TKACKS AND MAEKINGS — PEOTOZOA. 



15 



The Protozoa are animals of simple structure and usually G-allery X. 

 of minute size. In them the soft substance of the body is 

 not divided into cells, such as build up the body -tissues of 

 all other animals, and consequently they have no definite 

 tissues. They have been called unicellular animals, but a 

 single Protozoon often comprises more than is to be found 

 in any one cell of a multicellular animal. In the latter 

 each cell is trained for some special service and has dropped 

 the qualities not essential to that service, whereas the body 

 of the Protozoon has to fulfil all functions of the animal 

 economy. Microscopic investigation, moreover, has shown 

 that the minute drop of viscous protoplasm which con- 

 stitutes the body of even the simplest Protozoon has really a 

 most complicated structure. By keeping this in mind we 

 shall better appreciate the significance of those exquisite 

 skeletons formed by the Foraminifera and Eadiolaria and 

 frequently preserved as fossils. 



A fuller account of living Protozoa is given in the Guide 

 to the Coral Gallery in the Department of Zoology. It is 

 there explained how in some Protozoa the outer surface of 

 the body is hardened, so that the animal retains always a 

 definite shape, and can usually take in food only at one 

 permanent opening. Further, the surface is generally pro- 

 vided with definite lash-like or hair-like processes capable 

 of rapid rhythmic movement. Those Protozoa are called 

 Corticata, and since they have no skeleton to be fossilised, 

 need no further mention here. The rest of the Protozoa 

 have no hard skin, so that either the whole body may change 

 its form within certain limits, or portions of its protoplasm 

 may be extended as lobes or threads and again withdrawn. 

 By stretching out a lobe, and then as it were flowing into it, 

 the animal moves, and therefore these extensions are called 

 pseudopodia (false footlets). Fragments of food are caught 

 up in them and taken into the central body at any point. 

 These Protozoa are called Gymnomyxa (naked slimes) or 

 Rhizopoda (root-feet). They may be divided into the 

 Classes : I. Lobosa, with lobose pseudopodia, e.g. Amoeba, 

 II. Heliozoa or Sun Animalcules, with fine radiating pseudo- 

 podia. III. Foraminifera, with pseudopodia branching and 

 again uniting so as to form a network. IV. Radiolaria, 

 with fine radiating pseudopodia, as in Heliozoa, but with 

 the central protoplasm enclosed in a porous membrane or 

 capsule. In each of these groups there are some genera 

 that construct a skeleton and others that do not ; but in the 



