224 



SYSTEMATIC ZO( LOoY I OR TEACHERS. 



old; then compare with the old growth in fig. 108 C, in this the original 

 hollow membrane may be seen thiough the transparent walls. 



New twigs of the fiber arise from division at the extreme tip^i of the 

 of the new growths. (See fig. 109, C, and fig. 108 a.) The twigs thus 

 formed, although of an equal length at first, do not ah\ ays remain so, fo>" 

 the onward growth of one may assume a different direction from that of the 

 other, so that it may come in contact with the tip of another tube, forming 

 a comparatively small mesh in the net-work of fiber, while the other may 

 grow to a greater length, and then, anastomosing, foim a larger mssh, (see 

 fig. 110, K and S, where I give a life-size section of the growing skeleton 

 of tube sponge, and the different sizes of the meshes of the net- work may be 

 seen ) consequently this forms an irregular anastomosis. 



Fig. 108. 



Illustrating the anastomosing and twig division of the fiber of the tube sponge. B. a, division of 

 twig. C, a new twig before dividing; 0,0,0. in all is the anastomosing point 



As the -direction assumed by a new twig is rareiy the same as that ta- 

 ken by the one immediately behind it, sometimes the new twig comes in con- 

 tact with the thick skin covering of the outer surface of the sponge, or with 

 the lining membrane of the central tube; in either case the hollow at the tip 

 of the growing fiber becomes closed, and afterwards permanently sealed with 

 horn. For examples of these arrested growths, see fig. 109 A and 13, and 

 fig. 110 A to J. 



Thus it is easy to distinguish between what is continuous, or new growth 

 and an old, or arrested, growth. The growing twig being much more slen- 

 der than the old growth, is destitute of horny matter at the extreme tip, 

 and above all, the termination in the new twig remains constantly open. 

 while it is closed in the old growth. Thus by simply observing the condi- 



