40 THE PROTOZOA 



In addition to the passive protection afforded by a cuticle, organs 

 of active defence may be present in the ectoplasm in the form of 

 bodies kno\vn as trichocysts, found commonly in many ciliate In- 

 fusoria (p. 447, Fig. 187) ; they are little oval or spindle-shaped 

 bodies which on suitable stimulation are converted explosively into 

 a stiff thread which is shot out from the surface of the body. (For 

 the nematocyst-like organs known as " polar capsules," in Myxo- 

 sporidia and allied organisms, see p. 399, infra.} 



2. The ectoplasm is shown to be the seat of movement both by 

 the fact that motile organs arise from it and by the frequent 

 presence in it of special contractile mechanisms. The motile 

 organs which are found in the Protozoa are pseudopodia, flagella, cilia 

 with their various modifications, and undulating membranes ; any 

 of these structures may subserve the function of food capture in 

 addition to, or instead of, that of locomotion. These organs will 

 now be described in order, after which contractile mechanisms will 

 be dealt with. 



(1) Pseudopodia are organs of temporary nature, extruded from 

 the protoplasm when required, and retracted w r hen no longer needed. 

 They can be formed, probably, in all cases in which the body 

 protoplasm is naked, or limited only by a cuticle not of sufficient 

 thickness to inhibit the movements of the underlying protoplasm. 

 They arise simply as an eruption of the protoplasm at some point 

 at the surface of the body, forming an outgrowth or process which 

 varies greatly in different cases as regards size, length, width, com- 

 position, and activity. 



Pseudopodia always arise in the first instance from the ectoplasm, 

 and may consist throughout of this layer alone, in which case they 

 are relatively stiff er and more rigid ; or a core of endoplasm may 

 flow into the pseudopodium when it has grown to a certain length, 

 in which case the pseudopodium is more fluid and flexible. The 

 formation of a pseudopodium is best studied in a common amoeba, 

 such as Amoeba proteus (Fig. 2) or A. Umax (Fig. 20) ; it is then seen 

 to arise as a protrusion of the ectoplasm, forming a shallow promi- 

 nence at the surface of the body. The prominence continues to 

 grow out from the body, and is at first hyaline, transparent, and 

 free from granulations, since it consists of ectoplasm alone. In 

 some cases the pseudopodium may grow to a relatively very large 

 size, and still consist of clear ectoplasm alone, as in Entamceba 

 histolytica (Fig. 90), a form rather exceptional in this respect ; more 

 usually, so soon as the budding pseudopodium has reached a certain 

 not very great size, a core of granular endoplasm flows into it and 

 forms the axial part of the pseudopodium. It is then easier to study 

 the formation of the pseudopodium, since the granules in the endo- 

 plasm permit the characteristic flowing movements and currents to 



