57 



by the apposition of fresh layers of silica in length and in breadth. The silica- 

 layers precipitated on the surface of the growing spicule have the shape of cylin- 

 drical tubes or rather long, hollow cones, closed at the narrow end and there at- 

 tenuated to a point. Their lower tubular part is apposed to the sides of the ray 

 and causes its growth in thickness, the solid pointed terminal part covers the old 

 termination of the ray and causes its growth in length. 



Here and there adjacent spicules are secondarily glued together. This pro- 

 cess commences with a mere local concrescence of silica-layers apposed to diiferent 

 spicules and is then carried on by the deposition of fresh silica-layers on the place 

 of junction, this being thus converted into a stout bridge or column, the synapti- 

 culum. If the parts of spicules (rays) thus joined lie parallel to each other and if, 

 as is the rule in such cases, a number of synapticula are formed at nearly equal 

 intervals, those well known ladder-like structures are produced, which are so 

 well developed in the long and thin peduncles of Caulophacus and other Rossellids. 

 In Euplectella simplex' as in E. oweni such synapticular junctions of spicules 

 are however rather scarce and, even in older specimens, generally met with 

 only near the lower end of the body. 



To obtain a clearer insigh't into the mode of growth I have tried to ascer- 

 tain the number and arrangement of the transverse and longitudinal spicular 

 fibres of the main skeleton-net in all the specimens preserved in their entire 

 length. In these I find that the number of both the transverse and the long- 

 itudinal fibres distinctly increases, although not very greatly, with increasing 

 age. In the smallest specimen at my disposal, which is 30 mm. long and 5 mm. 

 broad, there are 25 transverse fibres and, in the middle of the body, 28 longi- 

 tudinal fibres. In the largest one preserved in its entire length, which is 11 cm. 

 long and 25 mm. broad, I counted 40 transverse, and, in the corresponding 

 central part of the body, 30 longitudinal fibres. In the following fractions, 

 arranged in order of the size of the specimens (increasing from left to right) 

 the upper figures give the number of transverse and the lower figures the 

 number of longitudinal fibres : — 



31 



We see that the transverse fibres increase in number pretty considerably 

 (from 25 to 40) and the longitudinal fibres only slightly, during the period of 

 growth represented by the specimens at my disposal. 



As regards the position of the fibres, particularly the distances between 

 adjacent ones in different parts of the sponge-body, one finds that the parallel 

 fibres retain their position and only move further apart and mcrease in thick- 

 ness during the growth of the sponge. The increase of the distance between 

 the longitudmal fibres is perfectly uniform throughout the length of the sponge, 

 8 



