PLATE LXXXV. 289 



surface, where they are usually in the most re- 

 gular condition, are exceedingly irregular and tor- 

 tuous in their course, and towards the inner portion of 

 the sponge they may be seen assuming every possible 

 direction. The secondary lines connecting the primary 

 ones with each other partake of the same irregularity ; 

 sometimes, but rarely, they are more or less multi- 

 spiculous, but the more usual mode of their dis- 

 position is in single spicula closely adjoining each 

 other, and frequently at different angles ; and they are 

 so numerous as to greatly complicate and confuse the 

 structures. 



The spicula of the primary lines are rather larger 

 and stouter than those of the secondary ones. An 

 average- sized one from the primary lines measured 

 2-Jy inch in length, with greatest diameter 50^0 

 inch, and the largest spiculum I could find measured 

 Y^y inch in length. 



The dermal membrane, although aspiculous, affords 

 the most distinctive character of the species when 

 mounted in Canada balsam, and viewed with a power 

 of about 400 linear. It is then seen to be abundantly 

 supplied with minute pellucid granules, uniform in 

 their form and size, and closely packed together ; but 

 it must be remembered that they are visible only 

 when the membrane is mounted in Canada balsam. 



ISODICTYA INVALIDA, BowerbanJc. 

 Plate LXXXV. 



Sponge parasitical on slender zoophytes, &c. Sur- 

 face smooth and even. Oscula simple, dispersed, 

 minute. Pores inconspicuous. Dermal membrane 

 thin, pellucid, aspiculous. Skeleton. Rete very open, 

 and cavernulous ; primary lines in accordance with the 

 long axes of the sponge, multispiculous, very tortuous ; 

 secondary lines varying from unispiculous to multi- 

 spiculous, irregularly disposed; spicula acuate, small 



19 



