Outlines of zoology / by J. Arthur Thomson.

Date:
1892
Catalogue details

Licence: Public Domain Mark

Credit: Outlines of zoology / by J. Arthur Thomson. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

    146/748 (page 118)
    small portions are sometimes set adrift from a sickly parent mass. Moreover, for commercial purposes, the bath sponge is cut in pieces and bedded out, the fragments reproducing the whole as do cuttings from many plants. The life-history of Spojigil/a, as told by Marshall, is one of interesting vicissitudes. In autumn, the sponge begins to suffer from the cold and the scarcity of food, and dies away. But throughout the moribund parent, clumps of cells combine into gemmules, which are furnished with capstan-like spicules, and are able to survive the winter. In April or May, they float away from the parental corpse and start new sponges. Some of these are short-lived males, others are more stable females. The ova produced by the latter, and fertilised by the cells of the former, develop into another generation of sponges, which in turn die away in autumn and give rise to gemmules. The life-history thus illustrates alternation of generations. Development.—The development of sponges varies con- siderably in the different kinds. We shall follow that of a calcareous form. The ovum lying within the middle stratum, but close to a canal, is fertilised by a male cell, borne to it by the water. Fertilisation is followed by repeated division of the ovum ; a hollow sphere of cells results, and this eventually escapes from the parent into the water. A short free-swimming life begins, in marked con- trast to the completely sedentary state which will follow. One half of the minute sphere consists of ciliated cells, the other consists of larger, granular cells without cilia. The sphere is soon dimpled in or invaginated, and the ciliated hemi- sphere is surrounded by the other. A gastrula stage is thus reached—a two-layered thimble-like embryo. But the cilia are now inside, and the embryo fixes itself, mouth or blasto- pore downwards. This very disadvantageous shut-in condi- tion cannot last; pores appear through the walls, perhaps somewhat pathologically at first; the water regains admission to the internal cavity ; the cilia, which seem meanwhile to have disappeared in the absence of stimulus, regain their activity ; an e.xhalent aperture (obviously in no sense the mouth) is ru])tured at the apex of the dome; a middle stratum is derived from the inner layer, and its cells begin to form spicules :—the young sponge is made.