XA TITHE STUDY IK SCHOOLS. 



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grown ; and that the form of the sponge is determiner! by it. The tendency 

 of the growth of the sponge is thus upward, and although, somewhat sing- 

 ularly, the upward growth sometimes considerably precedes the thickene f 

 lining membrane, (see fig. Ill, A) the inclination of growth is very rarely, 

 or never, inward. For proof of the assertion that the lining membram- ar- 

 rests any inward growth. See fig. Ill A and B, a, a, a, a, being the lin- 

 ing membrane of the tube, also in the young specimen, fig. 112 , B, where 

 the tendency of growth is outward so as to gradually form a wider tube. 



Keturning again to arrested growths of the twigs of the skeleton. \vi 

 find that those on the outer surface of the sponge differ as a rule from t fjos< 

 on the inner. The differences are slight, but very constant. Externally, the 

 growth app2ars to be checked gradually as it comes in contact with the out- 

 er skin, and as a consequence, the arrested growths are pointed. See fig. 

 110, B and C; the two twigs ia the latter named being normal, that at H 



Fig. 111. 



a a a 



A B 



Section of Tube Sponge. O* tube above which is the closing membrane. 



having become arrested on the external surface, and the sponge having become 

 finished in that direction, a kind of secondary growth appears that is quite 

 independent of the hollow structures of the fiber. These form minute, rath- 

 er opaque, often branching twigs, having their origin directly on the end of 

 the arrested growths. An example of this may be also se?n in fig. 11<» A. 

 Internally, or against the lining membrane of the central tube, we find 

 the growth much more suddenly arrested. Hence the twigs present a more 

 truncated appearance. (See fig. 110, F, G and J, and fig. 109, A ami <'». 

 In some cases, the arrested twigs of the interior present a cup- like depression 

 at the extremity. (See fig. 110, H, and the one directly above it. , In 

 these twigs the hollow membrane is sealed, but is covered with only a thin 

 layer of horny matter. This is evidently the consequence of the twig hav- 



