has been calculated by Dr. Grant, that as a 

 single square inch of an ordinary F lustra 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 transparency in the cell-wall, however, and the infrequency 

 with which the animal projects its body far beyond the rnouth of 

 the cell, renders 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 

 bases of Flustrse. 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 sub- 

 jected it to a far more minute examination than any Polyzoon 

 had previously received j 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 Alcyonidium), however, is among the most remarkable of 

 all the marine forms, for the comparatively large size of the ten- 

 tacular 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 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 withdrawn into their cells ; when these are expanded, however, 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 Halodac- 

 tylus, making it look as if it were covered with the most delicate 

 downy film, are in striking contrast with the larger, solid-looking 

 olypes of the Alcyonium. The opacity of the polyzoary of the 

 alodactylus renders it quite unsuitable for the examination ot 

 anything more than the tentacular crown and the oesophagus 

 which it surmounts; the stomach and the remainder of the visce- 

 ral apparatus being always retained within the cell. Several of 

 the fresh- water Polyzoa are peculiarly 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 cili- 

 ated tentacula are arranged upon a deeply crescentic or horse- 

 shoe-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 Infundibulata (funnel-like). 



330. The Infundibulata or Marine Polyzoa, constituting by far 

 the most numerous division of the class, are divided into four 



1 See his Memoir in the " Philosophical Transactions," for that year. 



32 



