176 



PORIFERA 



CHAP. 



be covered in various degrees with minute conical spines (Fig. 72, 

 a, b). These spicules are connected by means of a substam 

 termed spongin deposited around their overlapping ends, so as t< 

 form an irregular network of strands, of which some may be 

 distinguished as main strands or fibres, others as connecting fibres. 

 In the main fibres several spicules lie side by side, while in the 

 connecting fibres fewer or frequently single spicules form the 

 thickness of the fibre. The fibres are continuous at the base 

 with a plate or skin of spongin, which is secreted over the lower 

 surface of the sponge and intervenes between it and the sub- 



Fig. 70. Ephydatia jluviatilis. Section of flagellated chamber, showing the choanocytes 

 passing through the apopyle. (After Vosmaer and Pekelharing. ) 



stratum. Of the chemical composition of spongin we shall speak 

 later (see p. 237). It is a substance which reaches a great im- 

 portance in some of the higher sponges, and forms the entire 

 skeleton of certain kinds of bath sponge. Lying loose in the soft 

 parts and hence termed flesh spicules, or microscleres, are minute 

 spicules of peculiar form. These are the amphidiscs, consisting of 

 a shaft with a many-rayed disc at each end (Fig. 72). 



In addition to its habitat the fresh-water sponge is worthy 

 of attention on account of its methods of reproduction, which 

 have arisen in adaptation to the habitat. A similar adaptation 

 is widespread among fresh-water members of most aquatic in- 

 vertebrates. 1 



1 Sollas, Tr. Dublin Soc. (2) iii. 1884, p. 87. 



