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BLASTULARIA
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BLASTULARIA .
Dendriform See also:distribution of See also:animal See also:kingdom.
of cellular See also:embryology. On the one See also:hand, the true method of arriving at a knowledge of the genealogical See also:tree was recognized as lying chiefly in attacking the problem of the genealogical relationships of the smallest twigs of the tree, and proceeding from them to the larger branches. See also:Special studies of small families or orders of animals with this See also:object in view were taken in hand by many zoologists. On the other hand, a urvey of the facts of cellular embryology which were accumulated in regard to a variety of classes within a few years of See also:Kovalevsky's See also:work led to a generalization, independently arrived at by See also:Haeckel and Lankester, to the effect that a See also:lower grade of animals may be distinguished, the See also:Protozoa or Plastidozoa, which consist either of single cells or colonies of equiformal cells, and a higher grade, the Metazoa or Enterozoa, in which the See also:egg-See also:cell by " cell See also:division " gives rise to two layers of cells, the endoderm and the ectoderm, surrounding a See also:primitive See also:digestive chamber, the archenteron. Of these latter, two grades were further distinguished by Lankester—those which remain possessed of a single archenteric cavity and of two See also:primary cell-layers (the See also:Coelentera or Diploblastica), and those which by nipping off the archenteron give rise to two cavities, the coelom or See also:body-cavity and the metenteron or gut (Coelornata or Triploblastica). To the primitive two-cell-layered See also:form, the hypothetical ancestor of all Metazoa or Enterozoa, Haeckel gave the name Gastraea; the embryonic form which represents in the individual growth from the egg this ancestral See also:condition he called a " gastrula." The See also:term " diblastula. " was subsequently adopted in See also:England for the gastrula of Haeckel. The tracing of the exact mode of development, cell by cell, of the diblastula, the coelom, and the various tissues of examples of all classes of animals was in later years pursued with immense activity and increasing instrumental facilities.
Two names in connexion with See also:post-Darwinian taxonomy and the ideas connected with it require brief mention here. Fiatz Fritz See also: /at/oa. He is also responsible for the formulation of an important principle, called by Haeckel " the biogenetic fundamental See also:law," viz. that an animal in its growth from the egg to the adult condition tends to pass through a See also:series of stages which are recapitulative of the stages through which its ancestry has passed in the See also:historical development of the See also:species from a primitive form; or, more shortly, that the development of the individual (ontogeny) is an See also:epitome of the development of the See also:race (phylogeny). Pre-Darwinian zoologists had been aware of the class of facts thus interpreted by Fritz Muller, but the authoritative view on the subject had been that there is a See also:parallelism between (a) the series of forms which occur in individual development, (b) the series of existing forms from lower to higher, and (c) the series of forms which succeed 'one another in the strata of the See also:earth's crust, whilst an explanation of this parallelism was either not attempted, or was illusively offered in the shape of a See also:doctrine of See also:harmony of See also:plan in creation. It was the application of Fritz See also:Miller's law of recapitulation which gave the See also:chief stimulus to embryological investigations between 186 and 1890; and, though it is now recognized that " recapitulation " is vastly and bewilderingly modified by special adaptations in every See also:case, yet the principle has served, and still serves, as a See also:guide of See also:great value. Another important See also:factor in the See also:present condition of zoological knowledge as represented by See also:classification is the doctrine of degeneration propounded by Anton Dohrn. See also:Lamarck believed in a single progressive series of forms, whilst See also:Cuvier introduced Dohrn's the conception of branches. The first post-Darwinian doctrine systematists naturally and without reflexion accepted of degen- the See also:idea that existing simpler forms represent stages era[/on. in the See also:gradual progress of development—are in fact survivors from past ages which have retained the exact grade of development which their ancestors had reached in past ages. The See also:assumption made was that (with the rare exception of parasites) all the See also:change of structure through which the successive generations of animals have passed has been one of progressiveelaboration. It is Dohrn's merit to have pointed out 1 that this assumption is not warranted, and that degeneration or progressive simplification of structure may have, and in many lines certainly has, taken See also:place, as well