ANIMALS

BIOLOGY

UBRA? '

The Natural History
of Animals

BUTTERFLIES (RHOPALOCERA)

1. Giant Blue.

2. Resplendent Ptolemy.

3. Purple Emperor.

4. Peacock Butterfly.

5. Common Swallow-tail.

6. Imperial Swallow-tail.

BUTTERFLIES (Rhopalocerd)

Butterflies and Moths together constitute the order of Scale-
winged Insects (Lepidopterd) , in which the four wings are opaque
and coloured, owing to the presence of minute dust-like scales.
Typical Butterflies are diurnal in habit, their antennae are club-
shaped, and they bring the wings together above the back when
they settle. Those figured are among the most attractive of
tropical and British species, belonging to the Fritillaries (1-4) and
Swallow-tails (5, 6).

1. Giant Blue (Morpho cy-pris). \

} Tropical America.

2. Resplendent Ptolemy (M. neoptolemus). )

3. Purple Emperor (Apatura Iris), oakwoods in the southern

English counties.

4. Peacock Butterfly (Vanessa Io\ Europe (including Britain)

and N. Asia to Japan.

5. Common Swallow-tail (Papilio machaori), ranges from Britain

to the Himalayas.

6. Imperial Swallow-tail (Tinopalpus imperialis), male; in the

female each hind- wing has two tails. Sikkim.

The

Natural History
of Animals

The Animal Life of the World in its various
Aspects and Relations

BY

J. R. AINSWORTH DAVIS, M.A.

TRINITY COLLEGE, CAMBRIDGE

PROFESSOR IN THE UNIVERSITY OF WALES, AND PROFESSOR OF ZOOLOGY AND
GEOLOGY IN UNIVERSITY COLLEGE, ABERYSTWYTH

HALF-VOL. II

LONDON

THE GRESHAM PUBLISHING COMPANY

34 SOUTHAMPTON STREET, STRAND
1903

BIOLOGY

LIBRARY

3

CONTENTS

HALF-VOL. II

CHAPTER VI

STRUCTURE AND CLASSIFICATION OF FISHES (PISCES) AND
PRIMITIVE VERTEBRATES (PROTOCHORDATA)

FISHES (PISCES)

Page

Structure and Development of the Spotted Dog-Fish (Scyllium canicula), taken as

a Type - 257

Sub-Class I. LUNG-FISHES (Dipnoi)— Ceratodus, Protopterus, Lepidosiren - - 264

Sub-Class II. BONY FISHES (Teleostomi) - - 266

Order i. — GANOIDS (Ganoidei) - - 266

(i) Fringe-finned Ganoids— (2) Ray-fmned Ganoids.

Order 2.— BONY FISHES PROPER (Teleostei) - - 269

Structure of the Common Perch (Perca fluviatilis), taken as a Type - - 269

A, Teleosts in which the Swim-bladder (when present) has lost its con-

nection with the Gullet.

Sub-Order i.— SPINE-FINNED FISHES (Acanthopterygii) - 273

Sub-Order 2. — TUFT-GILLED FISHES (Lophobranchii) - 276

Sub-Order 3.— FIRM-JAWED FISHES (Plectognathi) - - 277

Sub-Order 4.— SOFT-FINNED FISHES (Anacanthini) 278

B, Teleosts in which the Swim-bladder retains its connection with the

Gullet.

Sub-Order 5.— TUBE-BLADDERED FISHES (Physostomi) - - - 280

Sub-Class III. SHARKS AND RAYS (Elasmobranchii) - - - 284

Order i.— SHARKS AND DOG-FISHES (Selachoidei) - - - 284

Order 2.— SKATES AND RAYS (Batoidei) - 287

Sub- Class IV. CHIMERAS (Holocephali) 290

Sub-Class V. ROUND-MOUTHS (Cyclostomata) - 291

(i) Lampreys — (2) Hag-Fishes.

PRIMITIVE VERTEBRATES (PROTOCHORDATA)

Essential Characters of the Chordata -- 292

Sub- Class I. LANCELETS (Cephalochorda) - - 293

Structure of the Common Lancelet (Amphioxus lanceolatus).
Sub-Class II. SEA-SQUIRTS OR ASCIDIANS (Urochorda or Tunicata) - - - 297

Structure and Development of a simple Sea-Squirt (Ascidia mentula), taken

as a Type — Fixed and Free-swimming Ascidians 297

Sub-Class III. WORM-LIKE PROTOCHORDATES (Hemichorda) - - 300

Structure of the Acorn-headed Worm (Balanoglossus).

5

vi CONTENTS

CHAPTER VII

BACKBONELESS ANIMALS (INVERTEBRATA). STRUCTURE AND
CLASSIFICATION OF NEMERTINES AND MOLLUSCS

Page

Distinctive Characters of Higher Invertebrates as compared with Vertebrates - 302
Main Groups or Phyla of the Invertebrates - 304

NEMERTINES (NEMERTEA)
Structure of a Typical Nemertine 305

MOLLUSCS (MOLLUSCA)

Structure of the Ormer or Sea-Ear (Haliotis tuberculata), taken as a Type 307

Essential Characters of Molluscs - 311

Class i.— HEAD-FOOTED MOLLUSCS (Cephalopoda) - - - 311

Structure of the Common Cuttle-Fish (Sepia officinalis), taken as a Type - 311

Sub-Class I. CUTTLE-FiSHES AND OCTOPI (Dibranchiata) 314

Sub-Class II. PEARLY NAUTILUS (Tetrabranchiata) - 316

Class 2.— SNAILS AND SLUGS (Gastropoda) • 317

Sub-Class i.— STREPTONEURA OR FORE-GILLED SNAILS (Prosobranchia) 318

Order i. — COMB-GILLED SNAILS (Ctenobranchia) - 318

Structure of the Periwinkle (Littorina littorea) and Purple-Shell (Purpura

lapillus), taken as Types.

Whelks, Mitre-Shells, Volutes, Olive-Shells, Harp-Shells, Cone-Shells,
Turret-Shells, Wing-Shells, Helmet-Shells, and Cowries; Heteropods.

Order 2.— SHIELD-GILLED SNAILS (Aspidobranchia) - 322
Trochidas, Ormers, Limpets.

Sub-Class 2.— EUTHYNEURA - - 324

Order i. — HIND-GILLED SNAILS (Opisthobranchia) 324

(i) Tectibranchs (Bubble-Shells, Sea-Hares, Wing-footed Snails or
Pteropods) — (2) Nudibranchs (Sea-Slugs).

Order 2. — LUNG SNAILS (Pulmonata) 326

Structure of the Garden Snail (Helix aspersa), taken as a Type - 326

Land Slugs, Fresh-water Pulmonates.

Class 3.— BIVALVE MOLLUSCS (Lamellibranchia or Pelecypoda) - 328

Structure of Fresh-water Mussels (Unio and Anodonta), taken as Types 328
(i) Cockles — (2) Gapers — (3) Razor-shells — (4) Rock-borers— (5) Sea Mussels —

(6) Ark-Shells— (7) Scallops— (8) Oysters— (9) Nucula Family 333

Class 4.— TUSK-SHELLS (Scaphopoda) 338

Structure of the Common Tusk-Shell (Dentalium vulgare).

Class 5.— PROTO-MOLLUSCS (Amphineura) 339

Mail-Shells (Chitons), Worm-like forms (Neomenia, Proneomenia, Chaeto-
derma).

CHAPTER VIII

STRUCTURE AND CLASSIFICATION OF JOINTED-LIMBED
ANIMALS (ARTHROPODA)

Essential Characters. Subdivisions - - 342

CONTENTS vii

A, Air-breathing Arthropods (Tracheata).

Page

Class i.— INSECTS (Insecta) - - 343
Structure and Development of the Cockroach (Periplaneta orientalis), taken

as a Type - 343

Order i.— BUGS (Hemiptera) - - 351

(1) Homoptera (Cicadas, Lantern-Flies, Frog-Hoppers, Plant-Lice).

(2) Heteroptera (Land-Bugs, Water-Bugs).

Order 2.— FRINGE-WINGED INSECTS (Thysanoptera) - - 355
Corn Thrips (Thrips cerealium).

