668 LIFE : OUTLINES OF GENERAL BIOLOGY 



co-operant cells, and (5) as an integrated multitude of protoplasmic 

 vortices in which diverse chemical reactions are going on with great 

 rapidity. It is the same in morpholog>^ for there are necessarily the 

 same levels of structural analysis. 



(1) The organism must be studied as an intact unity with a 

 certain shape and symmetry. (II) Scalpel in hand, in the case of 

 animals, the anatomist must disclose their larger organs, such as 

 brain and sensory structures, heart and lungs, liver and kidneys. 



(III) With hand lens and microscope he must then unravel their 

 web of tissues — nervous, muscular, glandular, and connective. 



(IV) But every component cell is a microcosm with great intricacy 

 of detail; and (V) the protoplasm itself has its colloidal film-work 

 and ultra-microscopic particles. 



FORM AND SYMMETRY.— The shape of a great mass of crystals, 



as in a snow-wreath, is partly determined by the molecular consti- 

 tution of the material, and partly by the environmental influences 

 that play on it while it is being formed. Similarly, but in a more 

 complex way, the form and shape of an organism depend on the 

 nature of the protoplasmic material, and on the surrounding 

 influences. For some reason that we cannot at present analyse, a 

 particular kind of sponge or coral has to make certain forms of 

 spicules, which determine the general architecture of the skeleton; 

 on the other hand, the shape of the sponge or the coral often varies, 

 according to the currents that play on it, or the substratum on 

 which it develops and grows. 



As regards symmetry, animals may be grouped as {a) radial, 

 (b) bilateral, and (c) asymmetrical, (a) In a radially symmetrical 

 animal, such as a jellyfish, the body can be divided into two mirroring 

 halves by a number of vertical planes. It is symmetrical around a 

 vertical median axis, and it cannot be said to have a right or a left 

 side. This is the symmetry of the frequently occurring gastrula 

 stage in development, and the symmetry of the hypothetical, but 

 highly probable, gastnea ancestor of multicellular animals. Going 

 farther back still, we find radial symmetry in a Protozoon colony 

 like Vol vox, with hundreds to thousands of cells united in a sphere, 

 though as a matter of fact one pole of the sphere is almost always 

 in front in the beautiful spiral swimming. Farther back still, many a 

 unicellular organism, both among Protozoa and Protophytes, is 

 radially symmetrical; very often it is spherical or oval in shape. 



It is necessary to distinguish, from thorough radial symmetry, 

 what may be called superficial radial symmetry. Thus although a 

 sea-anemone is a tubular animal, superficially quite radial, there 

 are only two cuts that will divide the body into two identical halves, 

 namely (i) the vertical cut which passes through the two ciliated 

 gullet-grooves or siphonogljT^hs, and (2) the vertical cut at right 



