ECHINODERMATA (_CONTINUED_): DEVELOPMENT AND PHYLOGENY 601
INDEX 625
SCHEME OF THE CLASSIFICATION ADOPTED IN THIS BOOK
_The names of extinct groups are printed in italics._
PROTOZOA (pp. 1, 48).
SARCODINA (p. 51)
+-- RHIZOPODA (p. 51)
| +-- Lobosa (p. 51).
| +-- Filosa (p. 52).
+-- FORAMINIFERA (p. 58)
| +-- Allogromidiaceae (p. 58).
| +-- Astrorhizidaceae (p. 59).
| +-- Lituolidaceae (p. 59).
| +-- Miliolidaceae (p. 59).
| +-- Textulariaceae (p. 59).
| +-- Cheilostomellaceae (p. 59).
| +-- Lagenaceae (p. 59).
| +-- Globigerinidae (p. 59).
| +-- Rotaliaceae (p. 59).
| +-- Nummulitaceae (p. 59).
+-- HELIOZOA (p. 70)
| +-- Aphrothoraca (p. 70).
| +-- Chlamydophora (p. 71).
| +-- Chalarothoraca (p. 71).
| +-- Desmothoraca (p. 71).
+-- RADIOLARIA (p. 75)
| +-- PORULOSA = HOLOTRYPASTA (p. 76)
| | +-- Spumellaria = Peripylaea (pp. 76, 77)
| | | +-- Collodaria (p. 77)
| | | | +-- Colloidea (p. 77).
| | | | +-- Beloidea (p. 77).
| | | +-- Sphaerellaria (p. 77)
| | | +-- Sphaeroidea (p. 77).
| | | +-- Prunoidea (p. 77).
| | | +-- Discoidea (p. 77).
| | | +-- Larcoidea (p. 77).
| | +-- Acantharia = Actipylaea (pp. 76, 78)
| | +-- Actinelida (p. 78).
| | +-- Acanthonida (p. 78).
| | +-- Sphaerophracta (p. 78).
| | +-- Prunophracta (p. 78).
| +-- OSCULOSA = MONOTRYPASTA (p. 76)
| +-- Nassellaria = Monopylaea (pp. 76, 78)
| | +-- Nassoidea (p. 78).
| | +-- Plectoidea (p. 78).
| | +-- Stephoidea (p. 78).
| | +-- Spyroidea (p. 78).
| | +-- Botryoidea (p. 79).
| | +-- Cyrtoidea (p. 79).
| +-- Phaeodaria = Cannopylaea
| = Tripylaea (pp. 76, 79)
| +-- Phaeocystina (p. 79).
| +-- Phaeosphaeria (p. 79).
| +-- Phaeogromia (p. 79).
| +-- Phaeoconchia (p. 79).
+-- PROTEOMYXA (p. 88)
| +-- Myxoidea (p. 89)
| | +-- Zoosporeae (p. 89).
| | +-- Azoosporeae (p. 89).
| +-- Catallacta (p. 89).
+-- MYCETOZOA (p. 90) --------+
+-- Acrasieae (p. 90).
+-- Filoplasmodieae (p. 90).
+-- Myxomycetes (pp. 90, 91).
SPOROZOA (p. 94)
+-- TELOSPORIDIA (p. 97)
| +-- Gregarinidaceae (pp. 97, 98)
| | +-- Schizogregarinidae (p. 97).
| | +-- Acephalinidae (p. 97).
| | +-- Dicystidae (p. 97).
| +-- Coccidiaceae (pp. 97, 99)
| +-- Coccidiidae (pp. 97, 99).
| +-- Haemosporidae (pp. 97, 102).
| +-- Acystosporidae (pp. 97, 102).
+-- NEOSPORIDIA (p. 97)
+-- Myxosporidiaceae (pp. 98, 106).
+-- Actinomyxidiaceae (p. 98).
+-- Sarcosporidiaceae (pp. 98, 108).
FLAGELLATA (p. 109) -----+
+-- Pantostomata (p. 109).
+-- Protomastigaceae (p. 110)
| +-- Distomatidae (p. 110).
| +-- Oikomonadidae (p. 111).
| +-- Bicoecidae (p. 111).
| +-- Craspedomonadidae (pp. 111, 121).
| +-- Phalansteridae (p. 111).
| +-- Monadidae (p. 111).
| +-- Bodonidae (p. 111).
| +-- Amphimonadidae (p. 111).
| +-- Trimastigidae (p. 111).
| +-- Polymastigidae (p. 111).
| +-- Trichonymphidae (pp. 111, 123).
| +-- Opalinidae (pp. 111, 123).
+-- Chrysomonadaceae (pp. 110, 125)
| +-- Coccolithophoridae (p. 114).
+-- Cryptomonadaceae (p. 110).
+-- Volvocaceae (pp. 110, 111)
| +-- Chlamydomonadidae (pp. 111, 125).
| +-- Volvocidae (pp. 111, 126).
+-- Chloromonadaceae (p. 110).
+-- Euglenaceae (pp. 110, 124).
+-- Silicoflagellata (pp. 110, 114).
+-- Cystoflagellata (pp. 110, 132).
+-- Dinoflagellata (pp. 110, 130).
INFUSORIA (p. 136)
+-- CILIATA (p. 137)
| +-- Gymnostomaceae (pp. 137, 152).
| +-- Aspirotrichaceae (pp. 137, 153).
| +-- Heterotrichaceae (pp. 137, 153).
| +-- Oligotrichaceae (pp. 137, 155).
| +-- Hypotrichaceae (pp. 137, 138).
| +-- Peritrichaceae (pp. 138, 155).
+-- SUCTORIA = TENTACULIFERA (p. 158).
PORIFERA (p. 163)
Class. Sub-Class. Order. Family. Sub-Family.
MEGAMASTICTORA (pp. 183, 184)
+-- CALCAREA (p. 184)
+-- HOMOCOELA (p. 185)
| +-- Leucosoleniidae (p. 185).
| +-- Clathrinidae (p. 185).
+-- HETEROCOELA (p. 187)
+-- Sycettidae (p. 187).
+-- Grantiidae (p. 192).
+-- Heteropidae (p. 192).
+-- Amphoriscidae (p. 192).
+-- Pharetronidae (p. 192)
| +-- Dialytinae (p. 192).
| +-- Lithoninae (p. 193).
+-- Astroscleridae (p. 194).
MICROMASTICTORA (pp. 183, 195)
+-- MYXOSPONGIAE (p. 196).
+-- HEXACTINELLIDA (p. 197)
| +-- AMPHIDISCOPHORA (p. 203).
| +-- HEXASTEROPHORA (p. 203).
| +---------------------- _Receptaculitidae_ (p. 207).
+-- _OCTACTINELLIDA_ (p. 208).
+-- _HETERACTINELLIDA_ (p. 208).
+-- DEMOSPONGIAE (p. 209)
+-- TETRACTINELLIDA (pp. 211, 212)
| +-- Choristida (p. 212).
| +-- Lithistida (pp. 212, 215).
+-- MONAXONIDA (pp. 211, 216)
| +-- Halichondrina (p. 217).
| +-- Spintharophora (p. 217).
+-- CERATOSA (pp. 211, 220)
+-- Dictyoceratina (p. 220)
| +-- Spongidae (p. 220).
| +-- Spongelidae (p. 220).
+-- Dendroceratina (pp. 220, 221).
COELENTERATA (p. 243)
Class. Order. Sub-Order. Family. Sub-Family.
HYDROZOA (p. 249)
+-- ELEUTHEROBLASTEA (p. 253).
+-- MILLEPORINA (p. 257).
+-- GYMNOBLASTEA (ANTHOMEDUSAE) (p. 262)
| +-- Bougainvilliidae (p. 269).
| +-- Podocorynidae (p. 270).
| +-- Clavatellidae (p. 270).
| +-- Cladonemidae (p. 270).
| +-- Tubulariidae (p. 271).
| +-- Ceratellidae (p. 271).
| +-- Pennariidae (p. 272).
| +-- Corynidae (p. 272).
| +-- Clavidae (p. 272).
| +-- Tiaridae (p. 273).
| +-- Corymorphidae (p. 273).
| +-- Hydrolaridae (p. 273).
| +-- Monobrachiidae (p. 274).
| +-- Myriothelidae (p. 274).
| +-- Pelagohydridae (p. 274).
+-- CALYPTOBLASTEA (LEPTOMEDUSAE) (p. 275)
| +-- Aequoreidae (p. 278).
| +-- Thaumantiidae (p. 278).
| +-- Cannotidae (p. 278).
| +-- Sertulariidae (p. 278).
| +-- Plumulariidae (p. 279)
| | +-- Eleutheroplea (p. 279).
| | +-- Statoplea (p. 279).
| +-- Hydroceratinidae (p. 279).
| +-- Campanulariidae (p. 280).
| +-- Eucopidae (p. 280).
| +-- _Dendrograptidae_ (p. 281).
+-- _GRAPTOLITOIDEA_ (p. 281)
| +-- _Monoprionidae_ (p. 282).
| +-- _Diprionidae_ (p. 282).
| +-- _Retiolitidae_ (p. 282).
| +-- _Stromatoporidae_ (p. 283).
+-- STYLASTERINA (p. 283)
| +-- Stylasteridae (p. 285).
+-- TRACHOMEDUSAE (p. 288)
| +-- Olindiidae (p. 291).
| +-- Petasidae (p. 294).
| +-- Trachynemidae (p. 294).
| +-- Pectyllidae (p. 294).
| +-- Aglauridae (p. 294).
| +-- Geryoniidae (p. 295).
+-- NARCOMEDUSAE (p. 295)
| +-- Cunanthidae (p. 296).
| +-- Peganthidae (p. 296).
| +-- Aeginidae (p. 296).
| +-- Solmaridae (p. 296).
+-- SIPHONOPHORA (p. 297)
+-- Calycophorae (p. 305)
| +-- Monophyidae (p. 306)
| | +-- Sphaeronectinae (p. 306).
| | +-- Cymbonectinae (p. 306).
| +-- Diphyidae (p. 306)
| | | Oppositae (p. 306).
| | +-- Amphicaryoninae (p. 306) --+
| | +-- Prayinae (p. 306) ---------+
| | +-- Desmophyinae (p. 307) -----+
| | +-- Stephanophyinae (p. 307) --+
| | | Superpositae (p. 307).
| | +-- Galeolarinae (p. 307) -----+
| | +-- Diphyopsinae (p. 307) -----+
| | +-- Abylinae (p. 307) ---------+
| +-- Polyphyidae (p. 307).
+-- Physophorae (p. 307)
+-- Physonectidae (p. 307)
| +-- Agalminae (p. 307).
| +-- Apoleminae (p. 307).
| +-- Physophorinae (p. 308).
+-- Auronectidae (p. 308).
+-- Rhizophysaliidae (p. 308).
+-- Chondrophoridae (p. 308).
SCYPHOZOA = SCYPHOMEDUSAE (pp. 249, 310)
+-- CUBOMEDUSAE (p. 318)
| +-- Charybdeidae (p. 318).
| +-- Chirodropidae (p. 319).
| +-- Tripedaliidae (p. 319).
+-- STAUROMEDUSAE (p. 320)
| +-- Lucernariidae (p. 320).
| +-- Depastridae (p. 321).
| +-- Stenoscyphidae (p. 321).
+-- CORONATA (p. 321)
| +-- Periphyllidae (p. 322).
| +-- Ephyropsidae (p. 322).
| +-- Atollidae (p. 322).
+-- DISCOPHORA (p. 323)
+-- Semaeostomata (p. 323)
| +-- Pelagiidae (p. 323).
| +-- Cyanaeidae (p. 324).
| +-- Ulmaridae (p. 324).
+-- Rhizostomata (p. 324)
+-- Cassiopeidae (p. 324) --+ = Arcadomyaria
| --+ (p. 324).
+-- Cepheidae (p. 324) --+ = Radiomyaria
| --+ (p. 324).
+-- Rhizostomatidae (p. 325)--+
+-- Lychnorhizidae (p. 325) | = Cyclomyaria
+-- Leptobrachiidae (p. 325) | (p. 325).
+-- Catostylidae (p. 325) --+
Class. Sub-Class. Grade. Order. Sub-Order. Family.
