96 PORIFERA. III. 



41. H. planca n. sp. 

 PI. X, Fig. i. 



Incrusting; surface densely hispid. Spicula: megasclera; the skeletal spiculcs acanthostyli zvifh- 

 out a distinct head, divided into two groups, large, only spined at the base, o-jjo-6j'"'", small, spined 

 about zn the lower half, 0-16 0-27""* / the dermal spicules tylotornota 0-21 0-29""" ; microsclera three 

 forms, chela arcuata 0-018 0-057""", sigmata of two forms, large, contorted, 0-021 0-028""", small, 

 plane, somewhat circularly curved, 0-014""". 



This species forms incrustations on Brachiopods, shells, Bryozoa and small stones; it reaches a 

 greatest extent of about i5 mm , with a thickness of about 0-5". The colour (in spirit) is white in all 

 specimens. The stir/ace is distinctly and densely hispid. The dermal membrane is very thin and 

 transparent; it is perforated by a multitude of close-lying circular openings of different sizes, which 

 are oscula and pores, but there is no such difference in size between them, that it can be decided 

 therefrom which of them are incurrent and which excurrent openings; the openings are seen only 

 when the sponge is somewhat dried; when lying in spirit close-standing openings of canals are seen 

 to shine through the membrane. 



The skeleton. The dermal skeleton consists of bundles of dermal spicules, which stretch 

 obliquely from the lower part of the main skeleton up to the dermal membrane; the bundles are small, 

 each not consisting of many spicules; the spicules all have the rounded ends inwards and the points 

 outwards; the spicules in the bundles are a little divergent outwards and bear the dermal membrane, 

 but they do not penetrate through it or only very slightly; as the longest skeletal styli reach the 

 dermal membrane and project through it, the bundles of dermal spicules are thus lying between the 

 ends of the long skeletal styli. The main skeleton is constructed in the ordinary way of vertical 

 acanthostyli with the heads based on the substratum; it is somewhat dense, and the longer spicules 

 reach as said to the dermal membrane and project beyond it At the base of the skeleton there is a 

 distinctly visible sponginous substance. 



Spicula: a. Megasclera. i. The skeletal spicules are acanthostyli; they are divided into two 

 distinct groups, large and small; the large styli are more or less curved near the base, this latter is 

 the thickest part of the style but there is no distinct head; they taper evenly into a long apex which 

 is a little more abruptly pointed outermost. They are only spined on a short basal part and the 

 spines are somewhat large. The small acanthostyli are straight and relatively more spined than the 

 large, the spines are continued to or near to the middle; the spines are also relatively more robust 

 than in the large styli. The length of the large styli is 0-35 0-65 mm with a diameter at the base of 

 0-021 0-028 mm , and of the small 0-160-27""" with a basal diameter of 0-017 0-022 mm . 2 - The dermal 

 spicules may be termed tylotornota; they are cylindrical and straight, one end is more or less 

 thickened or has an oblong swelling which goes however evenly over into the shaft, the other end has 

 a shorter or longer sharp point The length is 0-21 0-29 mm and the diameter in the middle 0-003 mm . 

 b. Microsclera are of three forms, chelae arcuatae and sigmata of two forms, i. The chelae have a 

 somewhat strongly curved shaft, the end-parts are relatively short, the alae are lobe-shaped and the 

 tooth elliptical. The length is very variable, from 0-018 to 0-057 mm , ^ ut tne shape remains the same. 



