SPONGES Day 
membrane of the central tube ; in either case the hollow at the tip of the grow- 
ing fiber becomes closed, and afterwards permanently sealed with horn. ( For 
examples of these arrested growths, see Fig. 16, A and B,and Fig, 17, A to 
J.) 
Thus it is easy to distinguish between what is a continuous, or new growth 
and an old, or arrested, growth. The growing twig, being much more slender 
Pigs Sue: 
B a b cL a a a 
ee os : f Cc le 
mr Kh Of 
\ Aw } 
Fiber of the Orange Tube Sponge. A and B, arrested twigs from the in- 
ner tube. C, new growths. D, E, and F, new growth anastomosing against 
old ; a, old growth ; 0, new. 
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 condition of the termination 