ANTHOZOA = ACTINOZOA (pp. 249, 326)
+-- ALCYONARIA (p. 329)
| +-- PROTOALCYONACEA (p. 342)
| | +-- Haimeidae (p. 342).
| +-- SYNALCYONACEA (p. 342)
| +-- Stolonifera (p. 342)
| | +-- Cornulariidae (p. 344).
| | +-- Clavulariidae (p. 344).
| | +-- Tubiporidae (p. 344).
| | +-- _Favositidae_ (p. 344).
| +-- Coenothecalia (p. 344)
| | +-- _Heliolitidae_ (p. 346).
| | +-- Helioporidae (p. 346).
| | +-- _Coccoseridae_ (p. 346).
| | +-- _Thecidae_ (p. 346).
| | +-- _Chaetetidae_ (p. 346).
| +-- Alcyonacea (p. 346)
| | +-- Xeniidae (p. 348).
| | +-- Telestidae (p. 348).
| | +-- Coelogorgiidae (p. 349).
| | +-- Alcyoniidae (p. 349).
| | +-- Nephthyidae (p. 349).
| | +-- Siphonogorgiidae (p. 349).
| +-- Gorgonacea (p. 350)
| | +-- Pseudaxonia (p. 350)
| | | +-- Briareidae (p. 350).
| | | +-- Sclerogorgiidae (p. 351).
| | | +-- Melitodidae (p. 351).
| | | +-- Coralliidae (p. 352).
| | +-- Axifera (p. 353)
| | +-- Isidae (p. 353).
| | +-- Primnoidae (p. 354).
| | +-- Chrysogorgiidae (p. 355).
| | +-- Muriceidae (p. 355).
| | +-- Plexauridae (p. 356).
| | +-- Gorgoniidae (p. 356).
| | +-- Gorgonellidae (p. 357).
| +-- Pennatulacea (p. 358)
| +-- Pennatuleae (p. 361)
| | +-- Pteroeididae (p. 361).
| | +-- Pennatulidae (p. 361).
| | +-- Virgulariidae (p. 362).
| +-- Spicatae (p. 362)
| | +-- Funiculinidae (p. 362).
| | +-- Anthoptilidae (p. 362).
| | +-- Kophobelemnonidae (p. 362).
| | +-- Umbellulidae (p. 362).
| +-- Verticilladeae (p. 363)
| +-- Renilleae (p. 363)
| | +-- Renillidae (p. 363).
| +-- Veretilleae (p. 364)
+-- ZOANTHARIA (pp. 329, 365)
| Edwardsiidea (p. 375)
| +-- Edwardsiidae (p. 377).
| +-- Protantheidae (p. 377).
| Actiniaria (p. 377)
| +-- Actiniina (p. 380)
| | +-- Halcampidae (p. 380).
| | +-- Actiniidae (p. 381).
| | +-- Sagartiidae (p. 381).
| | +-- Aliciidae (p. 382).
| | +-- Phyllactidae (p. 382).
| | +-- Bunodidae (p. 382).
| | +-- Minyadidae (p. 383).
| +-- Stichodactylina (p. 383)
| +-- Corallimorphidae (p. 383).
| +-- Discosomatidae (p. 383).
| +-- Rhodactidae (p. 383).
| +-- Thalassianthidae (p. 383).
+-- Madreporaria (p. 384)
| +----------- _Cyathophyllidae_ (p. 394).
| +----------- _Cyathaxoniidae_ (p. 394).
| +----------- _Cystiphyllidae_ (p. 394).
| +-- Entocnemaria (p. 394)
| | +-- Madreporidae (p. 395).
| | +-- Poritidae (p. 396).
| +-- Cyclocnemaria (p. 397)
| | Aporosa (p. 397).
| +-- Turbinoliidae (p. 398) ----+
| +-- Oculinidae (p. 399) -------+
| +-- Astraeidae (p. 399) -------+
| | A. Gemmantes (p. 400) |
| | A. Fissiparantes (p. 400) |
| +-- Trochosmiliacea -----------+
| | [Sub-Fam.] (p. 401) |
| +-- Pocilloporidae (p. 401) ---+
| | Fungacea (p. 402).
| +-- Plesiofungiidae (p. 403) --+
| +-- Fungiidae (p. 403) --------+
| +-- Cycloseridae (p. 404) -----+
| +-- _Plesioporitidae_ (p. 404) +
| +-- Eupsammiidae (p. 404) -----+
+-- Zoanthidea (p. 404) ---+
| +-- Zoanthidae (p. 404).
| +-- _Zaphrentidae_ (p. 406).
+-- Antipathidea = Antipatharia (p. 407)
| +-- Antipathidae (p. 408).
| +-- Leiopathidae (p. 409).
| +-- Dendrobrachiidae (p. 409).
+-- Cerianthidea (p. 409).
CTENOPHORA (p. 412).
Class. Order. Family.
TENTACULATA (p. 417)
+-- CYDIPPIDEA (p. 417)
| +-- Mertensiidae (p. 417).
| +-- Callianiridae (p. 417).
| +-- Pleurobrachiidae (p. 418).
+-- LOBATA (p. 418)
| +-- Lesueuriidae (p. 419).
| +-- Bolinidae (p. 419).
| +-- Deiopeidae (p. 419).
| +-- Eurhamphaeidae (p. 419).
| +-- Eucharidae (p. 420).
| +-- Mnemiidae (p. 420).
| +-- Calymmidae (p. 420).
| +-- Ocyroidae (p. 420).
+-- CESTOIDEA (p. 420)
| +-- Cestidae (p. 420).
+-- PLATYCTENEA (p. 421)
+-- Ctenoplanidae (p. 421).
+-- Coeloplanidae (p. 422).
NUDA (p. 423) Beroidae (p. 423).
ECHINODERMATA (p. 425).
Sub-Phylum. Class. Order. Sub-Order. Family. Sub-Family.
ELEUTHEROZOA (p. 430)
+-- ASTEROIDEA (pp. 430, 431)
| +-- SPINULOSA (pp. 461, 462)
| | +-- Echinasteridae (p. 462).
| | +-- Solasteridae (p. 462).
| | +-- Asterinidae (p. 463).
| | +-- Poraniidae (p. 464).
| | +-- Ganeriidae (p. 464).
| | +-- Mithrodiidae (p. 464).
| +-- VELATA (pp. 461, 464)
| | +-- Pythonasteridae (p. 464).
| | +-- Myxasteridae (p. 464).
| | +-- Pterasteridae (p. 466).
| +-- PAXILLOSA (pp. 461, 466)
| | +-- Archasteridae (p. 466).
| | +-- Astropectinidae (p. 467).
| | +-- Porcellanasteridae (p. 470).
| +-- VALVATA (pp. 461, 471)
| | +-- Linckiidae (p. 471).
| | +-- Pentagonasteridae (p. 471).
| | +-- Gymnasteridae (p. 471).
| | +-- Antheneidae (p. 471).
| | +-- Pentacerotidae (p. 471).
| +-- FORCIPULATA (pp. 462, 473)
| +-- Asteriidae (p. 473).
| +-- Heliasteridae (p. 474).
| +-- Zoroasteridae (p. 474).
| +-- Stichasteridae (p. 474).
| +-- Pedicellasteridae (p. 474).
| +-- Brisingidae (p. 474).
+-- OPHIUROIDEA (pp. 431, 477)
| +-- STREPTOPHIURAE (p. 494)
| +-- ZYGOPHIURAE (pp. 494, 495)
| | +-- Ophiolepididae (p. 495).
| | +-- Amphiuridae (p. 497).
| | +-- Ophiocomidae (p. 499).
| | +-- Ophiothricidae (p. 499).
| +-- CLADOPHIURAE (pp. 494, 500)
| +-- Astroschemidae (p. 501).
| +-- Trichasteridae (p. 501).
| +-- Euryalidae (p. 501).
+-- ECHINOIDEA (pp. 431, 503)
| +-- ENDOCYCLICA (pp. 529, 530)
| | +-- Cidaridae (p. 533).
| | +-- Echinothuriidae (p. 535).
| | +-- Saleniidae (p. 537).
| | +-- Arbaciidae (p. 538).
| | +-- Diadematidae (p. 538).
| | +-- Echinidae (p. 539)
| | +-- Temnopleurinae (p. 539).
| | +-- Echininae (p. 539).
| +-- CLYPEASTROIDEA (pp. 529, 542)
| +-- Protoclypeastroidea (p. 548).
| +-- Euclypeastroidea (p. 549)
| +-- Fibularidae (p. 549).
| +-- Echinanthidae
| | = Clypeastridae (p. 549).
| +-- Laganidae (p. 549).
| +-- Scutellidae (p. 549).
+-- SPATANGOIDEA (pp. 529, 549)
| | Asternata (p. 554).
| +-- Echinonidae (p. 553) ------+
| +-- Nucleolidae (p. 554) ------+
| +-- Cassidulidae (p. 554) -----+
| | Sternata (p. 554).
| +-- Ananchytidae (p. 554) -----+
| +-- Palaeostomatidae (p. 554) -+
| +-- Spatangidae (p. 554) ------+
| +-- Brissidae (p. 556) --------+
| +-- _Archaeocidaridae_ (p. 557).
| +-- _Melonitidae_ (p. 557).
| +-- _Tiarechinidae_ (p. 557).
| +-- _Holectypoidea_ (p. 558).
| +-- _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.
{4}It is not in detached fragments of the tissues of the higher animals
that we can best carry on this study: for here the cells are in singularly
close connexion with their neighbours during life; the proper appointed
work of each is intimately related to that of the others; and this
co-operation has so trained and specially modified each cell that the
artificial severance and isolation is detrimental to its well-being, if not
necessarily fatal to its very life. Again, in plants the presence of a
cell-wall interferes in many ways with the free behaviour of the cell. But
in the blood and lymph of higher animals there float isolated cells, the
white corpuscles or "leucocytes" of human histology, which, despite their
minuteness (1/3000 in. in diameter), are in many respects suitable objects.
Further, in our waters, fresh or salt, we may find similar free-living
individual cells, in many respects resembling the leucocytes, but even
better suited for our study. For, in the first place, we can far more
readily reproduce under the microscope the normal conditions of their life;
and, moreover, these free organisms are often many times larger than the
leucocyte. Such free organisms are individual Protozoa, and are called by
the general term "Amoebae." A large Amoeba may measure in its most
contracted state 1/100 in. or 250 µ in diameter,[3] and some closely allied
species (_Pelomyxa_, see p. 52) even twelve times this amount. If we place
an Amoeba or a leucocyte under the microscope (Fig. 1), we shall find that
its form, at first spherical, soon begins to alter. To confine our
attention to the external changes, we note that the outline, from circular,
soon becomes "island-shaped" by the outgrowth of a promontory here, the
indenting of a bay there. The promontory may enlarge into a peninsula, and
thus grow until it becomes a new mainland, while the old mainland dwindles
into a mere promontory, and is finally lost. In this way a crawling motion
is effected.[4] The promontories are called "pseudopodia" (=
{5}"false-feet"), and the general character of such motion is called
"amoeboid."[5]
[Illustration: FIG. 1.—_Amoeba_, showing clear ectoplasm, granular
endoplasm, dark nucleus, and lighter contractile vacuole. The changes of
form, _a-f_, are of the _A. limax_ type; _g_, _h_, of the _A. proteus_
type. (From Verworn.)]
The living substance, protoplasm,[6] has been termed a "jelly," a word,
however, that is quite inapplicable to it in its living state. It is
viscid, almost semi-fluid, and may well be compared to very soft dough
which has already begun to rise. It resembles it in often having a number
of spaces, small or large, filled with liquid (not gas). These are termed
"vacuoles" or "alveoles," according to their greater or their lesser
dimensions. In some cases a vacuole is traversed by strands of plasmic
substance, just as we may find such strands stretching across the larger
spaces of a very light loaf; but of course in the living cell these are
constantly undergoing changes. If we "fix" a cell (_i.e._ kill it by
{6}sudden heat or certain chemical coagulants),[7] and examine it under the
microscope, the intermediate substance between the vacuoles that we have
already seen in life is again found either to be finely honeycombed or else
resolved into a network like that of a sponge. The former structure is
called a "foam" or "alveolar" structure, the latter a "reticulate"
structure. The alveoles are about 1 µ in diameter, and spheroidal or
polygonal by mutual contact, elongated, however, radially to any free
surface, whether it be that of the cell itself or that of a larger alveole
or vacuole. The inner layer of protoplasm ("endoplasm," "endosarc")
contains also granules of various nature, reserve matters of various kinds,
oil-globules, and particles of mineral matter[8] which are waste products,
and are called "excretory." In fixed specimens these granules are seen to
occupy the nodes of the network or of the alveoli, that is, the points
where two or three boundaries meet.[9] The outermost layer ("ectoplasm" or
"ectosarc") appears in the live Amoeba structureless and hyaline, even
under conditions the most favourable for observation. The refractive index
of protoplasm, when living, is always well under 1.4, that of the fixed and
dehydrated substance is slightly over 1.6.
Again, within the outer protoplasm is found a body of slightly higher
refractivity and of definite outline, termed the "nucleus" (Figs. 1, 2).
This has a definite "wall" of plasmic nature, and a substance so closely
resembling the outer protoplasm in character, that we call it the
"nucleoplasm" (also "linin"), distinguishing the outer plasm as
"cytoplasm"; the term "protoplasm" including both. Within the nucleoplasm
are granules of a substance that stains well with the commoner dyes,
especially the "basic" ones, and which has hence been called "chromatin."
The linin is {7}usually arranged in a distinct network, confluent into a
"parietal layer" within the nuclear wall; the meshes traversing a cavity
full of liquid, the nuclear sap, and containing in their course the
granules; while in the cavity are usually found one or two droplets of a
denser substance termed "nucleoles." These differ slightly in composition
from the chromatin granules[10] (see p. 24 f.).
The movements of the leucocyte or Amoeba are usually most active at a
temperature of about 40° C. or 100° F., the "optimum." They cease when the
temperature falls to a point, the "minimum," varying with the organism, but
never below freezing-point; they recommence when the temperature rises
again to the same point at which they stopped. If now the temperature be
raised to a certain amount above 40° they stop, but may recommence if the
temperature has not exceeded a certain point, the "maximum" (45° C. is a
common maximum). If it has been raised to a still higher point they will
not recommence under any circumstances whatever.
Again, a slight electric shock will determine the retraction of all
processes, and a period of rest in a spherical condition. A milder shock
will only arrest the movements. But a stronger shock may arrest them
permanently. We may often note a relation of the movements towards a
surface, tending to keep the Amoeba in contact with it, whether it be the
surface of a solid or that of an air-bubble in the liquid (see also p. 20).
