THE ORGANIZATION OF THE PROTOZOA 31 



resting, inactive condition, or when the vital activity is temporarily 

 inhibited by some shock or stimulus, such as an electric current 

 suddenly turned on, physical forces reassert their sway, and under 

 the influence of surface-tension the pseudopodia are retracted, and 

 the body rounds itself off and returns to the spherical form. 



Apart, however, from temporary and variable departures from 

 the primary and fundamental spherical form, many unicellular 

 organisms exhibit a constant body-form which is often widely 

 different from the sphere, and which is characteristic of particular 

 species, or for the corresponding stages in the life-history of a 

 given species, and varies only within the narrowest limits, if at all. 

 The problem of form-production in Protozoa, like all other bio- 

 logical problems, may be considered from two points of view. In 

 the first place, there is the question why a particular species has 

 such and such a form. The answer to this question must be sought 

 in the habits and mode of life of the species and its relation to the 

 environment. In general it may be said that each species pos- 

 sesses, or tends to possess, the body-form best adapted to its par- 

 ticular mode of life, though it is not always easy to trace the 

 correlation of form and habit in special cases. A broad distinction 

 may be drawn, however, between species which move freely in 

 their environment and those which are fixed and sessile in habit. 

 In freely-moving species, again, a further distinction can be drawn 

 between those which float or swim in the medium, and those which 

 creep on a firm substratum. Free-swimming species tend to the 

 form of an ovoid, more or less elongated, with the longitudinal axis 

 lying in the direction of forward movement (Fig. 14). Creeping 

 forms tend to be more or less flattened, and spread, as it were, upon 

 the substratum, leading in extreme cases to the differentiation of a 

 ventral surface, in contact with the substratum, from a dorsal 

 surface on the opposite side. Sedentary forms tend to be more or 

 less vasiform, often with the point of attachment drawn out into a 

 stalk or peduncle of greater or less length. A frequent peculiarity 

 of the body-form in Protozoa, whether fixed or free, is the tendency 

 to a more or less pronounced spiral twist. Bilateral symmetry, on 

 the other hand, is a comparatively rare phenomenon in these 

 organisms ; examples are found among the Flagellata e.g., Lamblia 

 intestinalis (Fig. 117). 



The second question which arises is, Given a particular specific 

 form, how is the form developed and maintained, on physiological 

 or mechanical principles ? To this question the answer must be 

 sought in the structure of the individual, and more especially in 

 the formation and possession of special structural elements, more 

 or less rigid in nature, which determine the form and support the 

 soft body. Such structures may be external to the body, in the 



