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Show 1887.] COLLECTION F R O M CHRISTMA S ISLAND. 525 spicular axis. The diameter of the fibres varies much, averaging, say, about 012 millim. There are also numerous spicules outside the fibres, scattered loosely and irregularly 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 0'009 millim. Canal-System.-The canal-system is lacunar, and referable to Dr. Vosmaer's third type -. The pores (vide supra) lead into Pachychalina spinosissi7na; portion of a vertical longitudinal section, showing (1) the inhalant lacunas incompletely subdivided by strands of mesodermal tissue, (2) the exhalant lacunae, and (3) the subspherical flagellated chambers opening into the exhalant lacunas 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 lacunae, leading to the flagellated chambers. The ultimate ramifications of the inhalant lacunae are very peculiar, consisting of a system of spaces lying amongst and around the exhalant lacunae, 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). 1 Bronn's Klass. u. Ordn. cl. Thierreichs, Porifera. p. 130. P R O C ZOOL. Soc-1887, No. XXXV. 35 |