134 BRITISH SPONG1ADJE. 



The internal defensive spicula are especially charac- 

 teristic of the species, but they are by no means easy 

 of detection, as they are rather few in number and 

 very small, requiring a power of not less than about 

 1000 linear to exhibit their forms and proportions 

 distinctly, and when in sitii, immersed in the sarcode, 

 it is very rarely that they can be detected. When 

 portions of the sponge have been boiled in nitric acid 

 and the spicula mounted in Canada balsam, they may 

 by careful observation be detected sparingly dispersed 

 among the other spicula, but even then they may 

 readily escape observation from their minuteness and 

 from the strong resemblance they have to fragments 

 of broken spicula. Their form is regularly attenuated 

 from base to apex, and there are no spines upon any 

 other part than just at the basal extremity, and there 

 the spines are few in number and very inefficiently 

 produced, and the square truncated form of the base 

 gives them very much the appearance of a broken 

 basal termination, but when examined with a suffi- 

 ciently high power they are especially characteristic of 

 the species. Their average length is -^y inch, and 

 their greatest diameter ^foo inch. The degree of 

 their basal spination. also varies to some extent. The 

 one represented by Fig. 9 is the most profusely spined 

 one that I have seen. Fig. 10 exhibits about the average 

 amount of spination, and occasionally one may be seen 

 upon the base of which no spines can be detected. 



" The only species that is at all liable to be con- 

 founded with I. trunca is I. jugosa, but it is readily 

 distinguished by the total absence of retentive spicula, 

 which are sufficiently abundant in the last-named 

 species to at once distinguish it from the one under 