[Illustration: FIG. 2.—Ovum of a Sea-Urchin, showing the radially striated
cell-membrane, the cytoplasm containing yolk-granules, the large nucleus
(germinal vesicle), with its network of linin containing chromatin
granules, and a large nucleole (germinal spot). (From Balfour's
_Embryology_, after Hertwig.)]
If a gentle current be set up in the water, we find that the movements of
the Amoeba are so co-ordinated that it moves upstream; this must of course
be of advantage in nature, as keeping the being in its place, against the
streams set up by larger creatures, etc. (see also p. 21).
If substances soluble in water be introduced the Amoeba will, {8}as a rule,
move away from the region of greater concentration for some substances, but
towards it (provided it be not excessive) for others. (See also pp. 22,
23.) We find, indeed, that there is for substances of the latter category a
minimum of concentration, below which no effect is seen, and a maximum
beyond which further concentration repels. The easiest way to make such
observations is to take up a little strong solution in a capillary tube
sealed at the far end, and to introduce its open end into the water, and
let the solution diffuse out, so that this end may be regarded as
surrounded by zones of continuously decreasing strength. In the process of
inflammation (of a Higher Animal) it has been found that the white
corpuscles are so attracted by the source of irritation that they creep out
of the capillaries, and crowd towards it.
We cannot imagine a piece of dough exhibiting any of these reactions, or
the like of them; it can only move passively under the action of some one
or other of the recognised physical forces, and that only in direct
_quantitative_ relation to the work that such forces can effect; in other
words, the dough can have work done on it, but it cannot do work. The
Amoeba or leucocyte on the contrary does work. It moves under the various
circumstances by the transformation of some of its internal energy from the
"potential" into the "kinetic" state, the condition corresponding with this
being essentially a liberation of heat or work, either by the breaking down
of its internal substances, or by the combination of some of them with
oxygen.[11] Such of these changes as involve the excretion of carbonic acid
are termed "respiratory."
This liberation of energy is the "response" to an action of itself
inadequate to produce it; and has been compared not inaptly to the
discharge of a cannon, where foot-tons of energy are liberated in
consequence of the pull of a few inch-grains on the trigger, or to an
indefinitely small push which makes electric contact: the energy set free
is that which was stored up in the charge. This capacity for liberating
energy stored up within, in response to a relatively small impulse from
without, is termed "irritability"; the external impulse is termed the
"stimulus." The responsive act has been termed "contractility," because it
so often means an obvious contraction, but is better termed {9}"motility ";
and irritability evinced by motility is characteristic of all living beings
save when in the temporary condition of "rest."
Again, in the case of the cannon, the gunner after its discharge has to
replenish it for future action with a fresh cartridge; the Amoeba or
leucocyte can replenish itself—it "feeds." When it comes in contact with a
fragment of suitable material, it enwraps it by its pseudopodia (Fig. 3),
and its edges coalesce where they touch on the far side as completely as we
can join up the edges of dough round the apple in a dumpling. It dissolves
all that can be dissolved—_i.e._ it "digests" it, and then absorbs the
dissolved material into its substance, both to replace what it has lost by
its previous activity and to supply fuel for future liberation of energy;
this process is termed "nutrition," and is another characteristic of living
beings.
[Illustration: FIG. 3.—_Amoeba_ devouring a plant cell; four successive
stages of ingestion. (From Verworn.)]
Again, as a second result of the nutrition, part of the food taken in goes
to effect an increase of the living protoplasm, and that of every part, not
merely of the surface—it is "assimilated"; while the rest of the food is
transformed into reserves, or consumed and directly applied to the
liberation of energy. The increase in bulk due to nutrition is thus
twofold: part is the increase of the protoplasm itself—"assimilative
growth," part is the storage of reserves—"accumulative growth": these
reserves being available in turn by digestion, whether for future true
growth or for consumption to liberate energy for the work of the cell.
We can conceive that our cannon might have an automatic feed for the supply
of fresh cartridges after each shot; but not that it could make provision
for an increase of its own bulk, so as to gain in calibre and strength, nor
even for the restoration {10}of its inner surface constantly worn away by
the erosion of its discharges. Growth—and that growth "interstitial,"
operating at every point of the protoplasm, not merely at its surface—is a
character of all living beings at some stage, though they may ultimately
lose the capacity to grow. Nothing at all comparable to interstitial growth
has been recognised in not-living matter.[12]
[Illustration: FIG. 4.—_Amoeba polypodia_ in successive stages of equal
fission; nucleus dark, contractile vacuole clear. (From Verworn, after F.
E. Schulze.)]
Again, when an Amoeba has grown to a certain size, its nucleus divides into
two nuclei, and its cytoplasmic body, as we may term it, elongates, narrows
in the middle so as to assume the shape of a dumb-bell or finger-biscuit,
and the two halves, crawling in opposite directions, separate by the giving
way of the connecting waist, forming two new Amoebas, each with its nucleus
(Fig. 4). This is a process of "reproduction"; the special case is one of
"equal fission" or "binary division." The original cell is termed the
"mother," with respect to the two new ones, and these are of course with
respect to it the "daughters," and {11}"sisters" to one another. We must
bear in mind that in this self-sacrificing maternity the mother is resolved
into her children, and her very existence is lost in their production. The
above phenomena, IRRITABILITY, MOTILITY, DIGESTION, NUTRITION, GROWTH,
REPRODUCTION, are all characteristic of living beings at some stage or
other, though one or more may often be temporarily or permanently absent;
they are therefore called "vital processes."
If, on the other hand, we violently compress the cell, if we pass a very
strong electric shock through it, or a strong continuous current, or expose
it to a temperature much above 45° C., or to the action of certain chemical
substances, such as strong acids or alkalies, or alcohol or corrosive
sublimate, we find that all these vital processes are arrested once and for
all; henceforward the cell is on a par with any not-living substance. Such
a change is called "DEATH," and the "capacity for death" is one of the most
marked characters of living beings. This change is associated with changes
in the mechanical and optical properties of the protoplasm, which loses its
viscidity and becomes opaque, having undergone a process of _de_-solution;
for the water it contained is now held only mechanically in the interstices
of a network, or in cavities of a honeycomb (as we have noted above, p. 5),
while the solid forming the residuum has a refractive index of a little
over 1.6. Therefore, it only regains its full transparency when the water
is replaced by a liquid of high refractive index, such as an essential oil
or phenol. A similar change may be effected by pouring white of egg into
boiling water or absolute alcohol, and is attended with the same optical
results. The study of the behaviour of coagulable colloids has been
recently studied by Fischer and by Hardy, and has been of the utmost
service in our interpretation of the microscopical appearances shown in
biological specimens under the microscope.[13]
{12}The death of the living being finds a certain analogy in the breaking
up or the wearing out of a piece of machinery; but in no piece of machinery
do we find the varied irritabilities, all conducive to the well-being of
the organism (under ordinary conditions), or the so-called "automatic
processes"[14] that enable the living being to go through its
characteristic functions, to grow, and as we shall see, even to turn
conditions unfavourable for active life and growth to the ultimate weal of
the species (see p. 32). At the same time, we fully recognise that for
supplies of matter and energy the organism, like the machine, depends
absolutely on sources from without. The debtor and creditor sheet, in
respect of matter and energy, can be proved to balance between the outside
world and Higher Organisms with the utmost accuracy that our instruments
can attain; and we _infer_ that this holds for the Lower Organisms also.
Many of the changes within the organism can be expressed in terms of
chemistry and physics; but it is far more impossible to state them _all_ in
such terms than it would be to describe a polyphase electrical installation
in terms of dynamics and hydraulics. And so far at least we are justified
in speaking of "vital forces."
The living substance of protoplasm contains a large quantity of water, at
least two-thirds its mass, as we have seen, in a state of physical or loose
chemical combination with solids: these on death yield proteids and
nucleo-proteids.[15] The living protoplasm {13}has an alkaline reaction,
while the liquid in the larger vacuoles, at least, is acid, especially in
Plant-cells.[16]
METABOLISM.—The chemical processes that go on in the organism are termed
metabolic changes, and were roughly divided by Gaskell into (1) "anabolic,"
in which more complex and less stable substances are built up from less
complex and more stable ones with the absorption of energy; and (2)
"catabolic" changes in which the reverse takes place. Anabolic processes,
in all but the cells containing plastids or chromatophores (see p. 36)
under the influence of light, necessarily imply the furnishing of energy by
concurrent catabolic changes in the food or reserves, or in the protoplasm
itself.
Again, we have divided anabolic processes into "accumulative," where the
substances formed are merely reserves for the future use of the cell, and
"assimilative," where the substances go to the building of the protoplasm
itself, whether for the purpose of growth or for that of repair.
Catabolic processes may involve (1) the mere breaking of complex substances
into simpler ones, or (2) their combination with oxygen; in either case
waste products are formed, which may either be of service to the organism
as "secretions" (like the bile in Higher Animals), or of no further use
(like the urine). When nitrogenous substances break down in this way they
give rise to "excretions," containing urea, urates, and allied substances;
other products of catabolism are carbon dioxide, water, and mineral salts,
such as sulphates, phosphates, carbonates, oxalates, etc., which if not
insoluble must needs be removed promptly from the organism, many of them
being injurious or even poisonous. The energy liberated by the protoplasm
being derived through the breakdown of another part of the same or of the
{14}food-materials or stored reserves, must give rise to waste products.
The exchange of oxygen from without for carbonic acid formed within is
termed "respiration," and is distinguished from the mere removal of all
other waste products called "excretion." In the fresh-water Amoeba both
these processes can be studied.
RESPIRATION,[17] or the interchange of gases, must, of course, take place
all over the general surface, but in addition it is combined in most
fresh-water Protista with _excretion_ in an organ termed the "contractile"
or "pulsatile vacuole" (Figs. 1, 4, etc.). This particular vacuole is
exceptional in its size and its constancy of position. At intervals, more
or less regular, it is seen to contract, and to expel its contents through
a pore; at each contraction it completely disappears, and reforms slowly,
sometimes directly, sometimes by the appearance of a variable number of
small "formative" vacuoles that run together, or as in Ciliata, by the
discharge into it of so-called "feeding canals." As this vacuole is filled
by the water that diffuses through the substance, and when distended may
reach one-third the diameter of the being, in the interval between two
contractions an amount of water must have soaked in equal to
one-twenty-seventh the bulk of the animal, to be excreted with whatever
substances it has taken up in solution, including, not only carbon dioxide,
but also, it has been shown, nitrogenised waste matters allied to uric
acid.[18]
That the due interchanges may take place between the cell and the
surrounding medium, it is obvious that certain limits to the ratio between
bulk and surface must exist, which are disturbed by growth, and which we
shall study hereafter (p. 23 f.).
The Protista that live in water undergo a death by "diffluence" or
"granular disintegration" on being wounded, crushed, or sometimes after an
excessive electric stimulation, or contact with alkalies or with acids too
weak to coagulate them. In this process the protoplasm breaks up from the
surface inwards into a mass of granules, the majority of which themselves
finally dissolve. If the injury be a local rupture of the external pellicle
or {15}cuticle, a vacuole forms at the point, grows and distends the
overlying cytoplasm, which finally ruptures: the walls of the vacuole
disintegrate; and this goes on as above described. Ciliate Infusoria are
especially liable to this disintegration process, often termed
"diffluence," which, repeatedly described by early observers, has recently
been studied in detail by Verworn. Here we have death by "solution," while
in the "fixing" of protoplasm for microscopic processes we strive to ensure
death by "desolution," so as to retain as much of the late living matter as
possible. It would seem not improbable that the unusual contact with water
determines the formation of a zymase that acts on the living substance
itself.
We have suggested[19] that one function of the contractile vacuole, in
naked fresh-water Protists, is to afford a regular means of discharge of
the water constantly taken up by the crystalloids in the protoplasm, and so
to check the tendency to form irregular disruptive vacuoles and death by
diffluence. This is supported by the fact that in the holophytic
fresh-water Protista, as well as the Algae and Fungi, a contractile vacuole
is present in the young naked stage (zoospore), but disappears as soon as
an elastic cell-wall is formed to counterbalance by its tension the
internal osmotic pressure.
DIGESTION is always essentially a catabolic process, both as regards the
substance digested and the formation of the digesting substance by the
protoplasm. The digesting substance is termed a "zymase" or "chemical
ferment," and is conjectured to be produced by the partial breakdown of the
protoplasm. In presence of suitable zymases, many substances are resolved
into two or more new substances, often taking up the elements of water at
the same time, and are said to be "dissociated" or "hydrolysed" as the case
may be. Thus proteid substances are converted into the very soluble
substances, "proteoses" and "peptones," often with the concurrent or
ultimate formation of such relatively simple bodies as leucin, tyrosin, and
other amines, etc. Starch and glycogen are converted into dextrins and
sugars; fats are converted into fatty acids and glycerin. It is these
products of digestion, and not the actual food-materials (save certain very
simple sugars), that are really taken up by the protoplasm, {16}whether for
assimilation, for accumulation, or for the direct liberation of energy for
the vital processes of the organism.
