58 



the ratio of augmentation of the fibre-intervals being nearly the same in the 

 upper, central, and lower part of the body. In a specimen 45 mm. long the 

 longitudinal fibres are 500 /^ apart above, 1 mm. apart in the inflated centre and 

 500 At apart at the lower end, just above the root-tuft. In a specimen 11 cm. 

 lolig their distances are 1 mm. above, 2' 5 mm. in the centre and 1 mm. below. 

 With the transverse fibres it is somewhat different, in as much as the distances 

 between them increase, at least in young specimens, gradually from the upper to 

 the lower end of the body. In a specimen 45 mm. long, for instance, the in- 

 terval between the transverse fibres is in the upper cylindrical part of the body 

 200-400 /* , between the upper and middle third of the length 1, 5 mm. in the 

 centre 2 mm. and at the lower end 2*5 mm. In the larger specimens the dis- 

 tance between the transverse fibres increases continuously from the upper end 

 to the centre, whilst it remains pretty constant from there downwards. In the 

 specimen 11 cm. long, the transverse fibres are in the upper, cylindrical part of 

 the body, on an average 1 mm., between the upper and central third 2 mm., and 

 in the centre and from there downwards 5 mm. apart. The cause of this 

 remarkable difference in the arrangement of the longitudinal and transverse 

 fibres must be sought in the mode of growth of the sponge and particularly in 

 the difference of the regions, where the two kinds of fibres originate. 



The fibres multiply by fission, the daughter-fibres subsequently moving 

 away from each other. This can be gathered from the fact that one occasionally 

 finds bifurcated fibres, the simple stem of which is broad, and pairs of fibres of 

 the ordinary dimensions, quite close together, arrangements which differ very 

 essentially from the ordinary skeletal structure described above. It is remarkable 

 that the fissional structures of the longitudinal fibres are rare and quite irregu- 

 larly distributed, whilst transverse fibres multiplying by fission are pretty fre- 

 quently met with, three or four dividing transverse fibres being usually found in 

 each specimen. These are generally restricted to the upper part of the body, 

 in which these fibres are thinner and closer together than elsewhere. It follows 

 from this that the growth of the sponge in thickness is accompanied by an in- 

 crease of the distance between the longitudinal fibres and an occasional fission 

 of one or another of them, and further that the multiplication of the longitudi- 

 nal fibres by fission is not localised. The growth of the sponge-body in length 

 is on the other hand accompanied by an increase of the distance between the 

 transverse fibres particularly in the upper part of the body, and a multiplication 

 of them by fission, which is pretty much restricted to that region. AW the 

 parenchymal principalia are much longer and stouter in the lower than in the 

 upper part of the tubular body, the shortest and most slender principalia of 

 every kind being met with at the upper end, just below the marginalia-verticil. 

 From this it follows that the lower part of the body is older than the upper and 

 that the summit is the youngest part of the whole sponge. 



