Animal life and intelligence . and in some casescleavage. We will now proceed to pass in review the phenomenaof reproduction and development in animals. Attention has already been drawn to the differencebetween those lowly organisms, each of which is composedof a single cell—the protozoa, as they are termed—andthose higher organisms, called metazoa, in which thereare many cells with varied functions. Confining ourattention at first to the former group of unicellular animals,we find considerable diversities of form and habit, from [8 Animal Life and Intelligence. the relatively large, sluggish,
Animal life and intelligence . and in some casescleavage. We will now proceed to pass in review the phenomenaof reproduction and development in animals. Attention has already been drawn to the differencebetween those lowly organisms, each of which is composedof a single cell—the protozoa, as they are termed—andthose higher organisms, called metazoa, in which thereare many cells with varied functions. Confining ourattention at first to the former group of unicellular animals,we find considerable diversities of form and habit, from [8 Animal Life and Intelligence. the relatively large, sluggish, parasitic Gregarina, to theactive slipper-animalcule, or Paramcecium, or the beautiful,stalked bell-anirnalcule, or Vorticella; and from the small,slow-moving amoeba to the minute, intensely active many cases reproduction is by simple fission, as in theamoeba, where the nucleus first undergoes division; andthen the whole organism splits into two parts, each withits own nucleus. In other cases, also numerous, the. Protozoa. A, vorticella extended. B, the same contracted. C, D, monads. E, amoeba. F, Para-mecium. G, Gregarina. , contractile fibre ; , contractile vesicle; d., disc ; end., endo-plast; , food-vacuole; fl„ flagellum ; gu., gubernaculum ; n., nucleus; , potential anus;ps., (in A) peristome, (in E) pseudopodium ; vs., vestibule. organism passes into a quiescent state, and becomes sur-rounded with a more or less toughened cyst. The nucleusthen disappears, and the contents of the cyst break upinto a number of small bodies or spores. Eventually thecyst bursts, and the spores swarm forth. In the case ofsome active protozoa the minute creatures that swarm forthare more or less like the parent; but m the more sluggishkinds the minute forms are more active than the in the case of the gregarina, the minute spore-products are like small amoebae; while in other instances Reproduction and Development. the embryos, if so we may call them,
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