Not only food from without, but also reserves formed and stored by the
protoplasm itself, must be digested by some zymase before they can be
utilised by the cell. In all cases of the utilisation of reserve matter
that have been investigated, it has been found that a zymase is formed by
the cell itself (or sometimes, in complex organisms, by its neighbours);
for, after killing the cell in which the process is going on by mechanical
means or by alcohol, the process of digestion can be carried on in the
laboratory.[20] The chief digestion of all the animal-feeding Protista is
of the same type as in our own stomachs, known as "peptic" digestion: this
involves the concurrent presence of an acid, and Le Dantec and Miss
Greenwood have found the contents of food-vacuoles, in which digestion is
going on, to contain acid liquid. The ferment-pepsin itself has been
extracted by Krukenberg from the Myxomycete, "Flowers of tan" (_Fuligo
varians_, p. 92), and by Professor Augustus Dixon and the author from the
gigantic multinucleate Amoeba, _Pelomyxa palustris_ (p. 52).[21] The
details of the prehension of food will be treated of under the several
groups.
The two modes of Anabolism—true "assimilation" in the strictest sense and
"accumulation"—may sometimes go on concurrently, a certain proportion of
the food material going to the protoplasm, and the rest, after allowing for
waste, being converted into reserves.
MOVEMENTS all demand catabolic changes, and we now proceed to consider
these in more detail.
The movements of an Amoeboid[22] cell are of two kinds: "expansion,"
leading to the formation and enlargement of {17}outgrowths, and
"contraction," leading to their diminution and disappearance within the
general surface.[23] Expansion is probably due to the lessening of the
surface-tension at the point of outgrowth, contraction to the increase of
surface-tension. Verworn regards these as due respectively to the
combination of the oxygen in the medium with the protoplasm in diminishing
surface-tension, and the effect of combination with substances from within,
especially from the nucleus in increasing it. Besides these external
movements, there are internal movements revealed by the contained granules,
which stream freely in the more fluid interior. Those Protista that, while
exhibiting amoeboid movements, have no clear external layer, such as the
Radiolaria, Foraminifera, Heliozoa, etc., present this streaming even at
the surface, the granules travelling up and down the pseudopodia at a rate
much greater than the movements of these organs themselves. In this case
the protoplasm is wetted by the medium, which it is not where there is a
clear outer layer: for that behaves like a greasy film.
MOTILE ORGANS.—Protoplasm often exhibits movements much more highly
specialised than the simple expansion or retraction of processes, or the
general change of form seen in Amoeba. If we imagine the activities of a
cell concentrated on particular parts, we may well suppose that they would
be at once more precise and more energetic than we see them in Amoeba or
the leucocyte. In some free-swimming cells, such as the individual cells
known as "Flagellata," the reproductive cells of the lower Plants, or the
male cells ("spermatozoa") of Plants as high as Ferns, and even of the
Highest Animals, there is an extension of the cell into one or more
elongated lash-like processes, termed "flagella," which, by beating the
water in a reciprocating or a spiral rhythm, cause the cell to travel
through it; or, if the cell be attached, they produce currents in the water
that bring food particles to the surface of the cell for ingestion. Such
flagella may, indeed, be seen in some cases to be modified pseudopodia. In
other cases part, or the whole, of the surface of the cell may be covered
with regularly arranged short filaments of similar activity (termed
"cilia," from their resemblance to a diminutive eyelash), which, however,
instead of whirling round, bend sharply {18}down to the surface and slowly
recover; the movement affects the cilia successively in a definite
direction in waves, and produces, like that of flagella, either locomotion
of the cell or currents in the medium. We can best realise their action by
recalling the waves of bending and recovery of the cornstalks in a
wind-swept field; if now the haulms of the corn executed these movements of
themselves, they would determine in the air above a breeze-like motion in
the direction of the waves (Fig. 5).[24] Such cilia are not infrequent on
those cells of even the Highest Animals that, like a mosaic, cover free
surfaces ("epithelium cells"). In ourselves such cells line, for instance,
the windpipe. One group of the Protozoa, the "Ciliata," are, as their name
implies, ciliated cells pure and simple.
[Illustration: FIG. 5.—Motion of a row of cilia, in profile. (From
Verworn.)]
The motions of cilia and of flagella are probably also due to changes of
surface tension—alternately on one side and the other in the cilium, but
passing round in circular succession in the flagellum,[25] giving rise to a
conical rotation like that of a weighted string that is whirled round the
head. This motion is, however, strongest at the thicker basal part, which
assumes a spiral form like a corkscrew of few turns, while the thin lash at
the tip may seem even to be quietly extended like the point of the
corkscrew. If the tip of the flagellum adhere, as it sometimes does, to any
object, the motions induce a jerking motion, which in this case is
reciprocating, not rotatory. When the organism is free, the flagellum is
usually in advance, and the cell follows, rotating at the same time round
its longitudinal axis; such an anterior flagellum, called a "tractellum,"
is the common form in Protista that possess a single one (Figs. 29, 7, 8;
30, C). In the spermatozoa of Higher Animals (and some Sporozoa) the
flagellum is posterior, and is called a "pulsellum."
The cilium or flagellum may often be traced a certain distance into the
substance of the cytoplasm to end in a dot of denser, {19}readily-staining
plasm, which corresponds to a "centrosome" or centre of plasmic forces (see
below, pp. 115, 121, 141); it has been termed a "blepharoplast."[26]
Again, the cytoplasm may have differentiated in it definite streaks of
specially contractile character; such streaks within its substance are
called "myonemes"; they are, in fact, muscular _fibrils_. A "muscle-cell,"
in the Higher Animals, is one whose protoplasm is almost entirely so
modified, with the exception of a small portion of granular cytoplasm
investing the nucleus, and having mainly a nutritive function.
Definite muscular fibrils in action shorten, and at the same time become
thicker. It seems probable that they contain elongated vacuoles, and that
the contents of these vary, so that when they have an increased osmotic
equivalent, the vacuoles absorb water, enlarge, and tend to become more
spherical, _i.e._ shorter and thicker, and so the fibril shortens as a
whole. The relaxation would be due to the diffusion outwards of the
solution of the osmotically active substances which induced expansion.[27]
The MOTILE REACTIONS of the Protozoa[28] require study from another point
of view: they are either (1) "spontaneous" or "arbitrary," as we may say,
or (2) responsive to some stimulus. The latter kind we will take first, as
they are characteristic of all free cells. The stimuli that induce
movements of a responsive character are as follows:—(i.) MECHANICAL: such
as agitation and contact; (ii.) force of GRAVITY, or CENTRIFUGAL FORCE;
(iii.) CURRENTS in the water; (iv.) RADIANT ENERGY (LIGHT); (v.) changes in
the TEMPERATURE of the medium; (vi.) ELECTRIC CURRENTS through the medium;
(vii.) the presence of CHEMICAL SUBSTANCES in the medium.
These, or some of them, may induce one of three different results, or a
combination thereof: (1) a single movement or an arrest of motion; (2) the
assumption of a definite position; (3) movement of a definite character or
direction.
{20}(i.) MECHANICAL STIMULI.—Any sudden touch with another body tends to
arrest all motion; and if the shock be protracted or severe, the retraction
of the pseudopodia follows. It is to this reaction that we must ascribe the
retracted condition of the pseudopodia of most Rhizopods when first placed
on the slide and covered for microscopic examination. Free-swimming
Protista may, after hitting any body, either remain in contact with it, or
else, after a pause, _reverse_ their movement, turn over and swim directly
away. This combination of movements is characteristic as a reaction of what
we may term "repellent" stimuli in general.[29] Another mechanical reaction
is that to continuous contact with a solid; and the surface film of water,
either at the free surface or round an air-bubble, may play the part of a
solid in exciting it; we term it "thigmotaxy" or "stereotaxy." When
positive it determines a movement on to the surface, or a gliding movement
along it, or merely the arrest of motion and prolongation of contact; when
_negative_, a contact is followed by the retreat of the being. Thus
_Paramecium_ (Fig. 55, p. 151) and many other Ciliates are led to aggregate
about solid particles or masses of organic _débris_ in the water, which
indeed serve to supply their food. On contact, the cell ceases to move its
cilia except those of the oral groove; as these lash backwards, they hold
the front end in close contact with the solid, at the same time provoking a
backward stream down the groove, which may bring in minute particles from
the mass.
(ii.) Most living beings are able to maintain their level in water by
floating or crawling against GRAVITY, and they react in virtue of the same
power against centrifugal force. This mode of irritability is termed
(negative) "geotaxy" or "barotaxy." We can estimate the power of resisting
such force by means of a whirling machine, since when the acceleration is
greater than the resistance stimulated thereby in the beings, they are
passively sent to the sides of the vessel. The Flagellates, _Euglena_ and
_Chlamydomonas_, begin to migrate towards the centre when exposed to a
centrifugal force about equal to ½ G (G = 32.2 feet or 982 cm. per second);
they remain at the centre until the centrifugal force is increased to 8 G;
above that they yield to the force, and are driven passively to the sides.
The reaction ceases or is reversed at high temperatures.
{21}(iii.) RHEOTAXY.—This is the tendency to move against the stream in
flowing water. It is shown by most Protists, and can be conveniently
studied in the large amoeboid plasmodia of the Myxomycetes, which crawl
against the stream along wet strips of filter paper, down which water is
caused to flow. Most animals, even of the highest groups, tend to react in
the same way; the energetic swimming of Fishes up-stream being in marked
contrast with their sluggishness the other way; and every student of
pond-life knows how small Crustacea and Rotifers, no less than Ciliates,
swim away from the inrush of liquid into the dipping-tube, and so evade
capture. (See Vol. II. p. 216.)
(iv.) The movements of many Protozoa are affected greatly by LIGHT. These
movements have been distinguished into "photopathic," _i.e._ to or from the
_position_ of greatest luminosity; and "phototactic," along the _direct
path_ of the rays.[30] Those Protozoa that contain a portion of their
cytoplasm, known as a "plastid" or "chromatophore" (see pp. 36, 39),
coloured by a green or yellow pigment are usually "phototactic." They
mostly have at the anterior end a red pigment spot, which serves as an
organ of sight, and is known as an "eye-spot." In diffused light of low
intensity they do not exhibit this reaction, but in bright sunlight they
rise to the surface and form there a green or yellow scum.
Most of the colourless Protista are negatively phototactic or photopathic;
but those which are parasitic on the coloured ones are positively
phototactic, like their hosts.
Here, as in the case of other stimuli,[31] the absolute intensity of the
light is of importance; for as it increases from a low degree, different
organisms in turn cease to be stimulated, and {22}then are repelled instead
of being attracted. The most active part of the spectrum in determining
reactions of movement are the violet and blue rays of wave-length between
40 µ/10 and 49 µ/10, while the warmer and less refractive half of the
spectrum is inert save in so far as it determines changes in the
temperature of the medium.
(v.) The movements of many Protozoa are rendered sluggish by cold, and
active by a rise of TEMPERATURE up to what we may term the "optimum"; the
species becomes sluggish again as the temperature continues to rise to a
certain point when the movements are arrested, and the being is said to be
in a state of "heat-rigor." Most Protozoa, again, tend to move in an
unequally heated medium to the position nearest to their respective optimum
temperature. This is called "thermotaxy." The temperature to which Amoeba
is thermotactic is recorded as 35° C. (95° F.); that of _Paramecium_ is 28°
C. (82° F.).
(vi.) Most active Protozoa tend to take up a definite position in respect
to a current of ELECTRICITY passing through the medium, and in the majority
of cases, including most Ciliates, _Amoeba_, and _Trachelomonas_, they
orient their long diameters in the direction of the lines of force and swim
along these to assemble behind the cathode. The phenomenon is called
"galvanotaxy," and this particular form is "negative." _Opalina_ (Fig. 41,
p. 123), however, and most Flagellates are "positively galvanotactic," and
move towards the anode. H. H. Dale[32] has shown that the phenomenon may be
possibly in reality a case of chemiotaxy, for the direction of motion
varies with the nature and concentration of the medium. It would thus be a
reaction to the "ion" liberated in contact with the one or other extremity
of the being. Induction shocks, as we have seen, if slight, arrest the
movements of Protozoa, or if a little stronger determine movements of
contraction; if of sufficient intensity they kill them. No observation
seems to have been made on the behaviour of Protista in an electric field.
A magnetic field of the highest intensity appears to be indifferent to all
Protista.
(vii.) We have already referred to the effect of dissolved CHEMICAL
SUBSTANCES present in the water. If the substance is in itself not harmful,
and the effect varies with the concentration, we term the reaction one of
"tonotaxy," which combines {23}with that of "chemiotaxy" for substances
that in weak solution are attractive or repellent to the being.
_Paramecium_, which feeds on bacteria, organisms of putrefaction, is
positively chemiotactic to solutions of carbon dioxide, and as it gives
this off in its own respiration, it is attracted to its fellows. The
special case of reaction to gases in solution is termed "aerotaxy," or
"pneumotaxy," according as the gas is oxygen or carbon dioxide. We find
that in this respect there are degrees, so that a mixed culture of
Flagellates in an organic infusion sorts itself out, under the cover of a
microscopic preparation, into zones of distinct species, at different
distances from the freely aerated edge, according to the demands of each
species for oxygen and CO_{2} respectively.
Finally, we must note that the apparently "spontaneous movements" of
Protists can hardly be explained as other than due either to _external_
stimuli, such as we have just studied, or to _internal_ stimuli, the
outcome of internal changes, such as fatigue, hunger, and the like. Of the
latter kind are the movements that result in REPRODUCTION.