Order 3.— FLIES AND FLEAS (Diptera) - - - 355

Order 4.— MOTHS AND BUTTERFLIES (Lepidoptera) - - 358

Order 5.— BEETLES (Coleoptera) - - - 366

Order 6. — MEMBRANE-WINGED INSECTS (Hymenoptera) - 369

Order 7.— NET-WINGED INSECTS (Neuroptera) ... . 374

Order 8.— STRAIGHT-WINGED INSECTS (Orthoptera) - - 380

Order 9. — WINGLESS INSECTS (Aptera) - - 384

Class 2.— SPIDER-LIKE ANIMALS (Arachnida) 385

Structure of a Scorpion, taken as a Type - ... 385

Order i.— SCORPIONS (Scorpionidae) - 387

Order 2.— FALSE SPIDERS (Solpugidas) - - 387

Order 3.— FALSE SCORPIONS (Pseudoscorpionidae) - 388

Order 4.— WHIP-SCORPIONS (Pedipalpi) - 389

Order 5.— HARVESTMEN (Phalangidas) 390

Order 6. — SPIDERS (Araneidae) - 390

Order 7. — MITES (Acarina) - 393

Order 8.— TONGUE-WORMS (Linguatulidae) - 393

Order 9. — BEAR-ANIMALCULES (Tardigrada) - - 394

Class 3.— CENTIPEDES AND MILLIPEDES (Myriapoda) . . . 394

Class 4.— PRIMITIVE TRACHEATES (Prototracheata) 398
Structure and affinities of Peripatus.

B, Aquatic Arthropods (Branchiata).

Class 5.— CRUSTACEANS (Crustacea) 402

Structure of the Lobster (Homarus vulgaris), taken as a Type - 402

Sub-Class I. HIGHER CRUSTACEA (Malacostraca) - 410

Order i. — STALK-EYED CRUSTACEA (Thoracostraca) -.'••- - 410
(i) Ten-legged Crustacea (Decapoda) (Lobsters, Prawns, Shrimps, Cray-
fishes, Hermit-Crabs, Crabs) — (2) Opossum-Shrimps (Schizopoda) —
(3) Locust-Shrimps (Stomatopoda) — (4) Cumacea.

Order 2. —SESSILE-EYED CRUSTACEA (Arthrostraca) - - - 414
(i) Amphipods (Sand-Hoppers, Whale-Lice, Skeleton Shrimps) — (2) Isopods

(Slaters, Wood-Lice).

Order 3. — INTERMEDIATE CRUSTACEA (Leptostraca) — Mud-Shrimps - - 416

Sub-Class II. LOWER CRUSTACEA (Entomostraca) ------ 416

viii CONTENTS

Page

Order i. — BARNACLES (Cirripedia) - - 417

Order 2.— BIVALVE CRUSTACEA (Ostracoda)— Mussel-Shrimps 419

Order 3. — FORK-FOOTED CRUSTACEA (Copepoda) - - - 420

Order 4.— LEAF-FOOTED CRUSTACEA (Phyllopoda) 421

Class 6.— KING-CRABS (Xiphosura) - - - 422

Structure of the King- Crab (Limulus).

Class 7.— SEA-SPIDERS (Pycnogonida) - - - 424

CHAPTER IX

STRUCTURE AND CLASSIFICATION OF SEGMENTED WORMS,
SIPHON -WORMS, WHEEL -ANIMALCULES, MOSS - POLYPES,
AND LAMP-SHELLS

SEGMENTED WORMS (ANNELIDA)

Class i.— BRISTLE-WORMS (Chaetopoda) - - - 425

Structure of the Sea-Centipede (Nereis), taken as a Type 425

Order i. — MANY-BRISTLED WORMS (Polychasta) - 429

Order 2.— FEW-BRISTLED WORMS (Oligochaeta)— River- Worms, Earth- Worms 430

Order 3.— SIMPLE SEGMENTED WORMS (Archiannelida)— Dinophilus, Poly-

gordius 43 1

Class 2.— LEECHES (Discophora) 432

SIPHON-WORMS (GEPHYREA)
Bristle-Tail (Echiurus), Siphon-Worm (Sipunculus) - - 433

WHEEL-ANIMALCULES (ROTIFERA)
Structure of the Rose-coloured Rotifer (Philodina roseola) 434

MOSS-POLYPES AND LAMP-SHELLS (MOLLUSCOIDA)

Class i.— MOSS-POLYPES (Polyzoa or Bryozoa) 436

Sea-Mat (Flustra), Lace-Coralline (Membranipora), Plume Coralline (Pluma-
tella), Lophopus.

Class 2.— LAMP-SHELLS (Brachiopoda) 43$

Structure of a typical Lamp-Shell (Waldheimia).

CHAPTER X

STRUCTURE AND CLASSIFICATION OF FLAT-WORMS (PLATYHELMIA)
AND THREAD-WORMS (NEMATHELMIA)

FLAT-WORMS (PLATYHELMIA)

Class i.— TAPE- WORMS (Cestoda) - 44 1

Class 2.— FLUKES (Trematoda) 443

Class 3.— PLANARIAN WORMS (Turbellaria) - 445

CONTENTS ix

THREAD -WO RMS (NEMATHELMIA)

Page

Structure of a Round- Worm (Ascaris). Vinegar-Eel (Anguillula), Gordian-

Worm (Gordius), Thorn-headed Worm (Echinorhynchus) - - 447

CHAPTER XI
STRUCTURE AND CLASSIFICATION OF ECHINODERMS

(ECHINODERMATA)

Structure of the Common Star-Fish (Uraster rubens) - 450

Class i.— STAR-FISHES (Asteroidea) 454

Class 2.— BRITTLE-STARS (Ophiuroidea) 455

Class 3.— SEA-URCHINS (Echinoidea) - 456

Class 4.— SEA-LILIES AND FEATHER-STARS (Crinoidea) 459

Class 5.— SEA-CUCUMBERS (Holothuroidea) 462

CHAPTER XII

STRUCTURE AND CLASSIFICATION OF ZOOPHYTES (CCELENTERATA),
SPONGES (PORIFERA), AND ANIMALCULES (PROTOZOA)

ZOOPHYTES (CCELENTERATA)

Structure and Development of the Fresh-water Polype (Hydra), taken as a

Type. Comparison with higher forms. Cells and tissues - 465

Class i.— SEA-FLOWERS (Actinozoa or Anthozoa) 473

Structure of the Beadlet (Actinia mesembryanthemum), taken as a Type - 473

Order i.— SIX-RAYED SEA-FLOWERS (Hexactinia) - 474

(i) Sea-Anemones — (2) Corals.

Order 2.— EIGHT-RAYED SEA-FLOWERS (Octactinia) 476

Structure of the Dead-Man's Fingers (Alcyonium digitatum), taken as a
Type. Organ- Pipe Coral (Tubipora musica), Red Coral (Corallium
rubrum), Sea-Pens and Sea-Mats.

Class 2.— HYDROIDS (Hydrozoa) 4?8

Structure and Development of Obelia, taken as a Type- 478

Order i.— BUDDING HYDROIDS (Hydromedusae) 480

(i) Hydroid Zoophytes and "Naked-eyed" Medusas— (2) Compound Jelly-

Fish (Siphonophora).

Order 2. — SPLITTING HYDROIDS (Scyphomedusae) - - 481

Class 3.— COMB JELLY-FISH (Ctenophora) 483

Structure of Cydippe. Venus's Girdle, Beroe, Creeping Ctenophores.

SPONGES (PORIFERA)

Structure of a Simple Sponge - - - 484

(i) Calcareous Sponges — (2) Siliceous Sponges.

ANIMALCULES (PROTOZOA)

Protoplasm. Description of the Proteus Animalcule (Amoeba), taken as a

Type. Comparison of Protozoa and higher forms (Metazoa) - 484

x CONTENTS

Page

Group I.— INFUSORIANS (Infusoria) 492

Structure of Slipper Animalcule (Paramcecium) and Bell Animalcule (Vorticella).

(i) Ciliates— (2) Flagellates.

