BRITISH SPONGES 119 



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. 