REPRODUCTION.—We have noted above that the growth of an organism _which
retains its shape_ alters the ratio of the surface area to the whole
volume, so necessary for the changes involved in life. For the volume of an
organism varies as the cube of any given diameter, whereas the surface
varies with the square only. Without going into the arithmetical details,
we may say that the ratio of surface to volume is lessened to roughly
four-fifths of the original ratio when the cell doubles its bulk. As
Herbert Spencer and others have pointed out, this must reduce the
activities of the cell, and the due ratio is restored by the division of
the cell into two.[33] This accounts for what we must look on as the most
primitive mode of reproduction, as it is the simplest, and which we term
"fission" at Spencer's "limit of {24}growth." Other modes of reproduction
will be studied later (p. 30), after a more detailed inquiry into the
structure of the nucleus and of its behaviour in cell-division. All
cell-division is accompanied by increased waste, and is consequently
_catabolic_ in character, though the _anabolic_ growth of living
protoplasm, at the expense of the internal reserves, may be concurrent
therewith.
CELL-DIVISION
In ordinary cases of fission of an isolated cell the cell elongates, and as
it does so, like other viscid bodies, contracts in the middle, which
becomes drawn out into a thread, and finally gives way. In some cases
(_e.g._ that of the _Amoeba_, Fig. 4) the nucleus previously undergoes a
similar division by simple constriction, which is called direct or
"amitotic" division. But usually the division of the nucleus prior to
cell-division is a more complex process, and involves the co-operation of
the cytoplasm; and we must now study in detail the nucleus and its
structure in "rest" and in fission.[34]
We have noted above (p. 6, Fig. 2) the structure of the so-called "resting
nucleus,"[35] when the cell is discharging the ordinary functions of its
own life, with its wall, network of linin, chromatin-granules, and nucleole
or nucleoles. The chromatin-granules are most abundant at two periods in
the life of the cell, (1) when it is young and fresh from division, and (2)
at the term of its life, when it is itself preparing for division. In the
interim they are fewer, smaller, and stain less intensely. In many Protista
the whole or greater part of the chromatin is densely aggregated into a
central "nuclein-mass" or _karyosome_ {25}suspended in the linin network
(long regarded as a mere nucleole). Such a nucleus is often termed a
"vesicular nucleus".[36]
[Illustration: FIG. 6.—Changes in nucleus and cell in indirect (mitotic)
nuclear division. A, resting nucleus with two centrioles[37] in single
centrosphere (_c_); B, centrosphere divided, spindle and two asters (_a_)
forming; C, centrospheres separated, nuclear wall disappearing; D,
resolution of nucleus into chromosomes; E, mature plasmic spindle, with
longitudinal fission of chromosomes; F, chromosomes forming equatorial
plate (_ep_) of spindle. (From Wilson.)]
{26}When cell-division is about to take place the linin, or at least the
greater part of it, assumes the character of a number of distinct threads,
and the whole of the chromatin granules are distributed at even distances
along these (Fig. 6, A, B, C), so as to appear like so many strings of
beads. Each such thread is called a "chromosome." Then each bead divides
longitudinally into two. The thread flattens into a ribbon, edged by the
two lines of chromatin beads. Finally, the ribbon splits longitudinally
into two single threads of beads (Fig. 6, E). During these changes the
nucleole or nucleoles diminish, or even disappear, as if they had
contributed their matter to the growth of the chromatin proper. In Higher
Animals and Plants the nuclear wall next disappears, and certain structures
become obvious, especially in the cytoplasm of Metazoa. Two minute spheres
of plasm (themselves often showing a concentric structure), the
"centrosomes,"[38] which hitherto lay close together at the side of the
nuclear wall, now separate; but they remain connected by a spindle of clear
plasmic threads (Fig. 6, B-E) which, as the centres diverge, comes to lie
across the spot the nucleus occupied, and now the chromosomes lie about the
equator of this spindle (Fig. 6, F). Moreover, the surrounding cytoplasm
shows a radiating structure, diverging from the centrosome, so that spindle
and external radiations together make up a "strain-figure," like that of
the "lines of force" in relation to the poles of a magnet. Such we can
demonstrate in a plane by spreading or shaking iron filings on a piece of
paper above the poles of a magnet, or in space by suspending finely divided
iron in a thick liquid, such as mucilage or glycerin, and bringing the
vessel with the mixture into a strong magnetic field;[39] the latter mode
has the advantage {27}of enabling us to watch the changes in the
distribution of the lines under changing conditions or continued strain.
[Illustration: FIG. 7.—Completion of mitotic cell-division. G, splitting of
equatorial plate (_ep_); H, recession of daughter chromosomes; I, J,
reconstitution of these into new nuclei, fission of the centrioles and of
the cytoplasm. _if_, Central fibres of spindle; _n_, remains of old
nucleole. (From Wilson.)]
The chromosomes are now completely split, each into its two
daughter-segments, which glide apart (Fig. 7, G, _ep_), and pass each to
its own pole of the spindle, stopping just short of the centrosome (I).
Thus, on the inner side of either centrosome is found an aggregation of
daughter-segments, each of which is sister to one at the opposite pole,
while the number at either pole is identical with that of the segments into
which the old nucleus had resolved itself at the outset. The
daughter-segments shorten and thicken greatly as they diverge to the poles,
and on their arrival crowd close together.
A distinct wall now forms around the aggregated {28}daughter-chromosomes
(J), so as to combine them into a nucleus for the daughter-cell. The
reorganisation of the young nucleus certainly varies in different cases,
and has been ill-studied, probably because of the rapidity of the changes
that take place. The cytoplasm now divides, either tapering into a "waist"
which finally ruptures, or constricting by the deepening of a narrow
annular groove so as to complete the formation and isolation of the
daughter-cells.
We might well compare the cell-division to the halving of a pumpkin or
melon, of which the flesh as a whole is simply divided into two by a
transverse cut, while the seeds and the cords that suspend them are each
singly split to be divided evenly between the two halves of the fruit; the
flesh would represent the cytoplasm, the cords the linin threads of the
nucleus, and the seeds the chromatin granules. In this way the halving of
the nucleus is much more complete and intimate than that of the cytoplasm;
and this is the reason why many biologists have been led to regard the
nuclear segments, and especially their chromatic granules, as the seat of
the hereditary properties of the cell, properties which have to be equally
transmitted on its fission to each daughter-cell.[40] But we must remember
that the linin is also in great part used up in the formation of these
segments, like the cords of our supposed melon; and it is open to us to
regard the halving in this intimate way of the "linin" as the essence of
the process, and that of the chromatin as accessory, or even as only part
of the necessary machinery of the process. The halving or direct splitting
lengthwise of a viscid thread is a most difficult problem from a physical
point of view; and it may well be that the chromatin granules have at least
for a part of their function the facilitation of this process. If such be
the case, we can easily understand the increase in number, and size and
staining power of these granules as cell-division approaches, and their
atrophy or partial disappearance during their long intervening periods of
active cell life. Hence we hesitate to accept the views so commonly
maintained that the chromatin represents a {29}"germ-plasm" or "idioplasm"
of relatively great persistence, which gives the cell its own racial
qualities.[41]
The process we have just examined is called "mitosis," "karyomitosis," or
"karyokinesis"; and the nucleus is said to undergo "indirect" division, as
compared to "direct" division by mere constriction. In an intermediate
mode, common to many Protista, the nuclear wall persists throughout the
whole process, though a spindle is constituted within, and chromosomes are
formed and split: the division of the nucleus takes place, however, by
simple constriction, as seen in the Filose Rhizopod _Euglypha_ (Fig. 8).
[Illustration: FIG. 8.—Fission with modified karyokinesis in the Filose
Rhizopod _Euglypha_. A, outgrowth of half of the cytoplasm, passage of
siliceous plates for young shell outwards; B, completion of shell of second
cell, formation of _intra_-nuclear spindle; C, D, further stages. (From
Wilson, after Schewiakoff.)]
In many Sarcodina and some Sporozoa the nucleus gives off small fragments
into the cytoplasm, or is resolved into them; {30}they have been termed
"chromidia" by E. Hertwig. New nuclei may be formed by their growth and
coalescence, the original nucleus sometimes disappearing more or less
completely.
In certain cases the division of the nucleus is not followed by that of the
cytoplasm, so that a plurinucleate mass of protoplasm results: this is
called an "apocyte"; and we find transitional forms between this and the
uninucleate or true cell. Thus in one species of _Amoeba_ (_A. binucleata_)
there are always two nuclei, which divide simultaneously to provide for the
outfit of the daughter-cells on fission. Again, we find in some cases that
similar multinucleate masses may be formed by the union of two or more
cells by their cytoplasm only: such a union is termed "permanent
plastogamy," and the plurinucleate mass a "plasmodium."[42] Here again we
find intermediate forms between plasmodium and apocyte, for the nuclei of
the former may divide and so increase in number, without division of the
still growing mass. Both kinds of plurinucleate organisms are termed
"coenocytes" without reference to their mode of origin.
The rhythm of cell-life that we have just studied is called the
"Spencerian" rhythm. Each cell in turn grows from half the bulk of its
parent at the time it was formed to the full size of that parent, when it
divides in its own turn. Rest is rare, and assumed only when the cell is
exposed to such unfavourable external conditions as starvation, drought,
etc.; it has no necessary relation to fission.
MULTIPLE FISSION OR BROOD-FORMATION.—We may now turn to a new rhythm, in
strong contrast to the former: a cell after having attained a size, often
notably greater than its parents, divides: without any interval for growth,
the daughter-cells again divide, and this may be repeated as many as ten
times, or even more, so as to give rise to a number of small cells—4, 8,
16—1024,[43] etc., respectively. Such an assemblage of small cells so
formed is called a brood, and well deserves this name, for they never
separate until the whole series of divisions is completed. By this process
the number of individuals is rapidly {31}increased, hence it has received
the name of "sporulation." The term spores is especially applied to the
reproductive bodies of Cryptogams, such as Mosses, Fungi, etc.: the
resulting cells are called "spores," "zoospores" if active ("amoebulae" if
provided with pseudopodia, "flagellulae" if flagellate), "aplanospores," if
motionless. We prefer to call them by the general term "brood-cells," the
original cell the "brood-mother-cell," and the process, "multiple fission"
or "brood-formation." As noted, the brood-mother-cell usually attains an
exceptionally large size, and it in most cases passes into a state of rest
before entering on division: thus brood-formation is frequently the
ultimate term of a long series of Spencerian divisions. Two contrasting
periods of brood-formation may occur in the life cycle of some beings,
notably the Sporozoa.[44]
COLONIAL UNION.—In certain cases, the brood-cells instead of separating
remain together to form a "colony"; and this may enlarge itself again by
binary division of its individual cells at their limit of growth. Here,
certain or all of the cells may (either after separation, or in their
places) undergo brood-formation: such cells are often termed "reproductive
cells" in contrast with the "colonial cells."
Some such colonial Protista must have been the starting-points for the
Higher Animals and Plants; probably apocytial Protista were the
starting-points of the Fungi. In the Higher Animals and Plants, the
spermatozoa and the oospheres (the male and female pairing-cells) are alike
the offspring of brood-formation: and the coupled-cell (fertilised egg)
starts its new life by _segmentation_, which is a brood-formation in which
the cells do not separate, but remain in colonial union, to differentiate
in due course into the tissue-cells of the organism.
RETARDED BROOD-FORMATION.—The nuclear divisions may alternate with
cell-divisions, as above stated, or the former may be {32}completed before
the cytoplasm divides; thus the brood-mother-cell becomes temporarily an
apocyte,[45] which is then _resolved_ simultaneously into the 1-nucleate
brood-cells.
A temporary apocytial condition is often passed through in the formation of
the brood of cells by repeated divisions without any interval for
enlargement; for the nuclear divisions may go on more rapidly than those of
the cytoplasm, or be completed before any cell-division takes place (Figs.
31, 34, 35, pp. 95, 101, 104), the nuclear process being "accelerated" or
the cytoplastic being "retarded," whichever we prefer to say and to hold.
Thus as many as thirty-two nuclei may have been formed by repeated binary
subdivisions before any division of the cytoplasm takes place to resolve
the apocyte into true 1-nucleate cells.
In many cases of brood-formation the greater part of the food-supply of
the brood-mother-cell has been stored as reserve-products, which
accumulate in quantity in the cell; this is notably seen in the ovum or
egg of the Higher Animals. How great such an accumulation may be is
indeed well seen in the enormous yolk of a bird's egg, gorged as it were
to repletion. When a cell has entered on such course of "miserly"
conduct, it may lose all power of drawing on its own supplies, and
finally that of accumulating more, and passes into the state of "rest."
To resume activity there is needed either a change in the internal
conditions—demanding the lapse of time—or in the external conditions, or
in both.[46] We may call this resumption "germination."
