Zoology : an elementary text-book / [Sir A.E. Shipley].

  • Arthur Shipley
Date:
1920
Catalogue details

Licence: Public Domain Mark

Credit: Zoology : an elementary text-book / [Sir A.E. Shipley]. Source: Wellcome Collection.

    50/780 (page 26)
    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 micro spheric 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 wrhich show a good deal of resemblance in many Radiolaria. . ' 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 Tbalamophora, 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