British Sjjecies of Sponges. 335 



Microsclera^ two : — 



1. Toxa, very slender, with a short, abrupt, spiral curve in 

 the middle, the extremities long and straight. The tips are 

 generally smooth and very sharp, but in some cases a few 

 very minute spines may be detected on the spicule generally. 

 These spicules range from "250 up to '412 millim. in length, 

 the majority measuring between '3 and '4 millim., by an 

 average breadth of only "0015 to "002 millim. (tigs. A, 5 and 



6)- 



There are also a few much smaller toxa, among which are 



some in which the central curve is comparatively large and 



the long straight ends absent (fig. A, 7) . These last spicules 



are as stout as the longest ; their tips are generally micro- 



spined. Two measured were respectively '091 and '143 



millim. long. 



2. Isochela?, of the usual navicular type, minute, '017 to 

 •019 miUini. long (tigs. A, 9 and 10). 



The columns are composed of the larger kind of styles, 

 No. 1, and are sparsely echinated by the entirely spined 

 spicules, No. 2. The slender subclavate styles, No. 3, lie 

 imbedded in the substance of the sponge, as above stated, 

 generally parallel with and near to the surface. The chelie 

 and toxites are distributed throughout the sponge-substance, 

 and do not lie in any particular direction as regards the 

 columns or the surface. 



The megasclera of this sponge agree very closely in cha- 

 racter with those of the other species of Microciona proper, 

 and the chela is of the well-known shape which is character- 

 istic of a very considerable number of species in that and other 

 genera; the toxite, on the contrary, is of a peculiar and 

 striking form, which seems to call for a few remarks. 



In tiie other species of the genus, and, so far as I have 

 been able to ascertain, in all the species (with one exception) 

 of similar spiculation in other genera, the toxa lie flat in one 

 plane from end to end ; here and there perhaps one may be 

 found with a barely perceptible twist ; but in this sponge I 

 have not been able to tind one which can be focussed under a 

 ^-inch objective from end to end at the same time. In the 

 long straight-armed form the twist is nearly confined to the 

 central smus, and the conformation o£ this spicule may be 

 roughly imitated by laying a straight piece of wire along a 

 lead pencil and takmg one turn round the middle ; if then the 

 pencil be drawn out and the wire turned on its axis, it will be 

 found to present in different positions as regards the eye the 

 appearances shown in the figures ot the toxites (tigs. A, 5 and 

 6). The wire model would ditier from the actual form of 



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