3 8 PROTOZOA 



in abeyance for tlie most part, and there is formed an apocyte 

 with a continuous investment, sometimes, however, chambered at 

 intervals by partitions between multinucleate units of protoplasm. 

 We started with a purely j^hysiological consideration, and we 

 have now arri^■ed at a morphological distinction, very valid 

 among higher organisms. 



Higher Plants consist of cells for the most ^"'""'^ (^(«'h 

 isolated in its oion cell-ca^vity, save for the few slender threads of 

 communication. 



Higher AiYTMALS consist of cells that are rarely isolated in 

 this way, hut are mostly in mutucd contact over the greater j^art of 

 their surface. 



Again, Plants take in either food or else the material for 

 food in solution through their surface, and only by diffusion 

 through the cell-wall. Insectivorous Plants that have the power 

 of capturing and digesting insects have no real internal cavity. 

 Animal-feeding Protista take in their food into the interior of 

 their protoplasm and digest it therein, and the Metazoa have an 

 internal cavity or stomach for the same purpose. Here again 

 there are exceptions in the case of certain internal parasites, such 

 as the Tapeworms and Acanthocephala (Vol. II. pp. 74, 174), 

 which have no stomachs, li\dng as they do in the dissolved 

 food-supplies of their hosts, but still possessing the general tissues 

 and organs of Metazoa. 



Corresponding with the absence of mouth, and the absorption ^ 

 instead of the prehension of food, we find that the movements 

 of plant-beings are limited. In the higher Plants, and many 

 lower ones, the colonial organism is firmly fixed or attached, and 

 the movements of its parts are confined to flexions. These are 

 produced by inequalities of growth; or by inequalities of temporary 

 distension of cell-masses, due to the absorption of liquid into 

 their vacuoles, while relaxation is effected by the cytoplasm and 

 cell-wall becoming pervious to the liquid. We find no case of a 

 differentiation of the cytoplasm within the cell into definite 

 muscular fibrils. In the lower Plants single naked motile cells dis- 

 seminate the species ; and the pairing-cells, or at least the males, 

 have the same motile character. In higher Cryptogams, Cycads, 

 and Ginkgo (the Maiden-hair Tree), the sperms alone are free- 

 swimming ; and as we pass to Flowering Plants, the migratory 

 character of the male cells is restricted to the smallest limits. 



