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.

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    7^ ■ Jz aAyy^ Jz^ these movements is to be sought in the contractility of the surround- ing portions of protoplasm (Fig. 1). A pulsating space, the contractile vacuole, is not unfrequently to be found in the protoplasm, e.p., Diflugia, Actinophrys, Arcella (Fig. 2). Nuclei are usually to be made out, but there are forms in which no trace of a nucleus has yet been found. In such cases either our methods of observation are faulty, or the protoplasm of the nucleus is not yet differentiated as a separate structure (the Monera of E. Haeckel), or we have to do with a transient, non- nucleated stage in the life-history. The protoplasm usually secretes silicious or calcareous structures, either as fine spicula and hollow spines, which are directed from the centre to the periphery in regular order and number, or as lattice- work chambers {Radiolaria), which often bear points and spines, or finally as single and many-chambered shells with, in some cases, finely perforated walls {Foraminifera) and one larger opening. Through this last (Fig. 4), as well as through the countless pores of the small shells (Fig. 3), the slender threads of sarcode pass out to the exterior as pseudopodia, changing Avithout intermission in form, size, and number, and often joining themselves together in delicate networks. (Figs. 3, 4.) The pseudopodia, by their slow contraction, afford a means of locomotion, while they also serve for the taking up of nourishment by surrounding and transporting into the interior of the body small vegetable organisms as Bacillaria. Among the shell-bearing fonns, the reception and digestion of food takes place outside the shell in the peripheral threads and networks of sarcode ] for each spot on the surface can for the time being assume the functions of mouth, and also of anus, by rejecting the undigested remnants. While the power of emitting pseudopodia is characteristic of the form in Avhich the Gymnomyxa are usually met Avith, it is usual to find a stage in their life-history in Avhich locomotion is effected by means of flagella. To such flagellated forms the term mastigopod has been applied. Fio. 2.—Amoeba {Dactylosph(Bra) polypodia (after Fr. E. Schulze). N nucleus. Pv pulsating vacuole.