SPONGES 



485 



cells produce three-rayed calcareous spicules which form a very 

 characteristic skeleton (fig. 298). 



The majority of sponges, like corals (p. 476), are able to 

 produce colonies of the most varied shape by processes of budding 

 or splitting, and in such cases the boundaries between the indi- 

 viduals are usually very ill-marked, though the number of these 



Fig. 2q8. — Sponge Spicules, enlarged 



is generally to be told by counting the number of larger openings 

 or oscula which are present. There is also a very large amount 

 of variation in the nature of the skeleton and the extent to which 

 it is developed. It may, as in the simple case described, be made 

 up of scattered spicules, and these may either be calcareous or 

 siliceous. Such spicules are of many different shapes (fig. 298), 

 and they may be compacted into a firm, continuous mass, while 

 they may further be associated with, or replaced by, a complex 

 horny net -work. It should further be remarked that in the large 

 majority of cases the body-wall is much more complicated than 



