SPONGES ot 
inner. The differences are slight but very censtant. Externally the growth 
appears to be checked gradually as it comes in contact with the outer skin, and 
as a consequence the arrested growths are pointed. (Sce Fig. 17, C aid D: the 
two twigs in the latter named being normal, that at C having become arrested 
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Logger-head Sponge. Searlet Sponge. 
on the external surface, and the sponge has become finished, in that direction, « 
kind of secondary growth appears that is quite independent of the hollow 
structures of the fiber. These form minute, rather opaque, often branching 
twigs, having their origin directly on the end of the arrested growths. xam- 
ples of these may be seen in Fig. 17, A and B. 
Internally, or agains¢ the lining membrane of the central tube, we find the 
