328 
ARTHUR DENDY. 
grains of sand, broken sponge-spicules, &c., which form an 
axial core, surrounded and held together by concentric layers 
of spongin. The secondary fibres (fig. 3, s.f .) are short, and 
contain no foreign bodies. They run in various planes, and 
unite together adjacent primary fibres which they meet at 
various angles; they may also branch and anastomose inter 
se. They measure from about 0’048 to 0 - 08 mm. in diameter. 
Thus the skeleton is thoroughly typical in structure and 
arrangement, and essentially the same as that of the ordinary 
bath-sponge, only much coarser. The distinction between the 
primary and secondary fibres is well marked. There is good 
reason to believe that primary fibres are phylogenetically of 
earlier origin than secondaries, and really very distinct from 
them. The primitive skeleton of the horny Sponges and of 
the siliceous (excluding the Hexactinellida) was probably 
composed simply of large, radiating spicules, or bands of spicules, 
coming out from a common centre, the centre of the Sponge. 
Such a condition occurs now very frequently amongst the 
Clavulina, Tethyadse, and Tetractinellida, all of which 
groups we have reason to believe are more primitive than the 
Halichondrina and Ceratosa, which have reticulate skele- 
tons. Amongst the Clavulina, Tethyadae, and Tetracti- 
nellida there is usually little or no spongin present, but the 
spicules are very large, and arranged side by side in dense tufts 
radiating from a common centre. In the Halichondrina 
spongin is almost invariably present in considerable quantities, 
and the spicules gradually diminish in size as the amount of 
spongin increases, so that the fibres of the skeleton, instead of 
being stiff and rigid, become flexible and elastic. All stages 
in the gradual replacement of spicules by spongin may be 
traced in the large family of the Chalininse, as I have 
endeavoured to show in a former paper (8). It is obvious 
that while a skeleton, composed solely of radiating primary 
fibres, would be very efficient so long as the fibres remained 
stiff and rigid, yet when the fibres became soft and flexible 
owing to the replacement of the spicules by spongin, such a 
skeleton would be almost useless. Hence arose the necessity 