as progressive elaboration and in other cases continuous See also:maintenance of the status quo. The introduction of this conception necessarily has had a most important effect in the See also:attempt to unravel the genealogical See also:affinities of animals. It renders the task a more complicated one; at the same See also:time it removes some serious difficulties and throws a See also:flood of See also:light on every group of the animal kingdom. One result of the introduction of the new conceptions dating from Darwin was a healthy reaction from that attitude of mind which led to the regarding of the classes and orders recognized by authoritative zoologists as sacred institutions which were beyond the See also:criticism of See also:ordinary men. That See also:state of mind was due to the fact that the groupings so recognized did not profess to be simply the result of scientific reasoning, but were necessarily regarded as the expressions of the " insight " of some more or less gifted persons into a plan or See also:system which had been arbitrarily chosen by the Creator. Consequently there was a tinge of theological dogmatism about the whole See also:matter. Sub-Grade A. GCELENTERA. Grade 2. ENTEROZOA. V \ Grade I. PROTOZOA. A genealogical tree of animal kingdom (Lankester, 1884). To deny the Linnagan, or later the Cuvierian, classes was very much like denying the See also:Mosaic See also:cosmogony. But systematic See also:zoology is now entirely See also:free from any such prejudices, and the Linnaean taint which is apparent even in Haeckel and See also:Gegenbaur may be considered as finally expunged. _ There are, and probably always will be, See also:differences of See also:opinion as to the exact way in which the various kinds of animals may be divided into See also:groups and those groups arranged Lan-in such an See also:order as will best exhibit their probable kester's genetic relationships. 
The See also:main divisions which, system. See also:writing in 1910, the present writer prefers, are those adopted in his See also:Treatise on Zoology (See also:Part II. ch. ii.) except that Phylum r7, Diplochorda (a name doubtfully applicable to Phoronis) is replaced by Podaxonia, a term employed by Lankester in the 9th edition of this See also:encyclopaedia and now used to include a number of groups of doubtful but possible See also:affinity. 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 See also:branch or See also:divide into orders, orders into families, families into genera, genera into species. The See also:general purpose is to give something like an equivalence of importance to divisions or branches indicated by the same term, but it is not intended to imply that every phylum has the Ursprung der Wirbelthiere (See also:Leipzig, 1875) ; and Lankester, Degeneration (See also:London, 188o) Sub-Grade B CGELOMATA. Q • ~o 'e same range and distinctive See also:character as every other, nor to make such a proposition about classes, orders, families and genera. Where a further subdivision is desirable without descending to the next lower term of grouping, the prefix "sub" is made use of, so that a class may be divided first of all into sub-classes each of which is divided into orders, and an order into sub-orders each of which bears a group of families. The term " grade " is also made use of for the purpose of indicating the conclusion that certain branches on a larger or smaller See also:stem of the genealogical tree have been given off at an earlier See also:period in the See also:history of the See also:evolution of the stem in question than have others marked off as forming a higher grade. Thus, to begin with, the animal See also:pedigree is divided into two very distinct grades, the Protozoa and the Metazoa. The Metazoa form two main branches; one, Parazoa, is but a small unproductive stock comprising only the Phylum Porifera or See also: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). These relations are exhibited by the two following diagrams. /ARAZOA ENTEROZDA Grade B.METAZOA. Grade A PROTOZOA. See also:Diagram showing the primary grades and branches of the Animal Pedigree. Grade B. COELO1OGOELA. ' 9eeomPO°fie Cleo 000 e/ M tho3oa \ Grade A. ENTEROCOELA. Branch B. ENTEROZOA. Diagram to show the division of the great branch Enterozoa into two grades and the Phyla given off therefrom. The Phylum See also:Vertebrata in the above See also:scheme branches into the sub-phyla See also:Hemichorda, Urochorda, Cephalochorda and Craniata. The Phylum See also:Appendiculata similarly branches into sub-phyla, viz. the See also:Rotifera, the See also:Chaetopoda and the See also:Arthropoda. Certain additional small groups should probably be recognized as See also:independent lines of descent or phyla, but their relationships are obscure—they are the See also:Mesozoa, the See also:Polyzoa, the See also:Acanthocephala and the See also:Gastrotricha. We may now enumerate these various large groups in See also:tabular form. BIONTA—PHYTA, ANIMALIA. Additional information and CommentsThere are no comments yet for this article.
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