Very often in the study of a large and complex organism we are able to
find processes in action on a large scale which, depending as they must
do on the protoplasmic activities of its individual cells, reveal the
nature of similar processes in simple unicellular beings: such a clue to
the utilisation of reserves by a cell which has gorged itself with them
so as to pass into a state of rest is to be found in that common
multicellular organism, the Potato. This stores up reserves in its
underground stems (tubers); if we plant these immediately on the
completion of their growth, they will not start at once, even under what
would outwardly seem to be most appropriate conditions. A certain lapse
of time is an essential factor for sprouting. It would appear that in the
Potato the starch can only be digested by a definite ferment, which does
not exist when it is dug, but which is only formed very slowly, and not
at all until a certain time has supervened; and that sprouting can only
{33}take place when soluble material has been provided in this way for
the growth of the young shoots. We have also reason to believe that these
ferments are only formed by the degradation of the protoplasm itself. Now
obviously this degradation must be very slow in a resting organism; and
any external stimulus that will tend to protoplasmic activity will
thereby tend to form at the same time the digestive ferments and dissolve
the stored supplies, to render them available for the life-growth and
reproduction of the being. We now see why inactive "miserly" cells so
often pass into a resting state before dividing, and why they go on
dividing again and again when once they re-enter upon an active life, the
living protoplasm growing at the expense of the reserves.[47] Resting
cells of this type occur of course only at relatively rare intervals in
the animal-feeding Protozoa, that have to take into their substance the
food they require for their growth and life-work, and cannot therefore
store up much reserves. For they are constantly producing in the narrow
compass of their body those very ferments that would dissolve the
reserves when formed. Internal parasites and "saprophytes," that is,
beings which live on dead and decayed organic matter, on the other hand,
live surrounded by a supply of dissolved food; and rarely do we find
larger cells, richer in reserves, than in the parasitic Sporozoa, which
owe their name to the importance of brood-formation in their
life-history. In Radiolaria (p. 75 f.) a central capsule separates off an
inner layer of protoplasm; the outer layer is the one in which digestion
is performed, while the inner layer stores up reserves; and here
brood-formation appears to be the rule. But the largest cells of all are
the eggs of the Metazoa, which in reality lead a parasitic life, being
nurtured by the animal as a whole, and contributing nothing to the
welfare of it as an individual. Their activity is reduced to a minimum,
and the consequent need for a high ratio of surface to volume is also
reduced, which accounts for their inordinate size. But directly the
reserve materials are rendered available by the formation of a digestive
ferment, then protoplasmic growth takes place, and the need for an
extended surface is felt; cell-division follows cell-division with
scarcely an interval in the process of segmentation.
SYNGAMY.[48]—The essence of typical syngamy is, that two cells
("pairing-cells," "gametes") of the same species approach one another, and
fuse, cytoplasm with cytoplasm, and nucleus with nucleus, to form a new
cell ("coupled-cell," "zygote "). This process is called also "conjugation"
or "cytogamy." In the simplest cases the two cells are equal and attract
one another equally ("isogamy"), and have frequently the character of
zoospores.
In an intermediate type, the one cell is larger and more sluggish (female),
"megagamete," "oogamete," "oosphere," "egg"; the other smaller, more active
(male), "microgamete," "spermogamete," "spermatozoon," "sperm"; and in the
most specialised {34}cases which prevail among the Higher Animals and
Plants, the larger cell is motionless, and the smaller is active, ciliate,
flagellate, or amoeboid: the coupled-cell or zygote is here termed the
"oosperm."[49] It encysts immediately in most Protista except Infusoria,
Acystosporidae, Haemosporidae, and Trypanosomatidae.
As the size of the two gametes is so disproportionate in most cases that
the oosphere may be millions of times bigger than the sperm, and the latter
at its entrance into the oosphere entirely escape unaided vision, the term
"egg" is applied in lax speech, both (1) to the female cell, and (2) to the
oosperm, the latter being distinguished as the "fertilised egg," a survival
from the time when the union of _two_ cells, as the essence of the process,
was not understood.
We know that in _many_ cases, and have a right to infer that in _all_,
chemiotaxy plays an important part in attracting the pairing-cells to one
another. In Mammals and Sauropsida there seems also to be a rheotactic
action of the cilia lining the oviducts, which work downwards, and so
induce the sperms to swim upwards to meet the ovum, a condition of things
that was most puzzling until the nature of rheotaxy was understood. A
remarkable fact is that equal gametes rarely appear to be attracted by
members of the same brood, though they are attracted by those of any
other brood of the same species.[50] It may well be that each brood has
its own characteristic secretion, or "smell," as it were, slightly
different from that of other broods, just as every dog has his, so easily
recognisable by other dogs; and that the cells only react to different
"smells" to their own. Such a secretion from the surface of the female
cell would lessen its surface tension, and thereby render easier the
penetration of the sperm into its substance.
As a rule, one at least of the pair-cells is fresh from division, and it
would thus appear that the union of the nuclei is facilitated when one at
least of them is a "young" one. Of the final mechanism of the union of
the nuclei, we know nothing: they may unite in any of the earlier phases
of mitosis, or even in the "resting state." A fibrillation of the
cytoplasm during the process, radiating around a centrosome or two
centrosomes indicates a strained condition.[51]
{35}REGENERATION.—Finally, experiments have been made by several observers
as to the effects of removing parts of Protozoa, to see how far
regeneration can take place. The chief results are as follows:—
1. A nucleated portion may regenerate _completely_, if of sufficient size.
Consequently, multinucleate forms, such as _Actinosphaerium_ (Heliozoa,
Fig. 19, p. 72), may be cut into a number of fragments, and regenerate
completely. In Ciliata, such as _Stentor_ (Fig. 59, p. 156), each fragment
must possess a portion of the meganucleus, and at least one micronucleus
(p. 145), and, moreover, must possess a certain minimum size. A Radiolarian
"central-capsule" (p. 75) with its endoplasm and nucleus may regenerate its
ectoplasm, but the isolated ectoplasm being non-nucleate is doomed. A
"central capsule" of one species introduced into the ectoplasm of another,
closely allied, did well. All non-nucleate pieces may exhibit
characteristic movements, but appear unable to digest; and they survive
only a short time.[52]
"ANIMALS" AND "PLANTS"
Hitherto we have discussed the cell as if it were everywhere an organism
that takes in food into its substance, the food being invariably "organic"
material, formed by or for other cells; such nutrition is termed
"holozoic." There are, however, limits to the possibilities in this
direction, as there are to the fabled capacities of the Scillonians of
gaining their precarious livelihood by taking in one another's washing. For
part of the food material taken in in this way is applied to the supply of
the energies of the cell, and is consequently split up or oxidised into
simpler, more stable bodies, no longer fitted for food; and of the matter
remaining to be utilised for building up the organism, a certain proportion
is always wasted in by-products. Clearly, then, the supply of food under
such conditions is continually lessening in the universe, and we have to
seek for a manufactory of food-material from inorganic materials: this is
to be found in those cells that are known as "vegetal," in the widest sense
of {36}the word. In this, sense, vegetal nutrition is the utilisation of
nitrogenous substances that are more simple than proteids or peptones,
together with suitable organic carbon compounds, etc., to build up proteids
and protoplasm. The simplest of organisms with a vegetal nutrition are the
Schizomycetes, often spoken of loosely as "bacteria" or "microbes," in
which the differentiation of cytoplasm and nucleus is not clearly
recognisable. Some of these can build up their proteids from the free
uncombined nitrogen of the atmosphere, carbon dioxide, and inorganic salts,
such as sulphates and phosphates. But the majority of vegetal feeders
require the nitrogen to be combined at least in the form of a nitrate or an
ammonium salt—nay, for growth in the dark, they require the carbon also to
be present in some organic combination, such as a tartrate, a carbohydrate,
etc. Acetates and oxalates, "aromatic" compounds[53] and nitriles are
rarely capable of being utilised, and indeed are often prejudicial to life.
In many vegetal feeders certain portions of the protoplasm are specialised,
and have the power of forming a green, yellow, or brown pigment; these are
called "plastids" or "chromatophores." They multiply by constriction within
the cell, displaying thereby a certain independent individuality. These
plastids have in presence of light the extraordinary power of deoxidising
carbon dioxide and water to form carbohydrates (or fats, etc.) and free
oxygen; and from these carbohydrates or fats, together with ammonium salts
or nitrates, etc., the vegetal protoplasm at large can build up all
necessary food matter. So that in presence of light of the right
quality[54] and adequate intensity, such coloured vegetal beings have the
capacity for building up their bodies and reserves from purely inorganic
materials. Coloured vegetal nutrition, then, is a process involving the
absorption of energy; the source from which this is derived in the bacteria
being very obscure at present. Nutrition by means of coloured plastids is
{37}distinguished as "holophytic," and that from lower substances, which,
however, contain organically combined carbon, as "saprophytic," for such
are formed by the death and decomposition of living beings. The third mode
of nutrition (found in some bacteria) from wholly inorganic substances,
including free nitrogen, has received no technical name. All three modes
are included in the term "autotrophic" (self-nourishing).
Vegetal feeders have a great tendency to accumulate reserves in insoluble
forms, such as starch, paramylum, and oil-globules on the one hand, and
pyrenoids, proteid crystals, aleurone granules on the other.
When an animal-feeding cell encysts or surrounds itself with a continuous
membrane, this is always of nitrogenous composition, usually containing the
glucosamide "chitin." The vegetal cell-wall, on the contrary, usually
consists, at least primarily, of the carbohydrate "cellulose"—the vegetal
cell being richly supplied with carbohydrate reserves, and drawing on them
to supply the material for its garment. This substance is what we are all
familiar with in cotton or tissue-paper.
Again, not only is the vegetal cell very ready to surround itself with a
cell-wall, but its food-material, or rather, speaking accurately, the
inorganic materials from which that food is to be manufactured, may diffuse
through this wall with scarcely any difficulty. Such a cell can and does
grow when encysted: it grows even more readily in this state, since none of
its energies are absorbed by the necessities of locomotion, etc. Growth
leads, of course, to division: there is often an economy of wall-material
by the formation of a mere party-wall dividing the cavity of the old
cell-wall at its limit of growth into two new cavities of equal size. Thus
the division tends to form a colonial aggregate, which continues to grow in
a motionless, and, so far, a "resting" state. We may call this "vegetative
rest," to distinguish it from "absolute rest," when all other
life-processes (as well as motion) are reduced to a minimum or absolutely
suspended.
The cells of a plant colony are usually connected by very fine threads of
protoplasm, passing through minute pores where the new party-wall is left
incomplete after cell-division.[55] In a few plants, such as most Fungi,
the cell-partitions are {38}in abeyance for the 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 _physiological_ consideration, and we
have now arrived at a _morphological_ distinction, very valid among higher
organisms.
_HIGHER PLANTS consist of cells for the most part each isolated in its own
cell-cavity, save for the few slender threads of communication._
_HIGHER ANIMALS consist of cells that are rarely isolated in this way, but
are mostly in mutual contact over the greater part 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, living 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 disseminate 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. {39}though never wholly absent. Intracellular movements of
the protoplasm are, however, found in all Plants.
In Plants we find no distinct nervous system formed of cells and
differentiated from other tissues with centres and branches and
sense-organs. These are more or less obvious in all Metazoa, traces being
even found in the Sponges.
We may then define Plants as beings which have the power of manufacturing
true food-stuffs from lower chemical substances than proteids, often with
the absorption of energy. They have the power of surrounding themselves
with a cell-wall, usually of cellulose, and of growing and dividing freely
in this state, in which animal-like changes of form and locomotion are
impossible; their colonies are almost always fixed or floating; free
locomotion is only possible in the case of their naked reproductive cells,
and is transitory even in these. The movements of motile parts of complex
plant-organisms are due to the changes in the osmotic powers of cells as a
whole, and not to the contraction of differentiated fibrils in the
cytoplasm of individual cells. Plants that can form carbohydrates with
liberation of free oxygen have always definite plastids coloured with a
lipochrome[56] pigment, or else (in the Phycochromaceae) the whole plasma
is so coloured. Solid food is never taken into the free plant-cell nor into
an internal cavity in complex Plants. If, as in insectivorous Plants, it is
digested and absorbed, it is always in contact with the morphological
external surface. In the complex Plants apocytes and syncytes are rare—the
cells being each invested with its own wall, and, at most, only
communicating by minute threads with its neighbours. No trace of a central
nervous system with differentiated connexions can be made out.
Animals all require proteid food; their cyst-walls are never formed of
cellulose; their cells usually divide in the naked condition only, or if
encysted, no secondary party-walls are formed between the daughter-cells to
unite them into a vegetative colony. Their colonies are usually locomotive,
or, if fixed, their parts largely retain their powers of relative motion,
and are often provided on their free surfaces with cilia or flagella; and
many cells have differentiated in their cytoplasm contractile muscular
fibrils. Their food (except in a few parasitic groups) is always taken
{40}into a distinct digestive cavity. A complex nervous system, of many
special cells, with branched prolongations interlacing or anastomosing, and
uniting superficial sense-organs with internal centres, is universally
developed in Metazoa. All Metazoa fulfil the above conditions.
But when we turn to the Protozoa we find that many of the characters evade
us. There are some Dinoflagellates (see p. 130) which have coloured
plastids, but which differ in no other respect (even specific) from others
that lack them: the former may have mouths which are functionless, the
latter have functional mouths. Some colourless Flagellates are saprophytic
and absorb nutritive liquids, such as decomposing infusions of organic
matter, possibly free from all proteid constituents; while others, scarcely
different, take in food after the fashion of Amoeba. Sporozoa in the
persistence of the encysted stage are very plant-like, though they are
often intracellular and are parasitic in living Animals. On the other hand,
the Infusoria for the most part answer to all the physiological characters
of the Animal world, but are single cells, and by the very perfection of
their structure, all due to plasmic not to cellular differentiation, show
that they lie quite off the possible track of the origin of Metazoa from
Protozoa. Indeed, a strong natural line of demarcation lies between Metazoa
and Protista. Of the Protozoa, certain groups, like the Foraminifera and
Radiolaria and the Ciliate and Suctorial Infusoria are distinctly animal in
their chemical activities or metabolism, their mode of nutrition, and their
locomotive powers. When we turn to the Proteomyxa, Mycetozoa, and the
Flagellates we find that the distinction between these and the lower Fungi
is by no means easy, the former passing, indeed, into true Fungi by the
Chytridieae, which it is impossible to separate sharply from those
Flagellates and Proteomyxa which Cienkowsky and Zopf have so accurately
studied under the name of "Monadineae." Again, many of the coloured
Flagellates can only (if at all) be distinguished from Plants by the
relatively greater prominence and duration of the mobile state, though
classifiers are generally agreed in allotting to Plants those coloured
Flagellates which in the resting state assume the form of multicellular or
apocytial filaments or plates.