Group 2. — AMCEBA-LIKE PROTOZOA (Rhizopoda) - . - - 495

(i) Shell-bearing Amoebae — (2) Foraminifera— (3) Sun Animalcules (Heliozoa) —

(4) Ray Animalcules (Radiolaria) — (5) Mycetozoa.

Group 3.— GREGARINES (Sporozoa) - 498

Structure and Development of the Cockroach Gregarine (Clepsidrina blattarum).

LIST OF ILLUSTRATIONS

HALF-VOL. II

COLOURED PLATES

BUTTERFLIES (Rkopalocera).

From a Drawing by A. Fairfax Muckley Frontispiece.

WRASSES (Labrida).

From a Drawing by A. Fairfax Muckley 276

ECHINODERMS (Echinodermata).

From a Drawing by A. Fairfax Muckley 454

BLACK-AND-WHITE ILLUSTRATIONS

Skeleton of Dog-Fish (Scyllium canicula) -
Side-dissection of Dog-Fish (Scyllium) -
Dipnoi (after Giinther and Miall) - -
Ganoids (after Agassiz, Giinther, Goode,

and Dean) - -

Side-dissection of Perch (Perca fluviatilis)
Sea- Horse (Hippocampus] - -

Group of Fishes - - - -

GROUP OF FRESH-WATER FISHES - -
Dog-Fish and Sharks (after Couch) - -
Monk-Fish and Rays (mostly after Couch)
Chimaeroids (after Carman, Goode, and

Bean) - - -

Lampern (Petromyzon fluviatilis} - -
T anrpW ( Am-hhinYu<;\ (zttet Boveri) - -

Page

259
262
265

267
270
277
281
283
285
289

290
291
294

Olive (Oliva) -

Strombus

Cowry (Cyprcea}

Bubble- Shell (Bulla)

Diagram of a Tectibranch Snail (after Lang)

Eolis

Structure of a Garden Snail (Helix aspersa)
Shells of Fresh-water Mussel ( Unio)
Structure of Fresh-water Mussel (Anodonta)
Inside of Right Valve of a Sinupalliate

Shell

Sand-Gaper (Mya arenaria)
Razor-Shell (Solen) -

Piddocks (Pholas dactylus) in their burrows
Sea Mussel (Mytilus) (after Moebius)

WING to the nature of the subject, the full-page piates,
Coloured and Black -and -White, cannot be equally
distributed throughout the Work. Thus, while the Second
Divisional- Volume has Four Plates, there are Twelve in
Divisional- Volume I

ig and

(Peri-

I-

>ta ori-

Page
32I
322
322
324
325
326
327
329
329

334
335
335
336
337
337
339

340
344

347
352
352
353
355
356

Xll

LIST OF ILLUSTRATIONS

Page

Common Gnat ( Culex pipiens) - - - 356

Larva of Common Gnat (Culex pipiens) - 357

Scales from Wings of various Butterflies - 359

Heads and Proboscides of Butterflies - 359

Great Tortoise-shell Butterfly ( Vanessa) - 361

Cabbage Moth (Mamestra) (from Curtis) - 364

Adult Stage of a Clothes-Moth - - 365

Larvae of a Clothes-Moth - 365

Lady-Birds (Coccinella) (from Curtis) - 366

Flea-Beetle (Haltica) (from Curtis) - 366

Green Tiger Beetle ( Cicindela campestris} - 367

Great Water Beetle (Dytiscus marginalis) - 368

Devil's Coach Horse (Ocypus) (from Curtis) 368

Rose Chafer ( Cetonia aurata) (from Curtis) 369

Corn- Weevil (from Curtis) - - - 369

Structure of Hymenoptera - - - 370

Turnip Saw-Fly (Athalia) (from Curtis) - 371

Wood Wasp (Sirexgigas) • 371

Ichneumon-Fly (Hemiteles) (from Curtis) - 373

Humble Bees (from Curtis) - - - 374

Net- Winged Insects (Neuroptera) - - 375

Golden-Eyed Fly (Chrysopd) (from Curtis) 378

Common Earwig (Forficula auricularia) - 380

" Locust " (Acridiiim peregri num ) - - 382

Mole-Cricket (Gryllotalpa) (from Curtis) - 383

Aptera, magnified ----- 384
Scorpion (after Blanchard and Milne -

Edwards) - - 385

Common False Spider (Galeodes araneoides) 388

Book Scorpion ( Chelifer cancroides) - - 389

A Whip Scorpion (Phrynus) - - - 389

A Harvestman (Phalangium) (after Cuvier) 390

Garden Spider (Epeira diadema) and Web 391

Cheese-Mite ( Tyroglyphus siro) - - 393

Pentastomum (after Leuckart) - 393

A Bear- Animalcule (after Greeff) - 394

British Centipedes (from Curtis) 394
Structure of Centipede (after Vogt and

Yung) --- - 395

British Millipedes (from Curtis) - - 396

Cape Peripatus (after Balfour) - - - 399

Structure of Peripatus (after Balfour) - 400

Appendages of Lobster (Homarus vulgaris} 405

Dissection of Lobster (Homarus vulgaris} - 407

Mediterranean Crustacea - - - - 411

Rock-Lobster (Palinurus vulgaris) - - 412

A Hermit-Crab (Pagurus) (after Bell) - 412

Opossum-Shrimp (My sis) (after Gerstaecker) 413

Cuma (after Sars) ----- 414

Amphipods (after Mayer and Llitken) • 415

Isopods (after Stebbing and Shipley) • 415

Nebalia (after Milne-Edwards) - - 416

Page

A Nauplius Larva 417

Ship-Barnacles (Lepas) (after Schmarda) - 418
Small Fresh -water Crustacea (after Claus,

R. Hertwig, and Zenker) - - - 419

Apus (after Leunis-Ludwig) - - - 421

King-Crab (Linmlus) (partly after Leuckart) 423
Shore Pycnogon (Pycnogomim littorale)

(after Milne-Edwards) - 424

A Sea-Centipede (Nereis) (after Gosse) - 426
Structure of Sea-Centipede (Nereis) (after

Ehlers and R. Leuckart) - - - 427

Lugworm (Arenicola piscatorum) • 430

Gephyrea (partly after Greeff) - - 433

Rotifer (Philgdina) (after Hudson and Gosse) 434

Polyzoa - - - ... 437

Polyzoa, enlarged (after Kraepelin and Boas) 437
Lamp-Shell (Waldheimia) (after Boas and

Davidson) - - 439

Tape-Worm ( T&nia solium) (after Leuckart) 442

Liver-Fluke (Fasciola) (after Sommer) - 443

Planaria lactea (after O. Schmidt) - - 445

Leptoplana tremellaris .... 446

Round- Worm (Ascaris) (after Leuckart) - 448

Common Star- Pish (Ur aster rubens)- - 451

Common Star-Fish (Uraster rubens)- • 453

Brittle-Star (after Wyville Thomson) • 455
Edible Sea -Urchin (Echinus esculentus}

(partly after Hamann) - ... 456

Three-jawed Pedicellarise of Sea-Urchins - 458

Sea-Lily (Pentacrinus) (after J. Miiller) - 460

Feather- Star (Comatula), climbing - - 460
Stages in Development of Feather - Star

( Comatula) (after Wyville Thomson) - 461

A Sea-Cucumber (Cucumaria) - • - 462

Green Hydra (Hydra viridis) - - - 465

Structure of Hydra (partly after Jickeli) - 468

Epithelium (after Hatschek) - - - 470

Development of Hydra (after Brauer) - 472

Sea-Anemone 474

Skeletons of Arabian Corals (after Haeckel) 475
Dead - Man's Fingers (Alcyonium) (after

Cuvier and Hornell) - 477

A Hydroid Zoophyte (Obelia) (after Hornell) 479

A Compound Jelly-Fish (Sarsia) • 481

Development of Aurelia - - - 482

Lucernaria ... 482

Cydippe (after Chun) - 483

Sponge Spicules - 485

Bath -Sponge (Euspongia} • 486
Group of Calcareous Sponges - - -487

Protozoa (after various authors) - - 489

Protomyxa (after Haeckel) - - - 497

CHAPTER VI

STRUCTURE AND CLASSIFICATION OF FISHES AND
PRIMITIVE VERTEBRATES

The 7300 odd species of existing fishes known to science
are divided into the following five sub-classes: —

I. Lung-Fishes (Dipnoi).

II. Bony Fishes (Teleostomi).

III. Sharks and Rays (Elasmobranchii).

IV. Chimaeras (Holocephali).

V. Round-Mouths (Cyclostomata).

Probably the best idea of the general structure of the group
will be obtained by briefly describing such a common example
of the Shark kind (Elasmobranchs) as the Spotted Dog-Fish
(Scyllium canicula), abundant on our shores, especially during
the herring season. It may to all intents and purposes be
regarded as a small shark.

