CHAPTER XII.

THE TYPES OF CONSTRUCTION IN THE ANIMAL

KINGDOM.

Records of Evolution expressed chiefly in Generic and Specific Characters. From what has been said in the previous chapter it will be learned that the grand features and the great majority of the more important details of the structure of any living organism are of extreme antiquity. Not only so, but since very early geological time no new types of structure of as high as ordinal rank have been evolved in the majority of the branches of the Animal Kingdom.

In respect, therefore, to a great number of the more important characters of organisms the development of offspring has resulted in the repetition, without substantial modification, of the characters of the ancestors. This is the law of Heredity-the repetition in the offspring by generation of characters like those of its ancestors. Evolution has to do with the acquirement by organisms of morphological characters which their ancestors did not possess; hence we must seek for evidences of evolution chiefly among the characters of less than ordinal rank-those of ordinal and higher rank having been evolved almost at the beginning of the history.

Course of Individual Development supposed to have been Constant. It is not unreasonable to assume that all the course and the stages of development, of characters of ordinal and higher rank in the development of the individual, are repetitions of what has taken place since their first appearance at the beginning of the geological time-record. In the several types of organisms now living, the laws of individual development, as of the steps by which in each case diversity is elaborated out of simplicity of structure, may reasonably be regarded as applicable to all organismas of which we can study

the history. The reason why the course of development has been what it is may be no more evident than the reason why gold is yellow and heavier than sulphur; in a particular case the sufficient reason is that it is like that of its ancestors.

Beginning of Individual Life and Development.—In a previous chapter the stages of development of the individual are described. It is there shown how the simple cell is without distinction of parts, other than as protoplasm with cell-walls; a cell-nucleus, which is of great importance, and regarding which recent investigations with high power of the microscope are bringing out wonderful characters and functions; and a vacuole, often present, but the function of which is unknown. From such a cell the individual grows to the state of a complex, independent organism, such as the living Vertebrate, seen in its highest representative, Man.

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Hypotheses regarding the Phylogenetic Evolution of Races. -The term Ontogeny has been applied to this development, and to distinguish it therefrom, Phylogeny, or race-development, has been proposed to indicate the analogous passage from the simplest undifferentiated Protozoan, the Amoeba, or Monera, through the several stages of increasing complexity of organization to the most highly differentiated Vertebrate. Many attempts have been made to construct the history of the whole organic world on this basis, i.e., to construct phylogenetic trees of the ancestors of beings now living on the earth. Haeckel's History of Creation" is one of the earlier and most elaborate, and perhaps most artificial, of such treatises; for as science has developed, our knowledge of the true genetic relationship in some particular lines of organisms has greatly increased. When Haeckel's work was published (1868), the new methods of investigation, so greatly stimulated by the appearance of Darwin's "Origin of Species," had only begun to affect the students of fossil remains; and it is mainly since that date that the classification of organisms has been revised on the basis of genetic affinities determined by comparative studies of structure.

The analysis of organic structure, from the phylogenetic point of view, is very instructive and suggestive if it be not overdone. It helps us to attain general notions of organiza

tion, or what we may call the principles of construction of the Animal Kingdom.

The Undifferentiated Cell. From this point of view the primitive living organism is assumed to be an undifferentiated cell, having no tissues, no organs, no permanently specialized functions. If it moves, the motion is spontaneous, irregular, temporary motion; if it takes food, it is by attaching the food to itself; and in a sense such a protozoal cell is all mouth, all stomach, all everything necessary to living, but nothing particular in any part of itself is permanently different from any other part: it is an undifferentiated organism. The amoeba comes nearest to fulfilling these homogeneous conditions, but even there appear the nucleus and the contractile vacuoles, which are differentiated, and perform some, though not well understood, special functions.

FIG. 51.-Amaba proteus (after Grüber), greatly enlarged. cv contractile vacuole, n = nucleus, ps pseudopodium.

In the simplest form of the metazoal cell very considerable complexity is found at the earliest stage in which the cell is observed. The steps by which the cell reaches the organic structure which is characteristic of any of the metazoa when adult is explained in works on embryology and animal morphology.*

When we look at the progress more rapidly, and note the steps of progress in function rather than in structural mor

*See McMurrich, "Text-book of Invertebrate Morphology," chapter IX., Subkingdom Metazoa.

phology, we observe that in attaining differentiation from this simple state several systematic groups of differences are expressed. The first is concerned with general direction of motion, expressing itself in the arrangement of the body shape, or in its development.

Polarity. If we imagine the primitive form to be a globe, its motion is expressed by assuming polarity of directiona definite anteriority, or direction toward which motion approaches, and the opposite, posteriority, from which it goes. Every living animal having reached the first stage of differentiation (seen in the Metazoa, as the Coelenterata, for instance) expresses some degree of polarity. The longitudinal axis of the body, in the Metazoa of this simplest form, is clearly expressed, and the anterior end is primarily determined by the position of invagination in the growth of the embryo forming the gastrula.

FIG. 52.-A simple coral polyp (Me- Thus the simple coral polyp is

tridium marginatum Les.), rep

resenting the gastrula stage of dif- a mature animal representing the

ferentiation, in which the posterior

end of the body B is attached and Gastrula stage of embryonic de

the anterior end A is free.

velopment of higher animals.

In Fig. 52 the anterior end of the axis of the body, AB, is at A, which is the mouth or oral end of the enteron or digestive cavity. This is the centre of the free end of the body, and the opposite end, B, is in mature stage often fixed.

Antimeres and Metameres.-As such an organism is supposed to develop parts by differentiation, these parts are arranged in one of the following three ways: radially, or around the axis, when they are called Antimeres; or one after the other in the direction of the long axis, when they are called Metameres; or, third, without repetition of parts, except to express bilateral symmetry and a dorso-ventral opposition of parts.

Radiate Structure, Bilateral Symmetry, and Actinimeres.-The primary axis (AB in Fig. 52) is the one which is longitudinal

to the body, and the secondary axis is at right angles or transverse to this. In the course of growth repetition of parts is first noticed as as evidence

of elevation of rank, and the organism which has no duplication, or multiplication of parts, is lower in the scale, because less differentiated, than one in which there is multiplication of parts. Where there is multiplication of parts the simplest mode of arrangement is around the longitudinal axis. When each of the parts about the axis is alike there is

că

radiate structure (see the tentacles, t, FIG. 53.-Coral animal and its cal

of Fig. 53). This is the case in the coral animal, or in the starfish, and the separate parts are called antimeres; thus the tentacles of the coral, or the arms of the starfish (Fig. 54), are antimeres, or opposed parts.

careous base, Asteroides calycularis Lmk. Longitudinal section. s, cd calcareous skeletal base, the "coral; " o = mouth; t = tentacles; m = the mesentery folds of the body; c = cœnosarc connecting the individual polyps of the colony; f = chambers between the mesenteries. (After Steinmann and Döderlein.)

When there is difference

FIG. 54.-A typical radiate, Starfish, Asterias areniccla. (After Agassiz.) among these parts, and there are series of parts opposed to each other, the differentiation has progressed one step higher,