GASTROPODA, the second of the five classes of animals constituting the phylum Mollusca.
The phylum is subdivided as follows.
PLATYELMIA, a phylum of the animal kingdom which comprises three classes, the Planarians, Trematodes and Cestodes.
The Phylum Vertebrata in the above scheme branches into the sub-phyla Hemichorda, Urochorda, Cephalochorda and Craniata.
This revolution may be accomplished by adding the term " mutation ascending " or " mutation descending " for the minute steps of transformation, and the term phylum, as employed in Germany, for the minor and major branches of genetic series.
Ope yxta, gills), the fourth of the five classes of animals constituting the phylum Mollusca.
CHITON, the name 1 given to fairly common littoral animals of rather small size which belong to the phylum Mollusca, and, in the possession of a radula in the buccal cavity, resemble more especially the Gastropoda.
There is no doubt that the phylum of Angiosperms has not sprung from that of Gymnosperms.
This assemblage is now generally regarded as a great division (phylum or sub-phylum) of the animal kingdom and known by K.
They are not, therefore, like the wings of birds, modified from some pre-existing structures (the fore-limbs) common to their phylum; they are new and peculiar structures.
Of the animals included in the class Hexapoda of the great phylum (or sub-phylum) Arthropoda.
The suggestion to place Brachiopods with the Polyzoa, Phoronis, Rhabdopleura and Cephalodiscus, in the Phylum Podaxonia made in Ency.
The position of the Arachnida in the great sub-phylum Arthropoda, according to recent anatomical and embryological researches, is explained in the article Arthropoda.
The terms used for indicating groups are " Phylum " for the large diverging branches of the genealogical tree as introduced by Haeckel, each Phylum bears secondary branches which are termed " classes," classes again branch or divide into orders, orders into families, families into genera, genera into species.
The Metazoa form two main branches; one, Parazoa, is but a small unproductive stock comprising only the Phylum Porifera or Sponges; the other, the great stem of the animal series Enterozoa, gives rise to a large number of diverging Phyla which it is necessary to assign to two levels or grades - a lower, Enterocoela (often called Coelentera), and a higher, Coelomocoela (often called Coelomata).
P. Lamarck's term Annelides, now used to denote a major phylum or division of coelomate invertebrate animals.
Vertebrate palaeontologists were slow to grasp this principle; while the early speculative phylogenies of the horse of Huxley and Marsh, for example, were mostly displayed monophyletically, or in single lines of descent, it is now recognized that the horses which were placed by Marsh in a single series are really to be ranged in a great number of contemporaneous but separate series, each but partially known, and that the direct phylum which leads to the modern horse has become a matter of far more difficult search.
As the Turbellaria (Planarians) are the most primitive division of the Platyelmia, the problem of the affinities of this phylum resolves itself into that of the relationships of the Turbellaria.
Ray Lankester (preface to the English edition of C. Gegenbaur's Comparative Anatomy), and employed by the same writer in the 9th edition of this encyclopaedia (article "Zoology") to denote the eighth phylum, or major division, of coelomate animals.
For a discussion of the relationship of the Gastropoda to the remaining classes of the phylum, see MoLLUSCA.
For the class of the true insects included in this phylum, Linnaeus's old term Insecta, first used in a restricted sense by M.
Ii.) except that Phylum 17, Diplochorda (a name doubtfully applicable to Phoronis) is replaced by Podaxonia, a term employed by Lankester in the 9th edition of this encyclopaedia and now used to include a number of groups of doubtful but possible affinity.
The Phylum Appendiculata similarly branches into sub-phyla, viz.
TREMATODES, or flukes (as they are called from their fish-like shape), one of the three classes that compose the phylum Platyelmia.
The correctness of this association is questionable, and the Polyzoa are here treated as a primary division or phylum of the animal kingdom.
The main limitations of the sub-kingdom or phylum Mollusca, as laid down by Cuvier, and the chief divisions thus recognized within its limits by him, hold good to the present day.
Increase of knowledge has now, however, established the conclusion that the agreement of structure supposed to obtain between Polyzoa and true Mollusca is delusive; and accordingly they, together with the Brachiopoda, were removed from the Molluscan phylum by Lankester in his article in the 9th edition of this work (on the which present article is based).
(9th edit.), even though their numbers have been reduced by the elevation of the Sponges or Porifera to the rank of an independent Phylum under the title Parazoa (W.
AMPHIOXUS, or Lancelet, the name of small, fish-like, marine creatures, forming the class Cephalochorda, of the phylum Vertebrata.
Every great group or phylum of vascular plants, when it has become dominant in the vegetation of the world, has produced members with the tree habit arising by the formation of a thick woody trunk, in most cases by the activity of a cambium.
From the preceding discussion an idea may be formed of the primitive characters of the Phylum (From Gegenbaur.) FIG.