AFFINITIES OF THE GENUS SIPHONIA. 809 



ranged in radiating series, those of each row lying approximately 

 parallel with each other and concentric with the centre of the sponge, 

 i. e. transverse to the direction of the radiating canals : thus we have 

 hands of filigree and rows of smooth trabecular radiating towards the 

 centre of the sponge and regularly alternating with each other in a 

 vertical succession. This results, as will be seen in fig. 1, PI. XXVI,, 

 from the fact that the points of trifurcation of the spicules are con- 

 fined more or less to certain radiating lines, and that two out of the 

 three resulting rays, which diverge in a plane at right angles to the 

 shaft, remain short and divide into their clustered apophyses at 

 once, while the remaining ray is elongated in a concentric direction 

 for a certain distance (the breadth of the series) before it breaks up 

 into filigree ; and, to keep the series uniform, it often happens that 

 the points of trifurcation of the spicules are placed alternately on 

 opposite sides of the series, so that one spicule divides into its three 

 rays on the line where the long rays of the adjacent spicules termi- 

 nate, and sends its long arm to divide into tubercles on the same 

 line as that on which the trifurcation of its neighbours takes place. 



Transverse Section (PI. XXY. fig. 5). A similar arrangement is to 

 be seen about the radiating canals here, while in both the transverse 

 and longitudinal sections the circular ends of truncated shafts appear 

 scattered, isolated amidst the network, or attached to rays which 

 diverge from them. These show that the series of spicules exhibited 

 in transverse section are connected by more or less vertical shafts 

 with similar series above and below, and, similarty, that those series 

 shown in longitudinal section are connected by horizontal shafts with 

 similar series on each side— in other words, that the circles of the 

 transverse section represent the shafts of the longitudinal one, and 

 vice versa. 



From this it follows that the skeletal walls of the in current tubes 

 are composed of cylinders of a complex network consisting of parallel 

 bands of the interlocked terminations alternating with parallel rows 

 of the smooth rays of Lithistid spicules, the smooth rays lying con- 

 centric with the axis of the canals, but the series they form paral- 

 lel with them. Thus a minute observer entering one of these canals 

 would see around him, as it were, a number of ladders, the "rungs" 

 represented by the smooth arms, and the side pieces by the clustered 

 tubercles of the spicules ; and he could walk from end to end on the 

 same ladder without crossing from one to another, except where 

 two ladders might merge into one. 



The bands of network, although their direction is from the cir- 

 cumference towards the centre of the sponge, do not extend uninter- 

 ruptedly the whole way, but the3 T are exposed in the sections for a short 

 distance only ; this probably arises for the most part from the inter- 

 ference of the longitudinal canals with the course of the incurrent 

 ones, by displacing or absorbing them, and, to a less extent, in tic 

 case of the transverse sections from the fact that while the incui- 

 rent canals follow approximately lines radiating towards the centre 

 of the sponge, the sections, on the other hand, are taken simply at 

 right angles to its axis, and consequently if they pass, as they are 



