22<? 



SYSTEMATIC ZOOLOGY VOlt TEACHKfcS. 



astomosis appears to take place by the tip of one growing twig corning in 

 contact with the side of another growing twig. This combination must take 

 place near enough to the termination to have the inner membrane naked or 

 not .covered, even with a single layer of horny matter, then a perfect anasto- 

 mosis takes place; that is, the tip of the anastomosing fiber absorbs that 

 portion of the others tubular membrane with which it comes in contact; thus 

 the li >llo\v becom2s continuous from one fiber to another. The growth o/ 

 the twig coming in contact is at once checked and horny matter is deposited 

 over r lie place of jointure. See fig. 108 where I give three examples of the 

 anastomosis of growing twigs at A, B and C; o, being the point of jointure, 

 and as will be seen this is not always at the same angle, that at B being 

 at right angles, that at C being obliquely downward; while at A it is evi- 

 dent that the tips of two twigs came directly in contact, and upon anastomo- 

 sing the growth of both was checked. 



A 



B 



G 



Illustrating the skeleton of Tube Sponge. A, B. arrested twigs with secondary growtr.B; C. twig 

 irrested just oitcr dividing. D, arrested twigs from external sui:ace. F. G, J, arise frcm 8urlrce o x 

 central tube. H, and above, the same with cup-like depressions. K and S, portion of skeleton en- 

 larged lour times. M and N, flattened fibers from the filamentous growth. E, arreated twi£8 which 

 came in contact with a hard aurfaee. 



It is absolutely certain according to my most carefully made and often 

 repeated observations that no buds ever start from the side of a twig or 

 branch. Hence the interior of the skeleton of the tube sponge presents a 

 very finished appearance, there being no protruding twigs or arrested growth s 

 which have not anastomosed. See fig. 110, K, where I have given a portion 

 of the interior of the skeleton, enlarged. This is not true, however, where the 

 twigs have come in contact with the skin of the outer surface, or with the 

 membrane lining the central tube, for here, as I have previously remarked, 

 we may find many short, protruding twigs. 



As both sarcode and skeleton become arrested as soon as they become 

 covered with the lining membrane of the central tube, it will at once be 

 seen that there can be no inerease of growth inward after this membrane if* 



