SYNOPSIS 389 



PAGE 



30. Diptiyes. 



A free-swimming colony with polymorphic (nutritive, 

 reproductive, protective, and natatory) zooids 250 



31. Porpita. 



Extreme polymorphism of zooids giving the colony the 

 character of a single physiological individual 253 



V. SOLID AGGREGATES IN WHICH CELL-DIFFERENTIATION, AC- 

 COMPANIED BY CELL FUSION, TAKES AN IMPORTANT PART IN 

 PRODUCING GREAT COMPLEXITY IN THE ADULT ORGANISM. 



32. Polygordius. 



A triploblastic, ccelomate animal with metameric seg- 

 mentation : prostomium, peristomium, metameres, and 

 anal segment : besides ecto- and endoderm there is a 

 well developed mesoderm divided into somatic and 

 splanchnic layers separated by the coelome : differentia- 

 tion of cells into fibres, &c. : muscle-plates formed as 

 cell-fusions : necessity for distributing system for supply 

 of food to parts of the body other than the enteric canal, 

 and for the removal of waste matters : circulatory, res- 

 piratory, and excretory systems : high development of 

 nervous system brain and ventral cord, afferent and 

 efferent nerves : characteristic developmental stages 

 oosperm, polyplast, gastrula (diploblastic), trochosphere 

 (diploblastic with stomodaeum and proctodaeum), late 

 trochosphere (triploblastic but accelomate) 271 



33. Mosses. 



Cell-differentiation very slight, but the type necessary to 

 lead up to ferns : sclerenchyma and axial bundle : dis- 

 tributing system rendered necessary by carbon dioxide 

 being taken in by the leaves, water and mineral salts by 

 the rhizoids : alternation of generations the leafy plant 

 is the gamobium, the agamobium being represented by 

 the spore-producing sporogonium : developmental stages 

 oosperm and polyplast, the latter becoming highly diffe- 

 rentiated to form the sporogonium 332 



34. Ferns. 



Extensive cell-differentiation : formation of fibres (elon- 

 gated cells) and vessels (cell-fusions) ; general differentia- 

 tion of tissues into epidermis, ground-parenchyma, and 

 vascular bundles : presence of true roots : the leafy plant 

 is the amagobium and produces spores from which the 

 gamobium, in the form of a small prothallus, arises : 

 developmental stages oosperm, polyplast, and phyllula 

 (leaf- and root-bearing stage) . 344 



