164 AKTHUR DENDY. 



extremities of the chambers^ their place being taken by 

 pavement epithelium. This results in elongation and branching 

 of the exhalant canals^ and in corresponding shortening of 

 the chambers, which may be thus converted from the elon- 

 gated radial chambers of the Syconoid type to the short, 

 rounded, and irregularly arranged chambers of the Leuconoid 

 type. Branching of the primitively straight chambers is of 

 course also necessary in order to effect this change (compare 

 figs. 10, 19, 21, 17, 16). 



(7) By evagination or folding of the wall of the central 

 gastral cavity, which also results in elongation of the exhalant 

 canals, and possibly also in branching of the same (compare 

 fig. 7). As pointed out by SoUas (6), it is often extremely 

 difficult, if not impossible, to decide which of the two sets 

 of causes indicated in this and the preceding paragraph 

 respectively have operated in the production of a particular 

 type of canal system, though in some cases the arrangement 

 of the skeleton (fig. 7) may furnish a clue to the problem. 

 Though I do not doubt that in some cases a certain amount 

 of folding of the wall of the gastral cavity has taken place, 

 I do not believe that this cause has operated to any great 

 extent, in the production of the Leuconoid from the Syconoid 

 type of canal system. 



(8) By the fusion of different Syconoid or Leuconoid 

 individuals^ of a branching colony to form a compact whole, in 

 which the individuality of the different members is more or 

 less completely obliterated (compare fig. 15). 



Other minor causes have also doubtless aided in the produc- 

 tion of various modifications in the canal system; such are the 

 enormous dilatation of the gastral cavity and osculum in 

 Grrantia labyrinthica (9), and the strong development of 

 the mesoderm in the walls of the flagellated chambers which 

 takes place in some forms. The causes (or sets of causes) 



1 By the terms Sycouoid and Leuconoid individuals I mean simply in- 

 dividuals or " persons " consisting each of a single, central osculum-bearing 

 tube enclosing the gastral cavity, and surrounded by flagellated chambers and 

 canals arranged according to the " Sycon " or " Leucon " type, as the case 

 may be. 