On these grounds we should agree with Haeckel in distinguishing the living
world into the Metazoa, or Higher Animals, which {41}are sharply marked
off; the Metaphyta, or Higher Plants, which it is not so easy to
characterise, but which unite at least two or more vegetal characters; and
the Protista, or organisms, whose differentiation is limited to that within
the cell (or apocyte), and does not involve the cells as units of tissues.
These Protista, again, it is impossible to separate into animal and vegetal
so sharply as to treat adequately of either group without including some of
the other: thus it is that every text-book on Zoology, like the present
work, treats of certain Protophyta. The most unmistakably animal group of
the Protista, the Ciliata, is, as we have seen, by the complex
differentiation of its protoplasm, widely removed from the Metazoa with
their relatively simple cells but differentiated cell-groups and tissues.
The line of probable origin of the Metazoa is to be sought, for Sponges at
least, among the Choanoflagellates (pp. 121 f. 181 f.).
{42}CHAPTER II
PROTOZOA (_CONTINUED_): SPONTANEOUS GENERATION—CHARACTERS OF
PROTOZOA—CLASSIFICATION
THE QUESTION OF SPONTANEOUS GENERATION
From the first discovery of the Protozoa, their life-history has been the
subject of the highest interest: yet it is only within our own times that
we can say that the questions of their origin and development have been
thoroughly worked out. If animal or vegetable matter of any kind be
macerated in water, filtered, or even distilled, various forms of Protista
make their appearance; and frequently, as putrefaction advances, form after
form makes its appearance, becomes abundant, and then disappears to be
replaced by other species. The questions suggested by such phenomena are
these: (1) Do the Protista arise spontaneously, that is, by the direct
organisation into living beings of the chemical substances present, as a
crystal is organised from a solution: (2) Are the forms of the Protista
constant from one generation to another, as are ordinary birds, beasts, and
fishes?
The question of the "spontaneous generation" of the Protista was readily
answered in the affirmative by men who believed that Lice bred directly
from the filth of human skins and clothes;[57] and that Blow-flies, to say
nothing of Honey-bees, arose in rotten flesh: but the bold aphorism of
Harvey "omne vivum ex ovo" at once gained the ear of the best-inspired men
of science, and set them to work in search of the "eggs" that gave rise to
the organisms of putrefaction. Redi (ob. 1699) showed that Blow-flies never
arise save when other Blow-flies gain access to meat and deposit their very
visible eggs thereon. Leeuwenhoek, his {43}contemporary, in the latter half
of the seventeenth century adduced strong reasons for ascribing the origin
of the organisms of putrefaction to invisible air-borne eggs. L. Joblot and
H. Baker in the succeeding half-century investigated the matter, and showed
that putrefaction was no necessary antecedent of the appearance of these
beings: that, as well as being air-borne, the germs might sometimes have
adhered to the materials used for making the infusion; and that no
organisms were found if the infusions were boiled long enough, and corked
when still boiling. These views were strenuously opposed by Needham in
England, by Wrisberg in Germany, and by Buffon, the great French naturalist
and philosopher, whose genius, unballasted by an adequate knowledge of
facts, often played him sad tricks. Spallanzani made a detailed study of
what we should now term the "bionomical" or "oecological" conditions of
Protistic life and reproduction in a manner worthy of modern scientific
research, and not attained by some who took the opposite side within living
recollection. He showed that infusions kept sufficiently long at the
boiling-point in hermetically sealed vessels developed no Protistic life.
As he had shown that active Protists are killed at much lower temperatures,
he inferred that the germs must have much higher powers of resistance; and,
by modifying the details of his experiments, he was able to meet various
objections of Needham.
Despite this good work, the advocates of spontaneous generation were never
really silenced; and the widespread belief in the inconstancy of species in
Protista added a certain amount of credibility to their cause. In 1845
Pineau put forward these views most strongly; and from 1858 to 1864 they
were supported by the elder Pouchet. Louis Pasteur, who began life as a
chemist, was led from a study of alcoholic fermentation to that of the
organisms of fermentation and of putrefaction and disease. He showed that
in infusions boiled sufficiently long and sealed while boiling, or kept at
the boiling-point in a sealed vessel, no life manifested itself: objections
raised on the score of the lack of access of fresh air were met by the
arrangement, so commonly used in "pure cultures" at the present day, of a
flask with a tube attached plugged with a little cotton-wool, or even
merely bent repeatedly into a zigzag. The former attachment filtered off
all germs or floating solid particles from the air, the latter brought
about the settling of such particles in the elbows {44}or on the sides of
the tube: in neither case did living organisms appear, even after the lapse
of months. Other observers succeeded in showing that the forms and
characters of species were as constant as in Higher Animals and Plants,
allowing, of course, for such regular metamorphoses as occur in Insects, or
alternations of generations paralleled in Tapeworms and Polypes. The
regular sequences of such alternations and metamorphoses constitute,
indeed, a strong support of the "germ-theory"—the view that all Protista
arise from pre-existing germs. It is to the Rev. W. H. Dallinger and the
late Dr. Charles Drysdale that we owe the first complete records of such
complex life-histories in the Protozoa as are presented by the minute
Flagellates which infest putrefying liquids (see below, p. 116 f.). The
still lower Schizomycetes, the "microbes" of common speech, have also been
proved by the labours of Ferdinand Cohn, von Koch, and their numerous
disciples, to have the same specific constancy in consecutive generations,
as we now know, thanks to the methods first devised by De Bary for the
study of Fungi, and improved and elaborated by von Koch and his school of
bacteriologists.
And so to-day the principle "omne vivum ex _vivo_" is universally accepted
by men of science. Of the ultimate origin of organic life from inorganic
life we have not the faintest inkling. If it took place in the remote past,
it has not been accomplished to the knowledge of man in the history of
scientific experience, and does not seem likely to be fulfilled in the
immediate or even in the proximate future.[58]
PROTOZOA
_Organisms of various metabolism, formed of a single cell or apocyte, or of
a colony of scarcely differentiated cells, whose organs are formed by
differentiations of the protoplasm, and its secretions and accretions; not
composed of differentiated multicellular tissues or organs._[59]
{45}This definition, as we have seen, excludes Metazoa (including Mesozoa,
Vol. II. p. 92) sharply from Protozoa, but leaves an uncertain boundary on
the botanical side; and, as systematists share with nations the desire to
extend their sphere of influence, we shall here follow the lead of other
zoologists and include many beings that every botanist would claim for his
own realm. Our present knowledge of the Protozoa has indeed been largely
extended by botanists,[60] while the study of protoplasmic physiology has
only passed from their fostering care into the domain of General Biology
within the last decade. The study of the Protozoa is little more than two
centuries old, dating from the school of microscopists of whom the Dutchman
Leeuwenhoek is the chief representative: and we English may feel a just
pride in the fact that his most important publications are to be found in
the early records of our own Royal Society.
Baker, in the eighteenth century, and the younger Wallich, Carter,
Dallinger and Drysdale, Archer, Saville Kent, Lankester, and Huxley, in the
last half-century, are our most illustrious names. In France, Joblot,
almost as an amateur, like our own Baker, flourished in the early part of
the eighteenth century. Dujardin in the middle of the same century by his
study of protoplasm, or sarcode as he termed it, did a great work in laying
the foundations of our present ideas, while Balbiani, Georges Pouchet,
Fabre-Domergue, Maupas, Léger, and Labbé in France, have worthily continued
and extended the Gallic traditions of exact observation and careful
deduction. Otto Friedrich Müller, the Dane, in the eighteenth century, was
a pioneer in the exact study and description of a large number of forms of
these, as of other microscopic forms of life. The Swiss collaborators,
Claparède and Lachmann, in the middle of the nineteenth century, added many
facts and many descriptions; and illustrated them by most valuable figures
of the highest merit from every point of view. Germany, with her large
population of students and her numerous universities, has given many names
to our list; among these, Ehrenberg and von Stein have added {46}the
largest number of species to the roll. Ehrenberg about 1840 described,
indeed, an enormous number of forms with much care, and in detail far too
elaborate for the powers of the microscope of that date: so that he was led
into errors, many and grave, which he never admitted down to the close of a
long and honoured life. Max Schultze did much good work on the Protozoa, as
well as on the tissues of the Metazoa, and largely advanced our conceptions
of protoplasm. His work was continued in Germany by Ernst Haeckel, who
systematised our knowledge of the Radiolaria, Greeff, Richard Hertwig,
Fritz Schaudinn, and especially Bütschli, who contributed to Bronn's
_Thier-Reich_ a monograph of monumental conception and scope, and of
admirable execution, on which we have freely drawn. Cienkowsky, a Russian,
and James-Clark and Leidy, both Americans, have made contributions of high
quality.
Lankester's article in the _Encyclopædia Britannica_ was of epoch-making
quality in its philosophical breadth of thought.
Delage and Hérouard have given a full account of the Protozoa in their
_Traité de Zoologie Concrète_, vol. i. (1896); and A. Lang's monograph in
his _Vergleichende Anatomie_, 2nd ed. (1901), contains an admirable
analysis of their general structure, habits, and life-cycles, together with
full descriptions of well-selected types. Calkins has monographed "The
Protozoa" in the Columbia University Biological series (1901). These works
of Bütschli, Delage, Lang, and Calkins contain full bibliographies. Doflein
has published a most valuable systematic review of the Protozoa parasitic
on animals under the title _Die Protozoen als Parasiten und
Krankheitserreger_ (1901); and Schaudinn's _Archiv für Protistenkunde_,
commenced only four years ago, already forms an indispensable collection of
facts and views.
The PROTOPLASM of the Protozoa (see p. 5 f.) varies greatly in consistency
and in differentiation. Its outer layer may be granular and scarcely
altered in Proteomyxa, the true Myxomycetes, Filosa, Heliozoa, Radiolaria,
Foraminifera, etc.; it is clear and glassy in the Lobose Rhizopods and the
Acrasieae; it is continuous with a firm but delicate superficial pellicle
of membranous character in most Flagellates and Infusoria; and this
pellicle may again be consolidated and locally thickened in some members of
both groups so as to form a coat of mail, even with definite spines and
hardened polygonal plates (_Coleps_, Fig. 54, {47}p. 150). Again, it may
form transitory or more or less permanent pseudopodia,[61] (1) blunt or
tapering and distinct, with a hyaline outer layer, or (2) granular and
pointed, radiating and more or less permanent, or (3) branching and fusing
where they meet into networks or perforated membranes. Cilia or flagella
are motile thread-like processes of active protoplasm which perforate the
pellicle; they may be combined into flattened platelets or firm rods, or
transformed into coarse bristles or fine motionless sense-hairs. The
skeletons of the Protozoa, foreign to the cytoplasm, will be treated of
under the several groups.
Most of the fresh-water and brackish forms (and some marine ones) possess
one or more contractile vacuoles, often in relation to a more or less
complex system of spaces or canals in Flagellates and Ciliates.
The GEOGRAPHICAL DISTRIBUTION of Protozoa is remarkable for the wide, nay
cosmopolitan, distribution of the terrestrial and fresh-water forms;[62]
they owe this to their power of forming cysts, within which they resist
drought, and can be disseminated as "dust." Very few of them can multiply
at a temperature approaching freezing-point; the Dinoflagellates can,
however, and therefore present Alpine and Arctic forms. The majority breed
most freely at summer temperatures; and, occurring in small pools, can thus
achieve their full development in such as are heated by the sun during the
long Arctic day as readily as in the Tropics. In infusions of decaying
matter, the first to appear are those that feed on bacteria, the essential
organisms of putrefaction. These, again, are quickly followed and preyed
upon by carnivorous species, which prefer liquids less highly charged with
organic matters, and do not appear until the liquid, hitherto cloudy, has
begun to clear. Thus we have rather to do with "habitat" than with
"geographical {48}distribution," just as with the fresh-water Turbellaria
and the Rotifers (vol. ii. pp. 4 f., 226 f.). We can distinguish in
fresh-water, as in marine Protista, "littoral" species living near the
bank, among the weeds; "plankton," floating at or near the surface; "zonal"
species dwelling at various depths; and "bottom-dwellers," mostly
"limicolous" (or "sapropelic," as Lauterborn terms them), and to be found
among the bottom ooze. Many species are "epiphytic" or "epizoic," dwelling
on plants or animals, and sometimes choice enough in their preference of a
single genus or species as host. Others again are "moss-dwellers," living
among the root-hairs of mosses and the like, or even "terrestrial" and
inhabiting damp earth. The Sporozoa are internal parasites of animals, and
so are many Flagellates, while many Proteomyxa are parasitic in
plant-cells. The Foraminifera (with the exception of most Allogromidiaceae)
are marine, and so are the Radiolaria; while most Heliozoa inhabit fresh
water. Concerning the distribution in time we shall speak under the last
two groups, the only ones whose skeletons have left fossil remains.
