38 THE INVERTEBRATA 



cases (e.g. the " ciliospores " of the Suctoria) it is applied also to 

 products of budding. A cyst in which several spores are enclosed is 

 a sporocyst. Individual spores may be enclosed in spore cases^ when 

 they are chlamydospores (as those of the Mycetozoa, Fig. 73 A), or 

 naked, when they are gymnospores. The latter may be amoeboid 

 (amoebulae or pseudopodiospores, e.g. Amoeba, Polystomella, Fig. 66 C), 

 flagellate {flagellulae or flagellispores, e.g. Polystomella, Fig. 66 B, 

 Chlamydomonas), or ciliate {ciliospores, e.g. the Suctoria, Fig. 87 H). 

 Spores may be gametes (e.g. the Mycetozoa, Chlamydomonas), or 

 serve for the distribution of the species, when, if they are motile, they 

 are known as "swarm spores". The sporoblasts of many telosporidia 

 (e.g. Plasmodium, Fig. 75, 16-18) are spore-like bodies which are 

 not set free, but give rise under cover to another generation of spores. 

 The so-called spores of such sporozoa as Monocystis (Fig. 78 G, H) 

 are really minute sporocysts, enclosing several spores ("falciform 

 young"). 



The life history of the individual protozoon usually exhibits little 

 change save increase in size. Sporozoites and other spores, however, 

 may diff^er considerably from the adults into which they grow. This 

 difference reaches its height in the ciliospores of the Suctoria. 



The behaviour of a living being is that part of its life which consists 

 in action upon the outer world. Like the rest of life it comprises 

 activity of various kinds — mechanical, chemical, etc. — which in some 

 cases, as in the direction of locomotion to or from the light or the 

 shooting out of trichocysts, is immediately due to external circum- 

 stances {stimuli), while in others, as in the beating of cilia which 

 continues even when the organism is encysted, it is not. Both these 

 sorts of activity are so ordered that in normal circumstances they 

 conduce to the welfare of the organism. The reactions of the Protozoa 

 to stimuli are at least superficially analogous to the reflexes of the 

 Metazoa. Study of them has chiefly been directed to those which 

 result in locomotion. Such reflexes are of two kinds, topotaxis and 

 phobotaxis. In topotaxis the organism orientates itself in relation to 

 the stimulus, and moves either towards or away from it. This is the 

 less common mode of reaction in protozoa, but it appears to be per- 

 formed by some in the neighbourhood of food, by gametes in their 

 union, and by certain green flagellates {Volvox, sometimes Euglena, 

 etc.) in approaching the light. 



In phobotaxis , which has been studied in many protozoa of various 

 groups, the only circumstances which act as stimuli are those which 

 are "unfavourable", that is from which the organism withdraws; 

 and in doing this it is not repelled in a straight line, but turns away 

 at an angle which has no necessary relation to the direction of the 

 stimulus, and may again bring the individual into the unfavourable 



