Sponges sometimes increase by division, a part being separated from the parent mass and then developing into a complete colony; and they may be reproduced artificially to almost any extent by this method, each piece cut off, however small, producing a new sponge. They also increase by a process of ‘budding,’ the buds produced sometimes remaining attached to the original colony, thus increasing its size, but on other occasions becoming detached for the formation of new colonies on a different site. In addition to these methods of reproduction there are special cells in a sponge that possess the function of producing eggs which are ejected through the larger holes. The eggs are usually developed in the autumn, and, after being ejected, swim about freely for a time, after which they become fixed to rocks or weeds, and produce sponges in the following year. The eggs may often be seen towards the end of the summer by cutting through a sponge, or by carefully pulling it asunder. They are little rounded or oval bodies, of a yellowish or brownish colour, distinctly visible to the naked eye, occupying cavities in the interior. Sponges are classified according to the composition of the skeleton and the forms of the spicules, the chief divisions being:-- 1. The CALCAREOUS Sponges (_Calcarea_). Skeleton consisting of spicules of carbonate of lime in the form of needles and three-or four-rayed stars. 2. The SIX-RAYED SPONGES (_Hexactinellida_). Skeleton of six-rayed glassy spicules. 3. COMMON SPONGES (_Demospongia_). Skeleton horny, flinty, or entirely absent. The first of these divisions contains about a dozen known British species, which are to be found on the rockiest shores, attached to stones, weeds, or shells, generally hidden in very secluded holes or crevices, or sheltered from the light by the pendulous weeds. They should be searched for at the lowest spring tide, particular attention being given to the under surfaces of large stones, narrow, dark crevices, and the roofs of small, sheltered caves. They may be readily recognised as sponges by the numerous pores on the surface, though these are often hardly visible without a lens, and the calcareous nature of the skeleton may be proved by dropping a specimen into dilute hydrochloric acid, when the carbonate of lime will speedily dissolve, the action being accompanied by the evolution of bubbles of carbonic acid gas.