CLASSIFICATION.—The classification of the Protozoa is no easy task. We omit
here, for obvious reasons, the unmistakable Plant Protists that have a
holophytic or saprophytic nutrition; that are, with the exception of a
short period of locomotion in the young reproductive cells, permanently
surrounded with a wall of cellulose or fungus-cellulose, and that multiply
and grow freely in this encysted state: to these consequently we relegate
the CHYTRIDIEAE, which so closely allied to the Proteomyxa and the
Phycomycetous Fungi; and the Confervaceae, which in the resting state form
tubular or flattened aggregates and are allied to the green Flagellates;
besides the Schizophyta. At the opposite pole stand the INFUSORIA in the
strict sense, with the most highly differentiated organisation found in our
group, culminating in the possession of a nuclear apparatus with nuclei of
two kinds, and exhibiting a peculiar form of conjugation, in which the
nuclear apparatus is reorganised. The SPOROZOA are clearly marked off as
parasites in living animals, which mostly begin life as sickle-shaped cells
and have always at least two alternating modes of brood-formation, the
first giving rise to aplanospores, wherein is formed the second brood of
sickle-shaped, wriggling zoospores. The remainder, comprising the
SARCODINA, or RHIZOPODA in the old wide sense (including all {49}that move
by pseudopodia during the great part of their active life), and the
FLAGELLATA in the widest sense, are not easy to split up; for many of the
former have flagellate reproductive cells, and many of the latter can emit
pseudopodia with or without the simultaneous retraction of their flagella.
The RADIOLARIA are well defined by the presence in the body plasm of a
central capsule marking it off into a central and a peripheral portion, the
former containing the nucleus, the latter emitting the pseudopodia. Again,
on the other hand, we find that we can separate as FLAGELLATA in the strict
sense the not very natural assemblage of those Protista that have flagella
as their principle organs of movement or nutrition during the greater part
of their active life. The remaining groups (which with the Radiolaria
compose the Sarcodina of Bütschli), are the most difficult to treat. The
RHIZOPODA, as we shall limit them, are naked or possess a simple shell,
never of calcium carbonate, have pseudopodia that never radiate abundantly
nor branch freely, nor coalesce to form plasmatic networks, nor possess an
axial rod of firmer substance. The FORAMINIFERA have a shell, usually of
calcium carbonate, their pseudopodia are freely reticulated, at least
towards the base; and (with the exception of a few simple forms) all are
marine. The MYCETOZOA are clearly united by their tendency to aggregate
more or less completely into complex resting-groups (fructifications), and
by reproducing by unicellular zoospores, flagellate or amoeboid, which are
not known to pair. The HELIOZOA resemble the Radiolaria in their fine
radiating pseudopodia, but have an axial filament always present in each,
and lack the central capsule; and are, for the most part, fresh-water
forms. Finally, the PROTEOMYXA forms a sort of lumber-room for beings which
are intermediate between the Heliozoa, Rhizopoda, and Flagellata, usually
passing through an amoeboid stage, and for the most part reproducing by
brood-formation. Zoospores that possess flagella are certainly known to
occur in some forms of Foraminifera, Rhizopoda, Heliozoa, and Radiolaria,
though not by any means in all of each group.[63]
A. Pseudopodia the principal means of locomotion and feeding; {50}
flagella absent or transitory I. SARCODINA
(1) Plastogamy only leading to an increase in size, never to the
formation of "fructifications."
(_a_) Pseudopodia never freely coalescing into a network nor
fine to the base RHIZOPODA.
(*) Ectoplasm clear, free from granules; pseudopodia, usually
blunt RHIZOPODA LOBOSA
(**) Ectoplasm finely granular; pseudopodia slender,
branching, but not forming a network, passing into the
body by basal dilatation RHIZOPODA FILOSA
(_b_) Pseudopodia branching freely and coalescing to form
networks; ectoplasm granular; test usually calcareous or
sandy FORAMINIFERA
(_c_) Pseudopodia fine to the very base; radiating, rarely
coalescing.
(i.) Pseudopodia with a central filament HELIOZOA
(ii.) Pseudopodia without a central filament.
(*) Body divided into a central and a peripheral part by a
"central capsule" RADIOLARIA
(**) Body without a central capsule PROTEOMYXA
(2) Cells aggregating or fusing into plasmodia before forming a
complex "fructification" MYCETOZOA
B. Cells usually moving by "euglenoid" wriggling or by excretion of a
trail of viscid matter; reproduction by alternating modes of
brood-formation, rarely by Spencerian fission II. SPOROZOA
C. Flagella (rarely numerous) the chief or only means of motion and
feeding III. FLAGELLATA
D. Cilia the chief organs of motion, in the young state at least; nuclei
of two kinds IV. INFUSORIA
{51}CHAPTER III
PROTOZOA (_CONTINUED_): SARCODINA
I. SARCODINA.
_Protozoa performing most of their life-processes by pseudopodia; nucleus
frequently giving off fragments (chromidia) which may play a part in
nuclear reconstitution on division; sometimes with brood-cells, which may
be at first flagellate; but never reproducing in the flagellate
state.[64]_
1. RHIZOPODA
_Sarcodina of simple form, whose pseudopodia never coalesce into networks
(1),[65] nor contain an axial filament (2), which commonly multiply by
binary fission (3), though a brood-formation may occur; which may
temporarily aggregate, or undergo temporary or permanent plastogamic union,
but never to form large plasmodia or complex fructifications as a prelude
to spore-formation (4); test when present gelatinous, chitinous, sandy, or
siliceous, simple and 1-chambered (5)._
CLASSIFICATION.[66]
I. Ectoplasm distinct, clear; pseudopodia blunt or tapering, but not
branching at the apex
LOBOSA
_Amoeba_, Auctt.; _Pelomyxa_, Greeff; _Trichosphaerium_, A. Schneid.;
_Dinamoeba_, Leidy; _Amphizonella_, Greeff; _Centropyxis_, Stein;
_Arcella_, {52}Ehr.; _Difflugia_, Leclercq; _Lecqueureusia_,
Schlumberger; _Hyalosphenia_, Stein; _Quadrula_, F. E. Sch.;
_Heleopera_, Leidy; _Podostoma_, Cl. and L.; _Arcuothrix_, Hallez.
II. Ectoplasm undifferentiated, containing moving granules; pseudopodia
branching freely towards the tips
FILOSA
_Euglypha_, Duj.; _Paulinella_, Lauterb.; _Cyphoderia_, Schlumb.;
_Campascus_, Leidy; _Chlamydophrys_, Cienk.; _Gromia_, Duj. =
_Hyalopus_, M. Sch.
We have defined this group mainly by negative characters, as such are the
only means for their differentiation from the remaining Sarcodina; and
indeed from Flagellata, since in this group zoospores are sometimes formed
which possess flagella. Moreover, indeed, in a few of this group
(_Podostoma_, _Arcuothrix_), as in some Heliozoa, the flagellum or flagella
may persist or be reproduced side by side with the pseudopodia. The
subdivision of the Rhizopoda is again a matter of great difficulty, the
characters presented being so mixed up that it is hard to choose: however,
the character of the outer layer of the cytoplasm is perhaps the most
obvious to select. In LOBOSA there is a clear layer of ectosarc, which
appears to be of a greasy nature at its surface film, so that it is not
wetted. In the FILOSA, as in most other Sarcodina, this film is absent, and
the ectoplasm is not marked off from the endoplasm, and may have a granular
surface. Corresponding to this, the pseudopodia of the Lobosa are usually
blunt, never branching and fraying out, as it were, at the tip, as in the
Filosa; nay, in the normal movements of _Amoeba limax_ (Fig. 1, p. 5) the
front of the cell forms one gigantic pseudopodium, which constantly glides
forward. Apart from this distinction the two groups are parallel in almost
every respect.
There may be a single contractile vacuole, or a plurality; or none,
especially in marine and endoparasitic species. The nucleus may remain
single or multiply without inducing fission, thus leading to apocytial
forms. It often gives off "chromidial" fragments, which may play an
important part in reproduction.[67] In _Amoeba binucleata_ there are
constantly two nuclei, both of which divide as an antecedent to fission,
each giving a separate nucleus to either daughter-cell. _Pelomyxa
palustris_, the giant of the group, attaining a diameter of 1''' (2 mm.),
has very blunt pseudopodia, an enormous number of nuclei, and no
contractile vacuole, though {53}it is a fresh-water dweller, living in the
bottom ooze of ponds, etc., richly charged with organic débris. It is
remarkable also for containing symbiotic bacteria, and brilliant vesicles
with a distinct membranous wall, containing a solution of glycogen.[68]
Few, if any, of the Filosa are recorded as plurinuclear.
The simplest Lobosa have no investment, nor indeed any distinction of front
or back. In some forms of _Amoeba_, however, the hinder part is more
adhesive, and may assume the form of a sucker-like disc, or be drawn into a
tuft of short filaments or villi, to which particles adhere. Other species
of Lobosa and all Filosa have a "test," or "theca," _i.e._ an investment
distinct from the outermost layer of the cell-body. The simplest cases are
those of _Amphizonella_, _Dinamoeba_, and _Trichosphaerium_, where this is
gelatinous, and in the two former allows the passage of food particles
through it into the body by mere sinking in, like the protoplasm itself,
closing again without a trace of perforation over the rupture. In
_Trichosphaerium_ (Fig. 9) the test is perforated by numerous pores of
constant position for the passage of the pseudopodia, closing when these
are retracted; and in the "A" form of the species (see below) it is studded
with radial spicules of magnesium carbonate. Elsewhere the test is more
consistent and possesses at least one aperture for the emission of
pseudopodia and the reception of food; to avoid confusion we call this
opening not the _mouth_ but the "pylome": some Filosa have two
symmetrically placed pylomes. When the test is a mere pellicle, it may be
recognised by the limitation of the pseudopodia to the one pylomic area.
But the shell is often hard. In _Arcella_ (Fig. 10, C), a form common among
Bog-mosses and Confervas, it is chitinous and shagreened, circular, with a
shelf running in like that of a diving-bell around the pylome: there are
two or more contractile vacuoles, and at least two nuclei. Like some other
genera, it has the power of secreting carbonic acid gas in the form of
minute bubbles in its cytoplasm, so as to enable it to float up to the
surface of the water. The chitinous test shows minute hexagonal
sculpturing, the expression of vertical partitions reaching from the inner
to the outer layer.
{54}[Illustration: FIG. 9.—_Trichosphaerium sieboldii._ 1, Adult of "A"
form; 2, its multiplication by fission and gemmation; 3, resolution into
1-nucleate amoeboid zoospores; 4, development (from zoospores of "A") into
"B" form (5); 6, its multiplication by fission and gemmation; 7, its
resolution after nuclear bipartition into minute 2-flagellate zoospores or
(exogametes); 8, liberation of gametes; 9, 10, more highly magnified
pairing of gametes of different origin; 11, 12, zygote developing into "A"
form. (After Schaudinn.)]
Several genera have tests of siliceous or chitinous plates, formed in the
cytoplasm in the neighbourhood of the nucleus, and connected by chitinous
cement. Among these _Quadrula_ (Fig. 10, A) is Lobose, with square plates,
_Euglypha_ (Fig. 8, p. 29), and _Paulinella_[69] are Filose, with hexagonal
plates. In the latter they are in five longitudinal rows, with a pentagonal
oral plate, perforated by the oval pylome. In other genera again, such as
_Cyphoderia_ (Filosa), the plates are merely {55}chitinous. Again, the
shell may be encrusted with sand-grains derived directly from without, or
from ingested particles, as shown in _Centropyxis_, _Difflugia_ (Fig. 10,
D), _Heleopera_, and _Campascus_ when supplied with powdered glass instead
of sand. The cement in _Difflugia_ is a sort of organic mortar, infiltrated
with ferric oxide (more probably ferric hydrate). In _Lecqueureusia
spiralis_ (formerly united with _Difflugia_) the test is formed of minute
sausage-shaped granules, in which may be identified the partly dissolved
valves of Diatoms taken as food; it is spirally twisted at the apex, as if
it had enlarged after its first formation, a very rare occurrence in this
group. The most frequent mode of fission in the testaceous Rhizopods (Figs.
8, 10) is what Schaudinn aptly terms "bud-fission," where half the
protoplasm protrudes and accumulates at the mouth of the shell, and remains
till a test has formed for it, while the other half retains the test of the
original animal. The materials for the shell, whether sand-granules or
plates, pass from the depths of the original shell outwards into the naked
cell, and through its cytoplasm to the surface, where they become connected
by cementing matter into a continuous test. The nucleus now divides into
two, one of which passes into the external animal; after this the two
daughter-cells separate, the one with the old shell, the other, larger,
with the new one.
[Illustration: FIG. 10.—Test-bearing Rhizopods. A, _Quadrula symmetrica_:
B, _Hyalosphenia lata_; C, _Arcella vulgaris_; D, _Difflugia pyriformis_.
(From Lang's _Comparative Anatomy_.)]
If two individuals of the shelled species undergo bud-fission in close
proximity, the offspring may partially coalesce, so that a monstrous shell
is produced having two pylomes.
{56}Reproduction by fission has been clearly made out in most members of
the group; some of the multinucleate species often abstrict a portion,
sometimes at several points simultaneously, so that fission here passes
into budding[70] (Fig. 9, 2, 6).
Brood-division, either by resolution in the multinucleate species, or
preceded by multiple nuclear division in the habitually 1-nucleate, though
presumably a necessary incident in the life-history of every species, has
only been seen, or at least thoroughly worked out, in a few cases, where it
is usually preceded by encystment, and mostly by the extrusion into the
cyst of any undigested matter.[71]