Sponges reproduce by eggs formed in the mesoderm. The eggs escape as ciliated spheres and swim about until they find a place on which to attach themselves. As soon as they become fixed they grow with much rapidity into mature individuals.
Some species seem to prefer association with other animals and live as commensals with crabs. The crab /Dromia/ is always concealed under a sponge, which grows upon its back. Spider-crabs are often overgrown with sponges as well as seaweeds. In this case, however, the crab finds and plants the sponge himself. /Aplysella violacea/ overgrows worm-tubes. Many sponges afford shelter to numerous small animals which bore into their bodies for protection, no animal seeming to feed upon the sponge.
Sponges may be found in tide-pools, on the under side of stones, on seaweeds, and so on. A small bright-red incrusting sponge with irregular lobe-like branches is common on the New England coast; a thin yellow incrusting sponge also is found on the under side of stones. /Grantia ciliata/, a small urn-shaped species, having a large aperture at the summit, is found in tide-pools.
Perhaps the most singular in habit of any sponge is /Cliona sulphurea/, the boring-sponge, a common species found from Cape Cod to South Carolina and abundant in Long Island Sound. It [pg106] is bright sulphur-yellow in color, grows in irregular masses of considerable size and fine texture, and has low wart-like prominences. It lives on shells spreading over both surfaces, at first forming little burrows, but eventually penetrating the shell in every direction, honeycombing and at last completely destroying it by absorption. Sometimes it settles upon living shells and greatly irritates the animal, which constantly secretes new lime to cover the perforations in its shell.
These sponges are an important factor in the economy of the sea, as they disintegrate dead shells, which would otherwise accumulate in vast quantities.
^G. ciliata.^ Small, urn-shaped or oval, with large aperture at the summit, surrounded by a circle of projecting spicules. It is found in tide-pools and on piles of wharves from Rhode Island northward.
^L. botryoides.^ Tubular, branched. Occurs in the same places as /Grantia ciliata/.