306 A MONOGRAPH OF THE 



exceed about a line. The most remarkable character in its 

 structure is its hispidation, which is in part produced by 

 the projection of the terminations of the primary fasciculi 

 of the skeleton, which radiate slightly at their distal ex- 

 tremities ; but beside these defensive appliances, there are 

 numerous long, slender, attenuato-acuate defensive spicula, 

 which have their bases immersed in the body of the sponge, 

 frequently to about half the length of the spiculum ; in . 

 other cases, they have their origin at very slight distances 

 beneath the dermal membrane. They are not always based 

 on the primary lines of the skeleton ; but frequently spring 

 from the membranes, without contact with any portion of 

 the skeleton tissues. These spicula are very little more 

 than half the diameter of those of the skeleton, and are 

 three or four times their length. 



In the specimen under consideration, the large defensive 

 spicula appear to be very much more numerous on the 

 inner surface of the cup than on the outer one, and they 

 are very distinctly apparent by the aid of a lens of an inch 

 focus, but this difference between the two surfaces may 

 probably arise from the greater amount of exposure to in- 

 jury of those of the outer one, from the attrition of the 

 bodies surrounding the sponge in its natural condition. 

 The irregularity in the structure of the skeleton tissues is 

 very great ; the primary lines are frequently very loosely 

 fasciculated, and the secondary ones, when not multi- 

 spiculous, are so irregularly disposed, as to frequently render 

 it difficult to say whether they are properly skeleton spicula 

 or tension spicula, of the same size and form ; but as there 

 are no tension spicula on the dermal membrane, it is most 

 probable that they are disrupt-spicula of the secondary 

 portions of the skeleton tissue. 



22. Isodictya mammeata, Bowerbank. 



Sponge. Massive, sessile, parasitical on Fuci. Surface 

 smooth, mammillated. Oscula terminal on the mam- 

 millae, simple. Pores inconspicuous. Dermal mem- 



