In the young fixed larva the cells next the rock necessarily move over it to form a good disc of adhesion. Such movements may result in the whole young sponge changing its place ; a gem mule of Spongilla allowed to fix at the bottom of the glass side of the aquarium will crawl some little way up the side, until light and other reactions are less different on the upper and lower sides of its base.
The most remarkable change of form and position in cells is that which gives rise to the pores in Lace-chalks. The small afferent holes through which water enters the flagellate chambers consist in Calcareous sponges each of a single perforate cell (Bidder, 1891). In the Lace-chalks any one of the external cells can stretch down through the sponge-wall to the flagellate surface, push in between the collar-cells and perforate itself to make a pore (Minchin, 1892). These "porocytes" in unhealthy conditions crawl over and completely cover the collar-cells. The occasional vertical ray, which in certain species of Lace-chalks converts some of the three-rayed spicules into four-rayed spicules, is always added by a porocyte (Minchin).
In Banana-chalk (Leucandra aspera, fig. 6) 20 minutes after feeding with carmine the interstitial jelly between the collar-cells (fig. 5) is loaded with carmine particles and every collar-cell has several adhering to it or in vacuoles, generally near its base. After 4 hours the collar-cells are crowded with car mine, after 18 hours there are a few fine particles, but after a few hours more it has all disappeared. Food seems to be taken in at the sides of the collar-cell (9) (as in collar flagellates), but probably also inside the collar, and passes into vacuoles with an acid reaction. After about 6 hours the vacuole has become a faecal bolus of transparent mucilaginous substance (Cotte [1903, P. 459] observed the reaction to be alkaline) containing refringent granules, probably calcareous. This moves about in the upper part of the living collar-cell, from which it is presently cast out inside the collar and so into the efferent current. If a sponge, gathered from exposed rock at low-tide, be placed in fresh sea water, the stream from the vent, so soon as the current resumes, is seen to bear a number of white specks and clots, heavier than water, which are the aggregated faeces that have accumulated during the period of quiescence.
Increase of sponges is mainly by vegetative growth. Leucosolenia variabilis (fig. 4) sends out stolons along the rock from which tubular chimneys grow up in apparently endless succession. Clathrina coriacea (fig. 4), though complete in all its parts as a cushion a quarter-of-an-inch wide, will weave its lace over the rock till it covers many inches ; the crusting Needle-sponges such as Halichondria (fig. 4) or the fresh-water Ephydatia can extend their crusts indefinitely. Rate of growth varies enormously with conditions : a calcareous sponge hanging from a boat in harbour may add 40% a day to its weight (7, p. 315), while the same species on a tidal rock may only in crease 2% a day. The tidal sponges shown in fig. 4 are all annual, and in the most favourable situation Sycon or Grantia may grow to io inches instead of 2 inches long. Vosmaer's observation on the gemmule of the "Sea-orange" (13, P. 44o) gives an increase of about 17% a day, but Rathbun's artificial sponge-cuttings of Horny sponges in Florida from 21 cubic inches grew to 121 cubic inches in two months (see Bidder, 1896, p. 201), which is only 3% a day. Mr. L. R. Crawshay has told the writer that his sponge-cuttings in the Bahamas increase more slowly than this, growing in a year to 2 or
times their volume, and that the large Wool sponges increase in volume annually from o% to 8o% (say, 5o% a year); he considers the largest (21 feet across) to attain an age of at least 25 years. The Wool sponges, Velvet sponges and Mediterranean Honeycomb sponges are probably the largest of all sponges. Cresswell (15) describes a circular speci men of the last species 36 inches across and 12 inches high. The Needle-sponge "Neptune's Cup" (Cliona) forms in the East Indies a rigid stalked goblet a yard high and more than a foot across the rim ; it is one of the Pin-cushion sponges, and when young bores in soft limestone or sea shells, producing the worm-eaten appear ance often seen in oyster-shells ; in temperate seas its adult form is a yellowish corky sponge several inches in diameter, formerly known as Raphyrus.