28 ZOOLOGY FOR MEDICAL STUDENTS chap. 



disease from the projecting tumours which develop on the roots of plants 

 affected by it. The life-history begins with a small flagellate, somewhat 

 spindle-shaped, stage which hatches out of a spore. This makes its way 

 into one of the cells of the plant and there grows enormously, forming a 

 s)aicytium and causing the cell to swell up to an immense size. Eventually 

 the syncytium breaks up into numerous small spores which surround 

 themselves with a protective cyst and which when set free by the rotting 

 of the plant are scattered about and serve to infect other plants. 



The Protozoa mentioned up till now are linked together by the 

 character of their pseudopodia, which are soft and frequently branch. 

 This community is expressed by placing them in a group by themselves 

 — the Rhizopoda — separated from the Actinopoda in which the thin 

 tapering pseudopodia radiate stifHy from the body all round. Of the 

 first section of Actinopoda — the Heliozoa — we will consider first Actino- 

 sphaerium which is common in freshwater pools and excellently suited 

 for laboratory study. 



ACTINOSPHAERIUM 



A well-developed Actinosphaerium is clearly visible to the naked 

 eye as a round greyish-white spot when seen against a dark background. 

 Under a low magnification it is seen to float freely in the water, spherical 

 in form with stiff pseudopodia radiating out all round making it resemble 

 the conventional figure of the sun and justifying its popular name " Sun 

 animalcule." As in the floating types of Foraminifera the cytoplasm 

 (Fig. ii) is highly vacuolated, there being a superficial layer of vacuoles 

 of specially large size marking the ectoplasm, while in the endoplasm the 

 vacuoles are much smaller so that it has a more opaque appearance 

 than the ectoplasm. Certain of the vacuoles of the ectoplasm — ^as many" 

 as 15 there may be in a large specimen — are contractile and when at 

 the height of distension (diastole) they may be observed to bulge 

 conspicuously beyond the general surface (Fig. 11, c.v). 



The pseudopodia are extensions of the ectoplasm and examination 

 with a very high power shows that each pseudopodium is supported by 

 a stiff axial filament running up its centre and ensheathed in more fluid 

 cytoplasm which shows slow streaming movement. By careful focusing 

 on the extreme edge of the specimen the axial filament may be seen to 

 extend as far as the outer limit of the endoplasm. The pseudopodia 

 are used in feeding. If a small food-organism touches one it is paralysed 

 and killed almost instantly. It seems to adhere to the pseudopodium 

 which is reinforced by the neighbouring pseudopodia bending over to 



