1SODICTYA. 143 



total absence of retentive spicula in any part of the 

 sponge." 



57. ISODIOTYA INVOLUTA, BoiV. 9 U. Sp., IV, PI. X, figS. 



14. 



" Sponge massive, coating parasitically ; surface 

 smooth and even. Oscula simple, dispersed. Pores 

 inconspicuous. Dermal membrane abundantly spicu- 

 lous ; spicula acuate, slender, dispersed. Skeleton- 

 rete regular ; areas one spiculum wide ; primary line 

 mostly multispiculous ; secondary lines regularly dis- 

 posed, uni- or bispiculous ; spicula acuate, stout. In- 

 terstitial membranes spiculous ; tension spicula slender, 

 acuate, same as those of the dermis, rather numerous. 



" Colour. In the dried state, dark brown. 



" Habitat. Dredged in Westport Bay, Ireland ; Rev. 

 A. M. Norman. 



" Examined. In the dried state. 



" I received a single specimen of this species for exa- 

 mination from the Rev. A. M. Norman. It entirely 

 fills up the interstices of a small specimen of Nullipora 

 polymorpha an inch and a half in length by an inch 

 in breadth, leaving only the extremities of the small 

 branches visible. 



" The anatomical structures are simple, but very cha- 

 racteristic. The dermal membrane is abundantly sup- 

 plied with tension spicula of a very slender acuate 

 form, their diameter being not more than about one 

 third of that of the skeleton-spicula, while the lengths 

 of the two are very nearly equal ; their mode of disposi- 

 tion is quite irregular, crossing each other in every pos- 



