26 PEOTOZOA [CH. 



swim. The gametes escape from the shell and move freely about, 

 they coalesce with one another in pairs, and the resultant mass 

 or zygote acquires a shell and begins to bud off new chambers. 

 Since in spite of the fact that the zygote has resulted from the 

 union of two gametes it is much smaller than a product of ordinary 

 fission, the adult resulting from the growth of a zygote is dis- 

 tinguished from that resulting from fission by the small size of the 

 initial chamber. Hence we can distinguish a microspheric form 

 resulting from the development of gametes from a megalospheric 

 form resulting from fission. Lister has shown that in Polystomella 

 there is an alternation of fission and formation of gametes, but that 

 more than one generation which undergoes fission intervenes between 

 two which form gametes. 



We may pass now to the consideration of some Protozoa 

 which show a good deal of resemblance in many 

 points to the Thalamophora, though they have very 

 marked peculiarities of their own. These are the Radiolaria; 

 they have delicate threadlike pseudopodia, and their protoplasm 

 is divided into two parts an inner and an outer by a membranous 

 case pierced with pores, through which the two parts of the body 

 are connected with each other. This case, the central capsule, 

 may be compared to the Thalamophoran shell, and the protoplasm 

 outside it gives rise to the pseudopodia, but the interesting fact 

 is that these Radiolaria have in addition to this another skeleton 

 composed not of chalky calcareous but of flinty siliceous 

 substance, as are also some of the shells of the Thalamophora. This 

 flinty skeleton may consist simply of isolated needles sticking out 

 on all sides from the centre ; oftener, however, it consists of a 

 beautiful basketwork as in Heliosphaera inermis (Fig. 6), and some- 

 times we find several of these baskets one within the other, like the 

 Chinese ivory ball. The Radiolaria, like the free-swimming Thala- 

 mophora, have a vacuolated outer protoplasm, and often drops of oil 

 in the inner protoplasm ; these structures serve to sustain them and 

 they are found floating at the surface of the sea amongst Thala- 

 mophora. At the bottom, in medium depths, their flinty skeletons, 

 though mixed with the calcareous shells of the Thalamophora, do not 

 affect the general character of the chalky mud (called the Globigerina 

 ooze, from the name of one of the commonest Thalamophora found 

 in it), but at greater depths, owing to the enormous pressure, the 

 quantity of carbonic acid dissolved in the water increases very 

 much (on the same principle that the pressure inside a soda-water 



