ISODICTTA. 145 



envelopes irregularly the stem of a slender fucus for 

 about three inches in length, and a projecting portion 

 of it, about the size of a small hazel-nut, has a central 

 canal about the eighth of an inch in diameter running 

 through it and terminating at its summit as if it were 

 the termination of a cloacal cavity, but I could not, 

 without destroying the root of the sponge, determine 

 whether this character prevailed throughout the whole 

 of the sponge. The general mass is rather irregular 

 in its anatomical structure, and consists of but one 

 form of spiculum, very minute, slender, and purely 

 acuate, those of the skeleton varying from the 

 dermal ones only in being slightly the stouter of the 

 two. The extreme tenuity of these organs is, in 

 truth, the most striking specific character of the 

 sponge, and it readily separates it from other nearly 

 allied species, as it requires a power of not less than 

 about 400 linear to define either those of the skeleton 

 or the dermal membrane with certainty. An average- 

 sized spiculum of the dermal membrane measured 

 g-jy-o inch in length, and its greatest diameter TTO^JO" 

 inch. Two of skeleton-spicula measured yir inch in 

 length and TTFoiTo i ncn * n diameter, and yj-g- inch in 

 length and 8 / 52 inch in diameter. The dark amber- 

 coloured sarcode is sufficiently abundant in all parts of 

 the sponge to render it rather difficult to see the 

 spicula distinctly in situ; and their separation from the 

 sponge by boiling a portion of it in nitric acid, and 

 mounting the spicula in Canada balsam, is quite neces- 

 sary to obtain a clear idea of their forms and propor- 

 tions. I examined the membranous structures and 

 the spicula thus prepared and mounted with great 

 care, but I could not detect any other forms of 

 VOL. iv. 10 



