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).
The Phylum Vertebrata in the above scheme branches into the sub-phyla Hemichorda, Urochorda, Cephalochorda and Craniata.
The Phylum Appendiculata similarly branches into sub-phyla, viz.
Certain additional small groups should probably be recognized as independent lines of descent or phyla, but their relationships are obscure - they are the Mesozoa, the Polyzoa, the Acanthocephala and the Gastrotricha.
Sub-phyla: Rotifera, Chaetopoda, Arthropoda.
Sub-phyla: Hemichorda, Urochorda, Cephalochorda, Craniata.
This appears to indicate B that the Polyzoa are remotely allied to other phyla in which this type of larva prevails, and in particular to the Mollusca and Chaetopoda, as well as to the Rotifera, which are regarded as persistent Trochospheres.
MOLLUSCA, one of the great " phyla," or sub-kingdoms, of the animal pedigree or kingdom.
The structural features which the Mollusca do possess in common with other animals belonging to other great phyla of the animal kingdom are those characteristic of the Coelomata, one of the two great grades (the other and lower being that of the Coelentera) into which the higher animals; or Metazoa as distinguished from the Protozoa, are divided.
The Mollusca agree in being coelomate with the phyla Vertebrata, Platyhelmia (flat-worms), Echinoderma, Appendiculata (insects, ringed-worms, &c.), and others - in fact, with all the Metazoa except the sponges, corals, polyps, and medusae.
(1896), viii., 137, and the "Four Phyla of Oligocene Titanotheres," op. cit.
There are two main groups, or sub-phyla: the Graptoloidea or Graptolites proper, and the Dendroidea or tree-like Graptolites; the former is typified by the unbranched genus Monograptus and the latter by the many-branched genus Dendrograptus.
Agassiz; but it was the researches of Johannes Miller (1840-1850) that formed the groundwork of scientific conceptions of the group, proving it one of the great phyla of the animal kingdom.
But the study that should elucidate the fundamental similarities or homologies between the several classes, and should suggest the relations of the Echinoderma to other phyla, had scarcely begun.
The problem of the interrelations of the classes will thus be reduced to its simplest terms, and even questions as to the nature of the primitive Echinoderm and its affinity to the ancestors of other phyla may become more than exercises for the ingenuity of youth.
The small groups of recent plants forming the Psilotales and Ophioglossales are given independence in this scheme of classification owing to their exact affinities with the other phyla being at present doubtful.
So long, however, as our knowledge of these phyla is confined, as at present, to specialized forms, the nature of the relationship between them must remain to some extent hypothetical.
The important bearing of this question on the relationship of the Ophioglossaceae to the phyla of the Filicales and Lycopodiales will be obvious.
The several phyla of Pteridophyta having now been briefly described, their relationship to one another remains for con.
As to the relationship of the Filicales to the other phyla, evidence from extinct plants appears to be wanting.
If, as has been suggested by Bower, the strobiloid types are relatively primitive, the large-leaved Pteridophyta must be supposed to have arisenearly from such forms. The question cannot be discussed fully here, but enough has been said above to show that in the light of our present knowledge the main phyla of the Vascular Cryptogams cannot be placed in any serial relationship to one another.
ARTHROPODA, a name, denoting the possession by certain animals of jointed limbs, now applied to one of the three sub-phyla into which one of the great phyla (or primary branches) of coelomocoelous animals - the Appendiculata - is divided; the other two being respectively the Chaetopoda and the Rotifera.
It is the existence in each ring of the body of a pair of hollow lateral appendages or parapodia, moved by intrinsic muscles and penetrated by bloodspaces, which is the leading fact indicating the affinities of these great sub-phyla, and uniting them as blood-relations.
- The characters of the subphylum and those of the associated sub-phyla Chaetopoda and Rotifera have been given above, as well as the general characters of the phylum Appendiculata which comprises these great sub-phyla.
Diagram to show the division of the great branch Enterozoa into two grades and the Phyla given off therefrom.
Thus both invertebrate and vertebrate palaeontologists have reached independently the conclusion that the evolution of groups is not continuously at a uniform rate, but that there are, especially in the beginnings of new phyla or at the time of acquisition of new organs, sudden variations in the rate of evolution which have been termed variously " rhythmic," "pulsating," " efflorescent," "intermittent " and even " explosive " (Deperet).
The whole " system " or scheme of classification was termed a genealogical tree (Stammbaum); the main branches were termed " phyla," their branchings " sub-phyla "; the great branches of the sub-phyla were termed " cladi," and the " cladi " divided into " classes," these into sub-classes, these into legions, legions into orders, orders into sub-orders, suborders into tribes, tribes into families, families into genera, genera into species.