300 A MONOGRAPH OF THE 



ones were all closed, but very porous, and the pores large 

 and distinctly visible by the aid of a two-inch lens. The 

 fistulse are perfectly hollow ; their parietes are very thin, 

 and their bases are immediately over the confluent termi- 

 nations of two or more of the excurrent canals of the 

 sponge, which renders it probable that while the pores of 

 their parietes are incurrent organs ; those of their distal 

 terminations may be excurrent ones, as in the mammillae 

 of the genus Polymastia. The fistulas are usually simple 

 cylinders^ terminating more or less hemispherically, but 

 occasionally after passing from the sponge in this form for 

 half their length they divide into two or more short 

 branches. The pores are exceedingly abundant, and are so 

 large as to be readily visible by the aid of a lens of two 

 inches focus. The dermal membrane is a very beautiful 

 object beneath the microscope. The sides of the areas of 

 the network never exceed the length of a single speculum ; 

 the areas are various in form, but usually either three or 

 four sided. The hispidation does not consist of prolon- 

 gations of the primary lines of the skeletons, but it is pro- 

 duced by the projection of single spicula from the angles 

 of the dermal reticulation ; occasionally, but rarely, there 

 are two in place of one. The structure of the skeleton is 

 very fragile and irregular ; a few of the primary lines only 

 have more than a single series of spicula. 



In the second specimen the fistulse are few in number 

 and are not so well developed as in the larger ones ; but 

 in their place we have three large oscula, the largest termi- 

 nation a fistular projection six lines in height and about 

 the same in diameter. 



This species is readily distinguished from I. pallida in 

 the dried state by the hispidation of its dermal membrane 

 and by the greater size of its spicula, which, although of 

 the same form and diameter, are very considerably longer, 

 their proportions in that respect being as four to three. 



