| +-- _Echinoconidae_ (p. 558). | +-- _Collyritidae_ (p. 559). +-- HOLOTHUROIDEA (pp. 431, 560) +-- ASPIDOCHIROTA (p. 570). +-- ELASIPODA (p. 571). +-- PELAGOTHURIIDA (p. 572). +-- DENDROCHIROTA (p. 572). +-- MOLPADIIDA (p. 575). +-- SYNAPTIDA (p. 575). PELMATOZOA (pp. 430, 579) +-- CRINOIDEA (p. 580) | +---------------------- Hyocrinidae (p. 590). | +---------------------- Rhizocrinidae (p. 590). | +---------------------- Pentacrinidae (p. 591). | +---------------------- Holopodidae (p. 592). | +---------------------- Comatulidae (p. 594). | +-- _INADUNATA_ (p. 595). | +-- ARTICULATA (p. 595). | +-- _CAMERATA_ (p. 595). +-- _THECOIDEA_ = _EDRIOASTEROIDEA_ (pp. 580, 596). +-- _CARPOIDEA_ (pp. 580, 596). +-- _CYSTOIDEA_ (pp. 580, 597). +-- _BLASTOIDEA_ (pp. 580, 599). PROTOZOA BY MARCUS HARTOG, M.A., TRINITY COLLEGE (D.SC. LOND.) Professor of Natural History in the Queen's College, Cork. {3}CHAPTER I PROTOZOA—INTRODUCTION—FUNCTIONS OF PROTOPLASM—CELL-DIVISION—ANIMALS AND PLANTS THE FREE AMOEBOID CELL.—If we examine under the microscope a fragment of one of the higher animals or plants, we find in it a very complex structure. A careful study shows that it always consists of certain minute elements of fundamentally the same nature, which are combined or fused into "tissues." In plants, where these units of structure were first studied, and where they are easier to recognise, each tiny unit is usually enclosed in an envelope or wall of woody or papery material, so that the whole plant is honeycombed. Each separate cavity was at first called a "cell"; and this term was then applied to the bounding wall, and finally to the unit of living matter within, the envelope receiving the name of "cell-wall." In this modern sense the "cell" consists of a viscid substance, called first in animals "sarcode" by Dujardin (1835), and later in plants "protoplasm"[1] by Von Mohl (1846). On the recognition of its common nature in both kingdoms, largely due to Max Schultze, the latter term prevailed; and it has passed from the vocabulary of biology into the domain of everyday life. We shall now examine the structure and behaviour of protoplasm and of the cell as an introduction to the detailed study of the Protozoa, or better still Protista,[2] the lowest types of living beings, and of Animals at large.