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Key to the major animal groups. 



a' Organism composed of a single cell, or of a group, mass or string 

 of similar cells, or by continuous linear division gradually 

 becoming differentiated into more or less distinct tissues ; germ 

 cell never dividing into four in two planes at right angles to 

 each other 



b' mostly solitary, rounded, ovoid, or somewhat elongated, never 

 filiform ; in the few colonial types the individuals are irregularly 

 massed or more or less radially grouped ; if continuous cells 

 are formed the arrangement of these is spiral, in two or more 

 alternating series, or irregular, very exceptionally in a continuous 

 row ; chains are very rare, and are formed by the adherence 

 of a number of similar individuals : chiefly marine, but many 

 in fresh water, many parasitic, and many saprophytic Protozoa, 

 not usually solitary, and if so predominently elongated or 

 filiform ; the simpler types usually forming chains or films and 

 the hi^^her, by linear and subsequently ramifying growth from 

 the original germ cell, a more or less complicated thallus ; 

 mostly terrestrial or in fresh water, but many marine, especially 

 among the higher types ; many saprophytic and a number 



parasitic Thallophyta. 



a^ An 8-celled embryo is formed by the division of the original germ 

 cell and its derivatives in three planes each at right angles to 

 the other two 



b' the first cleavage plane divides the two sets of cells eventually 

 giving rise to the leaves and roots, which develop gradually 



and directly from them Bryophyta. 



Pteridophyta. 



b^ the 8-celled stage by a further division of the cells gives rise 

 to an embryo with a cortical (ectodermal or epidermal) layer and 

 an 'inner mass of cells, each group of cells giving rise to definite 

 and distinct structures 



c' development direct, without the segregation of mesoderm cells 



or the formation of a gastrula Phanerogamia. 



mesoderm cells are always segregated in the embryo, which 

 typically develops into a gastrula 

 d' primary axis of the gastrula becoming the axis of the adult, 

 which is radially or bilaterally symmetrical ; with an inde- 

 finite and diffuse nervous system and no excretory system 

 e' body consisting of a community of cells imperfectly inte- 

 grated and showing relatively little division of labor or 

 unified life, traversed by numerous small afferent canals 

 which lead into one or several much larger efferent canals, 

 with no definite organs, and including great numbers of 

 highly specialized calcareous or silicious spicules or a 

 mass of spongin fibers or of the two last combined, very 

 rarely with no supporting structures ; symmetry subradial 



Porifera. 



body a definite unit (or a group of distinct units) with a 

 gastrovascular cavity and a well developed muscular system, 

 and without inorganic inclusions ; skeletal structures, when 

 present, excreted 