External Characters (fig. 166). — The spindle-shaped body is
well suited for progression through the water, and its outline is con-
tinuous, there being no sharp boundary between head and trunk,
or trunk and tail. We have seen that in tadpoles and some adult
Amphibia (see p. 246) there is a membranous fringe bordering
the tail above and below and running forwards on the upper side
of the trunk. Such a longitudinal fringe running in the middle
line is known as an unpaired fin, and is especially characteristic
of Fishes, where, however, it is not a mere soft membrane as in
Amphibia, but is supported by firm rod-like structures, the Jin-
rays. We find that in the Dog-Fish, as in most fishes, this
membrane is not continuous, but is represented by a number of
separate pieces, each of which is named with reference to its
position. Here, for example, there is a caudal fin bordering the
tail, two dorsal fins in front of this above, and an anal or ventral
fin in front of it on the under side. Special attention may be called

VOL. I. 257 17

258 "XJrfAR'ACTfcitS'bF VERTEBRATE ANIMALS

to the tail-fin, which is unsymmetrical, consisting of a large upper
lobe into which the slender end of the body is continued, and a
smaller lower lobe. Such unequal or heterocercal (Gk. heteros,
diverse; kerkos, tail) tails are shown by reference to fossil forms
to be of very ancient type. There is good reason to believe that
remote fish-ancestors had a continuous unpaired fin, of which the
existing ones are fragments which have been retained and
enlarged to suit special purposes. The Dog- Fish, however, also
possesses paired fins, consisting of two large pectorals in front
and two smaller pelvics placed close together farther back. These
are the equivalents of the fore- and hind-limbs of the terrestrial
vertebrates so far described, but differ in important respects in
accordance with differences in use. The limbs of a Salamander,
for instance, have to support the weight of the body and are the
means of progression. The presence of digits is of obvious
advantage as regards the former, while locomotion would be
difficult and awkward were the limbs not transversely divided
into regions capable of being moved upon one another. But the
limbs of an ordinary fish do not support the body, and their chief
use seems to be that of steadying it in the water and directing
its movements. For these purposes the undivided paddle-like
shape which we associate with the paired fins of a fish appears
best adapted.

The large curved mouth is situated on the under side of the
head, and not far in front of it are the rounded nostrils, each of
which is connected by a groove with the corresponding corner
of the mouth. Far back on the under surface of the body, and
marking the junction of trunk and tail, is a rounded cloacal
aperture situated between the pelvic fins. On each side of this
opening is a small aperture known as an abdominal pore, of
unknown use, but commonly found in more than one group of
fishes. The oblique cat-like eyes are provided with imperfectly
movable upper and lower eyelids. A tadpole, it will be remem-
bered, has four gill-slits on each side of the throat, the cavity
of which is thus placed in communication with the exterior. Here
there are five gill-slits, and also a superseded gill-slit known as
the spiracular cleft and opening behind the eye by a small round
hole, the spiracle.

The skin is of a brownish hue, much darker above than
below, and marked with good-sized roundish spots. Projecting

FISHES

259

from the surface are the sharp points of innumerable small hard
structures, closely resembling teeth in structure and usually known
as placoid scales. The presence of these causes a peculiar rough-
ness suggestive of sand - paper. Numerous sense organs are
present in the skin, some of which will be alluded to farther on.

The first point of general interest to note with regard to
the internal skeleton (fig. 159) is that it is entirely made up of

HYOMANDIBULAR

Fig. 159.— Skeleton of Dog-Fish (Scyllium canicula)

A, Skull and part of vertebral column. B, End view of a caudal vertebra, and longitudinal section through centra of
two vertebrae, c, Pectoral fin (from below). D, Pelvic fin (from below).

cartilage and fibrous tissue, as generally in what are called
" cartilaginous fishes", while in ordinary " bony fishes ", including
all the common edible forms, a great deal of bone is present as
well as more or less cartilage.

The skull is extremely simple compared with that of the
higher animals so far considered, and consists of a brain-case
to which are attached protective capsules for nose and internal
ear, the framework of the jaws, and what is known as the

26o CHARACTERS OF VERTEBRATE ANIMALS

"visceral skeleton". Regarding the last two a little more may
be said, as they have much to do with the visceral arches and
clefts which are so characteristic of Vertebrates, and have been
so often alluded to in the foregoing pages (see pp. 62 and 242).

Examination of a young embryo of the Dog- Fish will show
that on each side of the head there are seven oblique bar-like
thickenings and six slits occupying the interspaces between them.
These are respectively termed visceral arches and clefts, the
hindermost five having the special name of gill arches and clefts
because the gills are developed in connection with them. The
first arch is the mandibular arch and the second the hyoid arch,
while the slit between them is naturally called the hyo -mandibular
cleft and is no other than the spiracular cleft of the adult. These
various arches are traversed and supported by firm jointed rods
forming part of the internal skeleton. The mandibular arch is
so called because it becomes the mandible or lower jaw, while
the upper jaw is a forward outgrowth from it. The skeleton of
the rest of the arches constitutes the visceral skeleton, which
stiffens the gill region and prevents the gill-slits from becoming
closed, giving also firm points of attachment to many muscles.
A special function is performed by the upper joint of the skeleton
of the second or hyoid arch, this being a stout piece of cartilage
(hyomandibular cartilage) which slings the jaws to the main
skull. In Vertebrates higher than Fishes this cartilage loses its
original function, for the jaws are directly attached to the skull,
and most probably some or all of the little ear-bones, which help
to conduct sound-waves across the drum of the ear in such
higher Vertebrates, correspond to this cartilage. This is one of
the best examples known of a change of function. The rest of
the visceral skeleton, here so important in connection with the
gill -clefts, dwindles in air-breathing Vertebrates to the "hyoid
apparatus", which has been so often mentioned (see pp. 29, 193,
and 239) as supporting the base of the tongue in these forms,
and to the cartilages which support the voice-box or larynx. In
the life-history of the frog the passage from a well-developed
visceral skeleton in the gill-bearing tadpole to such remnants in
the adult lung-possessing animal can be traced step by step.

In the vertebral column it is only possible to distinguish
between trunk-vertebrae and tail-vertebrae, and all these possess
biconcave centra, which may be regarded as the most primitive

FISHES 261

kind. Short ribs can be distinguished in the trunk, but there is
no trace of any sternum.

The skeleton of the paired fins exhibits many differences
from the supporting parts of the limbs of terrestrial Vertebrates,
and comparison between the two is extremely difficult. In the
pectoral fin there is a very simple shoulder-girdle, consisting of
a curved piece of cartilage running transversely and fusing with
its fellow in the middle line below. At the base of the free part
of the fin are three cartilages, followed by a number of others,
and these again by jointed fin-rays. In the pelvic fins the two
hip-girdles are represented by a simple transverse bar, while the
free fin is supported by a stout rod bearing a large number of
fin rays.

Digestive Organs (fig. 160). — The jaws are bordered by
numerous rows of small pointed teeth, all much alike, and
replaced by fresh ones during life as often as necessitated by
wear and tear. These teeth are not suited for chewing, but
for seizing such prey as small fish, crustaceans, &c., and after-
wards preventing their escape. Around the margins of the
mouth we find all gradations between ordinary placoid scales
and teeth, which is intelligible when we recollect that the cavity
of the mouth is developed as a pit on the surface of the body.
The lining of such a pit, or inpushing of the general surface,
so to speak, is evidently equivalent to skin, and teeth here and
elsewhere are simply more or less modified scales, developed
within the margins of the mouth. The tongue is merely an
immobile fold on the floor of the mouth. The nasal organs do
not possess internal nostrils as in the lunged vertebrates. The
cavity of the mouth passes behind into a wide pharynx, out of
which the spiracular cleft and gill -clefts open; and then follow
gullet, U-shaped stomach, and intestine opening into a cloaca.
There is no clear distinction, as in forms so far considered,
between small and large intestine. Within this intestinal part
of the gut is a so-called spiral valve, which is simply a pro-
jecting shelf winding round and round and presenting a large
surface for the absorption of digested food. A large liver pours
bile into the beginning of the intestine, and there is also a pancreas
opening not far from it.

