MORPHOLOGY, a term used in zoology and botany to cover all those studies in which the centre of interest is the form and structure of an organism. It is thus the antithesis of physi ology (q.v.).
The first step in morphology is to determine and record the shape of a whole animal. This shape never exists as a permanent quality; it is constantly varying with the animal's movements even in the case of animals with a hard skeleton such as the lamelli branchs or the crustacea.
There is nevertheless in the vast majority of cases a definite plan visible at any time for each individual animal. The creature may be free-moving or it may be fixed to the ground either per manently or for the time being. It may have no axis of symmetry and be indefinite in shape like an amoeba, or it may be bilaterally symmetrical like a fish, or radi ally symmetrical like a star fish. It may like a sea anemone be apparently radially symmetrical but actually possess an under lying bilateral symmetry. Such symmetry as it possesses may be externally perfect, or it may be modified by differences in the development of similar appendages on the opposite sides, as in the case of the great claws of a lobster.
Quick movement of an animal either through water or air, whether in contact with a substratum or not, can only be achieved if the animal is bilaterally symmetrical, because only then, the animal moving in one direction, will the impulses which the animal receives from the activities of its muscles be symmetrically disposed so as to ensure a straight track. On the other hand an animal which lives constantly attached to the sea bottom in most cases receives its food from all directions or from above, and must be so designed that it can catch it with equal ease whence soever it comes. Thus a radial symmetry is most suitable for a sedentary 'animal.
The analysis of the external form of a single animal can often be carried much further. The motion of a fish swimming in water is hindered by many factors, which have been disentangled by naval architects. Unless its shape conforms to a special geo
metrical figure, the "streamlined form," it will create eddies which absorb energy, it will produce how and tail waves, and the water will exert a definite frictional resistance to motion, varying with the smoothness of the skin and with some other qualities. Thus it should be possible to predict the ideal shape for a fish. But this exact form will be unrealisable in actual life, because it is necessarily modified to secure stability, both when the fish is motionless, and also in the sense of maintainance of its course when swimming. Furthermore the needs of feeding and of taking in and expelling the water which is necessary for respiration modify the shape of the anterior part, while the hinder end is conditioned by the area and shape of the tail fin and the muscles which move it.
Thus any explanation of the shape of a fish is necessarily com plicated, so many factors entering into it that anything more than a rough analysis is impossible.
But those features of the external form of an animal which have a directly adaptive significance are mere modelling carried out on a basis of fundamental structure which is in essence uniform throughout large groups of allied species.
The skeleton and somatic muscular system are only significant in relation to their function of bringing about movements, either of the animal as a whole, or of the jaws and other organs for such special purposes as the capture of food. Theoretically there is a most efficient shape for each bone and each muscle in the body of a fish, but exactly as in the case of the external form this ideal is never attained, in part because few organs or parts of organs are so isolated that they can be exclusively devoted to one function unmodified by the impact of neighbouring structures, and in part because their fundamental architecture is determined. not by present needs, but by those of a chain of ancestors which may be very remote and very different in habits.