Volume 1

A student's text-book of zoology / by Adam Sedgwick.

  • Sedgwick, Adam, 1854-1913.
Date:
1898-1909
Catalogue details

Licence: In copyright

Credit: A student's text-book of zoology / by Adam Sedgwick. Source: Wellcome Collection.

    26/640 (page 10)
    y ^ • are at first arranged spirally, but in the later stages of growth in a rectilinear series. Such forms are called dimorphous. Conjugation has been observed to take place in a few cases (Arcella, Dijfluqia, Euglyplia, etc.), but details concerning it are unknown. In the Polythalamous forms, it is 230ssible, as hinted above, that conjugation takes j^lace between the free-swimming zoospores. The Foraminifera jjresent four main varieties of shell: (1), the chitinous, e.g., Gromia, imperforate; (2), the ]3orcellaneous. e.g., Miliola, imj^erforate, and characterised by their opaque white colour and abundant organic basis; (3), the hyaline, e.g., Glohinerina. perforate, and with but little organic basis; and (4), the arenaceous. l)erforate, as in Psammosplicera, but generally inq^erforate. The last are formed of small foreign particles united by a cementing substance. It is the fact that perforate and imperforate arenaceous forms are found within the limits of the same family, which has ren- dered necessary the abandonment of the old division of the order into Perforata and Imperforata. Specimens of Biloculina ringens living in the red clay at 3000 fathoms, a dejjth at Avhich calca- reous organisms are generally absent, Avere found by Brady to have a shell composed of silica. Analysis of the calcareous shells shows that the mineral con- stituents consist of carbonate of lime and carbonate of magnesia, the latter varying from five to ten cent. There is besides a trace (generally under 0-5 j^er cent.) of silica. In spite of their small size, the shells of our simple organisms may lay claim to no small consequence, since they not only accumulate in enormous quantity in the sea sand (M. Schultze calculated their number for an ounce of sea sand from Molo di Gaiita at about one and a half millions), but are also found as fossils in different formations (the cretaceous and tertiary), and have yielded an essential material to the construction of rocks. tSilicious nodules of Pohjtlialamia are even found in Silurian deposits. The most remarkable, on account of their considerable size, are the Numnmlites (Fig. 6) in the thick formation of the so-called Num- niulite limestone (Pyrenees and elscAvhere). A coarse chalk of the Fig. 6.—Nuinmulitie Limestone, with horizontal .section of N. distaus (after Zittell).