A. Alcyonellea. Alcyonidium, Flustrella.
B. Vesicularina. Vesicularia, Amathia, Bowerbankia,
Farrella, Hypophorella, Triticella, Mimosella,
Victorella, Paludicella.
Order 2. Phylactolaemata.
Fredericella, Plumatella (including Alcyonella), Lophopus,
Cristatella, Pectinatella.
Even this classification, which deals only with the larger groups, must not be made use of without a word of warning. The division of the Cheilostomata is a matter of great difficulty; and no scheme which has yet been suggested can be regarded as more than tentative. The great number of forms included in this group makes its subdivision extremely desirable from the point of view of convenience; but a further knowledge of the anatomy and of the development of many of the forms of doubtful systematic position is probably necessary before any scheme which is likely to be permanent is put forward. Those who desire to make a further study of the classification of the Polyzoa should refer to the works of Hincks,[[590]] Busk,[[591]] MacGillivray,[[592]] and Gregory.[[593]]
The Polyzoa do not appear to lend any valuable assistance towards settling the disputed problems of Geographical Distribution. They are not in any case terrestrial, while the fresh-water species do not always respect the limits between the great zoogeographical regions. It has already been pointed out (p. [504]) that Plumatella, Fredericella, and Lophopus are believed to occur in Australia, and the first-named genus is practically world-wide in its distribution.
Many marine forms also have a surprisingly wide distribution. Thus among the British species which are described by Mr. Hincks as occurring from Norway to New Zealand are Membranipora pilosa, Scrupocellaria scruposa, Cellaria fistulosa, Microporella ciliata, and M. malusii. Even if it should be proved that specific differences do exist between the southern forms and our own, there can be no doubt of the wide distribution of certain species. It was pointed out by D'Orbigny that Bugula neritina has the habit of attaching itself to the bottoms of ships, a fact which may possibly account for the wide distribution of this species; although it would not be safe to assume this explanation of the facts in all cases. Other Polyzoa, on the contrary, have a more restricted range. Thus Catenicella is specially characteristic of the Australian region.