Circulatory Organs (fig. 160). — Here we have the same
conditions as in the Tadpole before the lungs begin to be of

262

CHARACTERS OF VERTEBRATE ANIMALS

use, and the conditions are consequently comparatively simple,
the problem of separating two kinds of blood not having yet
arisen. The heart consists of a thin-walled venous sinus, which
receives the impure blood of the body and passes it on to a
single auricle, by which it is squeezed into a thick-walled
ventricle, continued again into a muscular tube, the arterial
cone. Valves to prevent the blood from running back the
wrong way are placed at the points of junction between sinus,

1ST. DORSAL FIN

MOUTH

VENTRAL AORTA

OONU8 ARTERIOSUS

HEARTS AURICLE

VENTRICLE

SINUS VENOSUS

Fig. 160.— Side-dissection of Dog- Fish (Scyllium canicula]. Left half of liver has been removed
i, Internal opening of spiracular cleft. 2, 3, 4, 5, 6, Internal openings of gill-clefts.

auricle, and ventricle, and several transverse rows of valves are
present in the arterial cone. From the ventral aorta, which
runs forward from the cone, five pairs of afferent gill-arteries
run out and carry impure blood to the gills, from which five
pairs of efferent gill-arteries conduct the purified blood, uniting
above to form the dorsal aorta that runs back to the end of
the tail, giving off numerous vessels by which the various organs
and regions receive their pure blood -supply. Each afferent
artery, with its corresponding efferent one, may be looked upon
as constituting an aortic arch (see p. 242).

Breathing- organs (fig. 160). — Upon the walls of the five
pairs of gill-slits, or pouches, to speak more correctly, numerous
closely-set gill-folds are placed, and in these folds there are
very numerous capillary blood-vessels, with which the gill-
arteries are in communication. Water is constantly taken in

FISHES 263

at the mouth, and, passing back, streams outwards through the
gill -pouches — thus passing over these folds, which collectively
present a large surface over which the oxygen dissolved in the
water can diffuse into the blood, while the waste carbon dioxide
of the blood can diffuse outward into the water and be carried
away to the exterior.

In this connection great interest attaches to the spiracular
cleft, which looks very much like a narrow gill-pouch, .and on
close examination is seen to have a number of small folds on its
front wall. These are so gill-like that they have been collectively
called a false gill (pseudobranch), and when it is added that some
few fishes possess a properly-developed set of gill-folds in this
place, it will be clearly seen that the spiracular cleft is really a
gill-cleft which is losing its function. There is every reason
to believe that the cavity of the middle ear in higher Verte-
brates, together with the Eustachian tube, is equivalent to the
spiracular cleft. Here, then, is a structure which once had to
do with breathing and is now concerned with sound-conduction —
another excellent example of change of function.

Nervous System and Sense Organs (fig. 160). — The brain is
pretty well developed, a peculiar feature being that the two
cerebral hemispheres are represented by an unpaired swelling,
while the olfactory lobes are placed on stalks. The cerebellum
is much larger than in Amphibia.

The eyeball is flattened on the outside and its crystalline
lens is approximately spherical, as in aquatic animals generally.
Probably everyone has noticed, some time or other, the lens in
the eye of a cooked bony fish, such as salmon or herring,
looking, as it does, like a sugar-coated pill. Needless to say,
the opacity is the result of cooking. Here, too, the lens is
spheroidal. The organs of hearing consist simply of the internal
ear or membranous labyrinth, contained in a gristly capsule on
either side of the back end of the brain-case. In shape the
labyrinth is somewhat simpler than in the Amphibia. The skin
contains a large number of sense-organs, some of which are
sunk in a tube which runs along each side of the body and
opens to the exterior at intervals. An external streak, the
lateral line, marks the position of either tube, but this is much
better seen in a bony fish than in the Dog-Fish. There are
also peculiar jelly-tubes which open by regularly arranged pores

264 CHARACTERS OF VERTEBRATE ANIMALS

on the under side of the head, and undoubtedly have to do with
sensation. Little is known of the use of these organs, but they
no doubt respond to certain vibrations in the surrounding water.
It is important to avoid the common error, made when dealing
with the sense-organs and sensations of lower animals, of trying
to explain them by reference to ourselves.

Development. — The Dog- Fish is developed from an egg, which
looks almost like the " yolk " of a fair-sized bird's egg were it not
for its greenish tint. This egg is enclosed in a horny " purse ",
the corners of which are drawn out into tendril-like threads which
curl round sea- weeds and other firm objects. *

We may now proceed to the consideration of the chief groups
of Fishes.

SUB-CLASS I.— LUNG-FISHES (DIPNOI)

Of all known fishes these come nearest to the Amphibia, and
some zoologists place them in a class of their own, distinct from
that containing more ordinary fishes. They are a very ancient
group, now represented by only three genera, all of which are
found in fresh water. They are the Barramunda or Burnett
Salmon (Ceratodus] from the Burnett and Mary rivers in
Queensland; the African Mud- Fish {Protopterus\ native to
some of the rivers of tropical Africa; and the South American
Mud- Fish (Lepidosircn), found in the Amazon and upper part
of the Paraguay rivers and their tributaries (fig. 161).

There are certain characters common to all three forms.
Taking first the external characters, the head is somewhat
amphibian-like, and has been compared to that of a salamander,
while the tail tapers considerably and is symmetrically margined
by a fin which is not expanded into lobes (protocercal or diphy-
cercal caudal fin). There is no spiracle, but gill-clefts are present,
protected by a flap or gill-cover. The paired fins are much
elongated, and the body is covered by thin overlapping scales.

The most characteristic feature of the internal organs is the
presence of one or two bag-like lungs opening on the under
side of the pharynx, so that these animals possess, like some
adult Amphibia, both gills and lungs at the same time, and it
is from this circumstance that they derive the name of Dipnoi
(Gk. (Us, twice; pnoe, breath).

FISHES

265

Since the heart receives pure blood from the lung or lungs,
as well as impure blood from the general body, the problem of
separating the two kinds demands solution; but this is only
effected in an imperfect manner, for there is not even, as in
Amphibia (see p. 240), a complete separation of the two auricles
which are here present.

These are the only fishes in which there are internal nostrils
as well as external ones. They open just within the margin of

Fig. 161. — Dipnoi
A, Australian Lung- Fish (Ceratodus], xf B, African Lung- Fish [Protopterus], Xj.

the upper lip. The mouth is provided with large dental plates
of peculiar form.

Ceratodus is a large broad fish, attaining the length of 4 or
5 feet, and with its body covered by very large scales. The
paired fins are broad and paddle-shaped, and each of them is
supported by a central axis made up of numerous joints, and of
a series of fin-rays attached to the axis in front and behind in
a feather-like way. Protopterus is much more slender in form
and its paired fins are extremely narrow, the fin-rays on the
central axis being largely suppressed. Some specimens are said
to reach a length of 6 feet. There are small external gills as
well as the gill-folds on the walls of the gill-clefts. Lepidosiren
is an eel-like form which may be as much as 6 feet long. Its

266 CHARACTERS OF VERTEBRATE ANIMALS

paired fins are even narrower than those of Protopterus, and
supported only by a central jointed axis.

