1887.] 



COLLECTION FROM CHRISTMAS ISLAND. 



525 



spicular axis. The diameter of the fibres varies much, averaging, 

 say, about 01 2 miilim. There are also numerous spicules outside 

 the fibres, scattered loosely and irregidarly through the choanosome. 

 (b) Dermal : a very well-developed reticulation of stout spiculo- 

 fibre, branching and anastomosing in such a manner as to give rise 

 to a number of irregularly polygonal meshes. These coarser meshes 

 are further subdivided by a much finer, unispicular, or sub-uni- 

 spicular reticulation (Plate XLIV. fig. 2), lying at a slightly higher 

 level. 



Spicules. — Slightly curved oxea (Plate XLIV. fig. 3), sharply and 

 fairly gradually pointed at each end ; size of full-grown spicules 

 about 0-16 by 0009 miilim. 



Canal-System. — The canal-system is lacunar, and referable to 

 Dr. Vosmaer's third type*. The pores (vide supra) lead into 



Fi-. 6. 



Pmhychalma spinoxissima ; portion of a vertical longitudinal section, showing 

 (1) the inhalant lacunre incompletely subclivided by strands of mesodermal 

 tissue, (2) the exhalant laeunEB, and (3) the subspherical flagellated 

 chambers opening into the exhalant lacuna! by means of wide mouths. 



irregularly shaped subdermal cavities, lying immediately beneath the 

 dermal membrane. Many pores lead into each of the subdermal 

 cavities, which are merely the proximal extremities of a system of 

 ramifying inhalant lacunse, leading to the flagellated chambers. 



The ultimate ramifications of the inhalant lacunse are very 

 peculiar, consisting of a system of spaces lying amongst and around 

 the exhalant lacunse, and themselves incompletely subdivided by 

 numerous delicate strands of mesodermal tissue, which, branching and 

 anastomosing, run across and across, from wall to wall (vide woodcut, 

 fig. 6). 



* Bronn's Klass. u. Ordn. d. Tliierreiehs, Porifeia, p. 130. 



Proc. Zool. Soc— 1887, No. XXXV. 35 