SUB-CLASS II.— BONY FISHES (TELEOSTOMI)

This large group of fishes includes a vast number of recent
and fossil forms which present such varying characters that it
is difficult to give a satisfactory definition. There is, however,
a good deal of bone in the skeleton, and the jaws, instead of
being mere bars of gristle, as in a Dog- Fish, are ensheathed by
tooth-bearing bones in the same way as in higher forms. The
gill-clefts are very near together, and the gill-arches between
them are comparatively narrow, so that the gill-folds as seen
in a Dog- Fish are not supported along the whole of their length,
but project more or less to the exterior as free gill filaments.
The gill-clefts are covered over and protected by a flap, the
gill-cover or operculum, which is supported by bones. Two
orders may be distinguished: — i. Ganoids, and 2. Bony Fishes
proper or Teleosts.

Order i. — GANOIDS (Ganoidei)

Under this name are included a number of recent genera,
widely scattered over the globe, and for the most part limited
to fresh water (fig. 162). They are the last surviving remnants
of groups which were once of great importance, but which have
been unable to compete with more highly -organized fishes, and
have greatly declined in consequence. Of recent forms the
two most primitive are the Bichir (Polypterus) of the Nile and
some other African rivers, and the Reed- Fish (Calamoichthys)
from the rivers of Old Calabar. The former is a remarkable-
looking creature of respectable size, being as much as 4 feet
long. Running along the back are a considerable number of
little dorsal fins, each with a strong spine in front, while the
last of them abuts against the rounded protocercal fin of the
tail, close to which, on the under side, is an anal fin. The
paired fins consist of a thickened basal part, supported by
cartilages something like those described for Dog- Fish (see p. 261),
and fringed by a thinner region supported by radiating fin-rays.
The body is covered by thick lozenge-shaped bony plates (ganoid

Fig. 162.— Ganoids

A, Bichir (Pofypterus), xj. B, Reed-Fish (Calamoichthys], xf c, Gar-Pike (Lefiidosteus}, xf D, Common
Sturgeon (Acipenser), X ^. E, Shovel-nose Sturgeon (Scaphirhynchus), xf F, Slender-beaked Sturgeon
(Psephurus), Xi G, Spoonbill Sturgeon (Spatularia or Polyodon], xf. H, Bow- Fin (^wi«), X A.

267

268 CHARACTERS OF VERTEBRATE ANIMALS

scales), united firmly by their edges and having a very regular
arrangement in oblique rows, while the head is covered by paired
bony plates.

The Reed-Fish is what a small Bichir might be imagined to
become if it were pulled out into an eel-like form and lost its
pelvic fins.

Both these forms are distinguished as "Fringe-finned" Ganoids
on account of the structure of the paired fins ; while the remaining
recent Ganoids are " Ray-finned ", i.e. the paired fins have lost
the thickened basal portion and consist of a fan-like expansion
strengthened by the numerous diverging fin-rays, which may be
compared to the sticks of the fan. These ray-finned forms include
the Gar- Pikes, Sturgeons, and Bow- Fins of the present day.

The Gar- Pike (Lepidosteus) is common in the fresh waters
of North America, and may be as much as 6 feet long. Covered
with firm armour like the Bichir, it differs not only in the structure
of the paired fins, but also in the presence of a single dorsal only,
while the head is drawn out into long but powerful jaws.

Sturgeons are large fishes which have lost more or less of the
dermal armour, have a much elongated snout, and an unsym-
metrical tail like that of the Dog- Fish (see p. 258). The mouth
is on the under side of the body, at the base of the snout. The
Common Sturgeon (Acipenser) includes some twenty species, of
which the largest, a Russian form, is as much as 30 feet in length.
It is distinguished by its broad pointed snout, on the under side
of which are four sensitive thread-like structures (barbels), and the
presence of rows of broad keeled plates in the skin. Sturgeons
of this kind are widely distributed through the fresh waters and
along the coasts of the Northern Hemisphere. One species
(Acipenser sturio] is British, and may occasionally be se<tn in
fishmongers' shops.

The Shovel -nose Sturgeon (Scaphirhynchus], native to the
Mississippi and the rivers of Central Asia, is of more elongated
form than the common form, and its dermal armour is more
complete, especially in the hinder part of the body.

The two remaining kinds of Sturgeon both have teeth, while
the sorts so far mentioned are toothless; and they are further
distinguished by enormous snouts and the complete, or almost
complete, absence of armour. One, the Slender-beaked Sturgeon
(Psephurus) of the Yang-tse-Kiang and Hoang-ho rivers has a

FISHES 269

conical snout, and the other a broad flattened one, as is indicated
by the name of Paddle- Fish or Spoon-bill Sturgeon (Spatularia).
It is found in the Mississippi.

The last remaining recent Ganoid, the Bow -Fin (Amia\
resembles the ordinary bony fishes in many respects, and was
for long confused with them. Its body is covered by thin over-
lapping flexible scales. In distribution it corresponds to the
Gar- Pike.

One structural feature in which all these Ganoids differ from
the Dog- Fish is the possession of an elongated air-containing sac,
the swim -bladder or air-bladder •, which opens into the pharynx,
usually on the upper side. In the Bichir, however, the air-
bladder, like the lungs of terrestrial Vertebrates, is paired, and
opens into the under side of that region. The original use of
the swim-bladder is apparently that of a balancing organ, but
in some Ganoids (Bichir, Gar-Pike, Bow- Fin) it assists in
breathing, and in the Dipnoi has been converted into a lung.
It is highly probable that the lungs of the higher Vertebrates
are simply to be regarded as modified swim-bladders, and if so,
an extremely interesting example of change of function is afforded.

Order 2. — ORDINARY BONY FISHES (Teleostei)

Here are included the ordinary bony fishes of the present
day, which include some three thousand existing species. The
most important features of the group may be understood by
briefly describing an average form, such as the common fresh-
water Perch, and noting the differences between it and the
Dog- Fish (see pp. 257-264).

The Common Perch (Perca fluviatilis) (fig. 163) is widely
distributed through the fresh waters of Europe and North Asia,
and is very beautifully coloured. The upper parts are of a warm
reddish-brown, passing into golden on the sides and again into
white below. Several broad dark bands run across the body,
tapering off on the sides. Well-grown specimens in this country
attain a length of 9 or 10 inches, or even a foot, but much
larger specimens have been recorded.

The body is much flattened from side to side and the outline
is spindle-shaped. It has been compared to a rounded wedge,
eminently adapted for rapid progression through the water by

270

CHARACTERS OF VERTEBRATE ANIMALS

means of the powerful lateral strokes of the large tail-fin, while
the remaining fins act as balancers and steerers. The fairly
large mouth, its jaws supported by bones bearing small teeth,
is placed at the end of the pointed head, and above it may be
seen two external nostrils, one on either side of the snout.
Farther back still come the large, round, expressionless eyes,

DORSAL FINS

OLFACTORY LOBE
OLFACTORY
NERVE

LATERAL LINE

GILLS

VENTRAL AORTA
BULBUS ARTERIOSUS
ENTRICLE

INUS VENOSUS }— HEART

AURICLE

fYLORIC CAECA

Fig. 163. — Side-dissection of Perch (Percajluviatilis)

devoid of eyelids, but there are no spiracles behind them. No
gill-slits are externally visible, but these may be seen by lifting
up the firm gill-cover or operculum placed at the side of the
head as in a Ganoid. Four such slits are present, instead of
five as in the Dog- Fish (see p. 258), with four narrow bar-like
gill-arches bearing the red, comb-like gills, the separate teeth
of the combs being the gill-filaments. The gill-openings here
are really slit-like, and not pouches as in Dog- Fish, and their
existence is practically demonstrated by every school-boy who
strings his small catches on a piece of grass, which he threads
through the mouth and gill -slits under the gill-cover to the
exterior. It is further of great interest to notice that on the
inner side of the gill-cover, close to the first gill-slit, there is
a small reddish projection. This false gill (pseudobranch)
is the vestige of the first gill present in a Dog- Fish, and must
not be confused with the false gill of that animal (see p. 263),
which is on the front wall of the spiracular cleft, here entirely
absent.

The unpaired fins consist of two large dorsals, of which the

FISHES 271

first is supported by exceedingly sharp spines, of an externally
symmetrical (homocercal) caudal fin shaped like a fan, and an
anal fin, quite close to which is the external opening of the
intestine, for there is here no cloaca. Fan-shaped pectoral and
pelvic fins are present, of the rayed type found in living Ganoids
except the Bichir and Reed- Fish, and it may be noted that the
latter are placed very far forward, much more so than in many
other Teleosts. The lateral line, marking the position of im-
portant sense-organs (see p. 263), is indicated by a dark streak
running along either side of the body about the level of the
eye. The body is covered by thin overlapping scales.

The internal skeleton is complicated by the presence of a
large number of bones, and contrasts strongly with the com-
paratively simple cartilaginous skeleton in Dog- Fish (see p. 259).
The skull is particularly elaborate, and it must 'suffice to mention
once more the presence of tooth-bearing bones bordering the
jaws, which are attached at the back to the main skull by a
hyo-mandibular bone equivalent to the cartilage of the same
name in Dog-Fish (see p. 260). And as the Dog-Fish possesses
no operculum, it has nothing corresponding to the bones which
support that structure here.

The skeleton by which the base of the tail is supported
presents some points of great interest. It will be remembered
that in the Dog- Fish the tail is unsymmetrical (heterocercal), the
vertebral column bending upwards and running into its upper
lobe. Here, on the contrary, the expanded tail is externally
symmetrical (homocercal), and the backbone apparently stops
short at its base. Examination of certain very young Teleosts,
however, shows that in them the tail is unsymmetrical, the back-
bone bending up in the characteristic way, and close examination
of the apparently symmetrical tail of the adult shows that behind
the last joint of the backbone is a little bony rod which turns
upwards, and marks what is really the posterior end of the
body. The homocercal tail, therefore, is really a modification
of the old-fashioned heterocercal one, and may be looked upon
as an improved type which has arisen from it.

Digestive Organs (fig. 163). — Small pointed teeth are present
on the roof of the mouth-cavity as well as on the margins of
the jaws, and the usual regions of the gut are present, i.e.
mouth -cavity itself, pharynx out of which the gill -slits open,

272 CHARACTERS OF VERTEBRATE ANIMALS

gullet, stomach, and intestine, the last not opening into a
cloaca as in Dog- Fish, nor provided with a spiral valve (see
p. 261). A large liver is present, but the pancreas appears to
be represented physiologically by some blindly -ending tubes
(pyloric cceca] which open into the beginning of the small
intestine.

The Circulatory Organs, broadly speaking, are constructed
on the same type as in Dog-Fish (see p. 261), though important
differences are seen in the heart, which no longer possesses
an arterial cone with numerous rows of valves, but presents a
new structure, the arterial bulb (fig. 163),* which succeeds the
ventricle, and is really the swollen beginning of the ventral
aorta, which, as before, runs forwards on the floor of the gill-
region, and gives off the afferent branchial arteries that carry the
impure blood to the gills to be purified.

The only organs of respiration are the gills, of which enough
has been said to give an idea of their arrangement.

Nervous System and Sense Organs (fig. 163). — The brain
presents the same regions as in a Dog-Fish (see p. 263), but is
shorter and broader. The olfactory lobes are not placed on
stalks, and there are two cerebral hemispheres instead of an
unpaired projection, while the cerebellum is smooth and tongue-
shaped.

Two obvious points as regards the sense-organs may be
mentioned. One is that the nasal sacs have no internal openings
into the mouth-cavity as in Dipnoi, but are distinguished by the
possession of double external nostrils; while the other peculiarity
is a negative one and consists in the absence of the numerous
jelly-tubes so characteristic of the Dog-Fish (see p. 263).

A membranous swim-bladder (fig. 163) is present, situated
above the other internal organs, close to the under surface of
the backbone, but though it contains air, it has lost in the adult
all connection with the gullet, and therefore cannot play even a
subsidiary part in respiration.

Little more can be done here than briefly mention some of
the more important or interesting types of bony fishes, but even
so, it will make things clearer to indicate the chief groups, which
are as follows: —

A. — Teleosts in which the swim-bladder when present has
lost its connection with the gullet.

FISHES 273

Sub-order i. — Spine-firmed Fishes (Acanthopterygii).
Sub-order 2. — Tuft-gilled Fishes (Lophobranchii).
Sub-order 3. — Firm-jawed Fishes (Plectognathi).
Sub-order 4. — Soft-firmed Fishes (Anacanthini).

B. — Teleosts in which the swim-bladder retains its connection
with the gullet by a pneumatic duct or tube.

Sub-order 5. — Tube-bladdered Fishes (Physostomi).

Sub-order i. — SPINE-FINNED FISHES (Acanthopterygii)

Here as in the Perch, which is a type of the sub-order, some
or all of the fin-rays which support the dorsal, anal, and pelvic
fins are sharp unjointed spines. Some 3650 different species
are here included, divided into about 60 families, of which only
a few can be mentioned.

1. Perch Family (fig. 163). — This is a group of carnivorous
fishes, almost entirely confined to fresh water and distributed
through the northern hemisphere.

2. Bass Family. — This is a large group of fishes, which are
for the most part marine though some inhabit fresh water, and
which have a very wide distribution. A well-known British
species is the Common Bass (Morone labrax), which is much
like the Perch, but more slender in build and not so brilliantly
coloured, being bluish-grey above, gradually shading off to white
on the under surface.

3. Sword-Fish Family. - - The members of this family are
large fishes, mostly found in the open sea, and deriving their
name from the formidable sword-like weapon into which the
upper jaw is produced. The Common Sword- Fish (Xiphias
gladius) is sometimes taken in British seas, and occurs on both
sides of the Atlantic. Other species are found in the Atlantic
and Pacific. These fishes have become notorious from the fact
that their swords have been found broken off in the timbers of
ships, penetrating to a depth of as much as 22 inches (in a
specimen exhibited in the British Museum). The force with
which such blows have been given may be imagined.

4. Dory Family. - - This includes marine fishes of extra-
ordinary form, found in the East Atlantic, Mediterranean, and
the seas of Australia and New Zealand. The John Dory (Zeus
faber) is not uncommonly found in British seas. The body

is much flattened, and with the head has a broad oval outline.

VOL. I. 18

274 CHARACTERS OF VERTEBRATE ANIMALS

From behind each of the sharp spines of the large first dorsal
fin projects a long filament. A small part of the upper side
is dark -brown, which shades into golden -yellow farther down,
and again into brownish white. On each side of the body there
is a large round blotch, dark in colour with a lighter margin.
By the ancient Romans it was regarded as sacred to Neptune.

5. Mackerel Family. - - This is an important group of car-
nivorous food-fishes, abundant in the temperate and tropical seas
of both hemispheres. They are remarkable for their powers of
swimming. The Common Mackerel (Scomber scomber) suggests
the lines of a racing-boat in the beautifully graduated curves of
its rather slender body, terminated behind by a well-developed
caudal fin shaped like the head of a broad arrow. Rows of
pointed finlets fringe the body in the spaces between the tail
and the anal and second dorsal fins.

Another member of the family, sometimes taken off the
south of England, and the object of an important Mediterranean
fishery, is the Tunny (Thynnus thynnus], which may attain the
length of 10 feet and a weight of more than 1000 pounds.
Smaller species of the same genus are the Bonito (Thynnus
pelamys] and Albicore (Thynnus albicora), which prey largely on
Flying- Fishes, and, being rendered conspicuous by this habit, are
often mentioned in accounts of voyages.

6. Angler- Fish Family. — This embraces widely-distributed
fishes of extraordinary form, some of which frequent shallow
water, while others drift with sea-weed in the open ocean, and
still others live at great depths. A well-known British species
is the Fishing- Frog {Lophius piscatorius), with its body dwarfed
by an enormous head, the wide gaping mouth of which, armed
with numerous teeth, is a regular death-trap for unwary little
fishes of all sorts. The first dorsal is reduced to its long spines,
the foremost of which terminates in a soft flap. The upper side
is of a blackish brown and the under side white.

7. Bull-Head Family. - - These are small ground fishes of
wide distribution, and mostly found near to land, while there
are also fresh-water forms. A common British species is the
Bull-head or Miller's Thumb (Coitus gobio\ common in brooks,
as every school-boy knows; while just as common along our
shores is the Sea-Scorpion (Coitus scorpius], which has much the
same appearance.

FISHES 275

8. The Gurnards are larger relatives of the preceding and
are distinguished by their curiously -shaped bony heads, and by
the fact that the first three rays of the pectoral fin have become
distinct and form " fingers " used as organs of touch and in
progression along the sea - bottom. They are fishes of wide
distribution and often of bright coloration. The commonest
British species is the Red Gurnard (Trigla pini), which is bright
red in colour above and gleaming white below and on the sides.

9. Goby Family. — Gobies and their allies are small fishes
common along the coasts of both tropical and temperate seas,
while some are estuarine and a few inhabit fresh water. There
are nearly a dozen British species, of which the largest and
probably on that account best known is the Rock Goby (Gobius
niger\ In these fishes the pelvic fins are united into a funnel-
shaped sucker.

10. Blenny Family. --'The fishes of this family are mostly
small and have the same wide distribution as the Gobies. The
pelvic fins are very much reduced, and there is a single dorsal
running along the whole length of the back, and equivalent to
the two dorsals of perch fused together. A common British
species is the Smooth Blenny or Shanny (Blennius pholis], in
which the long, low dorsal fin is not broken into sections. The
Wolf- Fish (Anarrhichas lupus], common on the north British
coasts and ranging to Norway and Greenland, may be over
6 feet long, and resembles the Shanny on a large scale. The
margins of its jaws and the roof of the mouth are studded with
blunt teeth, adapted for breaking the coverings of molluscs and
Crustacea.

11. Grey Mullet Family. — Grey Mullets are common along
the coasts of tropical and temperate regions, frequenting estuaries.
The first dorsal fin is small, with only four spines, and the lateral
line is absent. The best-known British species is the Common
Grey Mullet (Mugil capita].

12. Mackerel-Pike Family. — Here are included Gar-Fishes
and Flying- Fishes. The former are distributed throughout tropical
and temperate seas, and the Common Gar- Fish (B clone vulgaris]
is common on the British coasts. The body is much elongated
and the snout is lengthened into a narrow pointed beak, well
suited for seizing small fish. Flying-Fishes include a considerable
number of species limited to the warmer parts of the ocean. The

276 CHARACTERS OF VERTEBRATE ANIMALS

Common Flying- Fish (Exoccetus volitans), found in all tropical
seas, has immensely elongated pectoral fins, which constitute the
so-called wings.

13. Stickleback Family. — The familiar little fishes consti-
tuting this family are mainly, but not entirely, inhabitants of
fresh water, and are only found in the Arctic and North
Temperate zones. The name is derived from the fact that the
first dorsal fin is represented only by its spines, of which there
are a variable number. There are three common British species,
all of them nest-builders. Of these the largest is the Sea Stickle-
back (Gastrosteus spinachia), with fifteen spines. It also frequents
brackish water. The other two kinds live in fresh or brackish
water, the smaller being the Ten-spined Stickleback (Gastrosteus
pungitius] or ''Tinker", while the other is the Three-spined

Stickleback (Gastrosteus aculeatus\

14. Wrasse Family. — Wrasses are widely -distributed shore
fishes, being absent, however, from the polar regions, and are
especially numerous on rocky coasts and coral reefs. Most of
them are handsomely coloured, and some are pre-eminent among
fishes in this respect. The bones which constitute the bases of
the skeleton supporting the gill-arches (lower pharyngeal bones)
are fused together into a single tooth - bearing piece. Many
members of the group are distinguished by the possession of
thickened lips, and all have blunt conical teeth suitable for
crushing the shells of molluscs and Crustacea, of which the food
consists. There are a number of British species, of which one
of the commonest is the Ballan Wrasse (Labrus macula tus), a
stoutly-built fish of some 15 inches long. The colour is of a
bright brown with numerous whitish spots, and touches of green
on the fore-part of the head and the bases of the fins. Some
individuals, however, are of a greenish colour all over. The
spiny first dorsal is long and low, while the soft second dorsal
is much shorter and higher.

Sub-order 2. — TUFT-GILLED FISHES (Lophobranchii)

This small sub-order includes about 120 species of curiously
modified fishes, in which pelvic fins usually, and anal and caudal
fins commonly, have disappeared. The snout is drawn out into
a tube, at the end of which the small rounded toothless mouth

WRASSES (Labridce)

Wrasses are tropical and temperate shore fishes, possessed of
thick lips and crushing teeth well suited to the food, which consists
of molluscs. Many of them are beautifully coloured. All four of
the species figured are native to the seas of Britain. Their names
are: —

1. Rainbow Wrasse (Coris julis}.

2. Ballan Wrasse (Labrus maculatiis).

3. Cuckoo Wrasse (L. mixtus\ male.

4. Corkwing (Crenilabrus melops}.

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FISHES 277

is situated. Scales are absent, but their place is taken by bony
plates developed in the deeper part of the skin and arranged in
regular transverse rings. The name of the group (Gk. lophos,
a tuft; branchia, gills) has reference to the gill-filaments, which
are arranged in tufts and not in combs as in ordinary Teleosts.
The male fish usually has a pouch in the skin on the under side
of the body in which the eggs are developed and the young
protected.

The Great Pipe- Fish (Syngnathus acus] is
a common British species, with much-elongated
cylindrical body and a small caudal fin. A
much more extraordinary looking species is the
Short-snouted Sea- Horse (Hippocampus anti-
quorum) (fig. 1 64), which has a very wide range
and is sometimes found off the British coast.
The name is suggested by the shape of the
head, which is sharply bent on the trunk and
separated from it by a sort of neck. The
animal maintains a vertical position in the Fig. i64.-sea-Horse (#/#*•
water, both when swimming and also when
attached to sea-weed by means of its prehensile tail, which is
devoid of a caudal fin.

Sub-order 3. — FIRM-JAWED FISHES (Plectognathi)

This also is a small group, embodying about 180 species,
most of which have a well-developed external skeleton. There
is as a rule nothing to correspond to the spiny first dorsal and
pelvic fins of a perch, except, perhaps, a few spines. The gill-
cover is not a large free flap, but is united with the surrounding
parts so as to leave only a small aperture through which the
water which has passed over the gills can flow out to the exterior.
The internal skeleton is deficient in bony matter, but the bones
of the upper jaw are very firm and fused to the main mass of
the skull, a feature which the name of the sub-order suggests
{Gk. plektos, woven together; gnathos, jaw). The included species
are characteristic of tropical seas, though not confined to them,
and include: i. File-Fishes; 2. Coffer- Fishes; 3. Globe-Fishes;
and 4. Sun-Fishes.

i. File- Fishes constitute a widely distributed family, in which

278 CHARACTERS OF VERTEBRATE ANIMALS

the skin is rough and there is a small spiny first dorsal fin, of
which the first spine is ridged like a file on its front surface.
The firm jaws, provided with strong teeth, are well adapted for
breaking open the shells of molluscs, or nipping off pieces of
coral. The Mediterranean File- Fish (Batistes capriscus] has
occasionally been taken in British seas.

2. Coffer-Fishes are curious-looking creatures in which the
body is almost entirely protected by numerous six-sided plates
united by their edges. The Four-horned Coffer- Fish (Ostracion
quadricornis] has been taken off the coast of Cornwall.

3. Globe-Fishes mostly have their bodies covered with strong
spines, and possess the power of dilating the gullet with air,
when the body swells out into a globe-like form and the spines
are erected, furnishing a formidable protection. In this condition
they are unable to swim, but are drifted along with the under
side turned upwards. In the genus Diodon there is a bony plate
in the front of each jaw, while in the allied genus Tetrodon each
of these plates is divided into two, giving the appearance of four
large front teeth.

4. Sun- Fishes are remarkable for the shortness and depth
of the body, the caudal fin forming a border to the hinder end,
which does not taper as in an ordinary fish. Adjoining the
caudal fin is a long pointed dorsal above and a similar anal below.

Sub-order 4. — SOFT-FINNED FISHES (Anacanthini)

In these fishes the rays which support the various fins are
all soft and jointed, and the pelvic fins are situated very far
forwards. There are some 370 species, of which many are of
great economic importance. Only two of the included families
need be mentioned here, i.e.: i. the Cod Family, and 2. that in
which the Flat- Fishes are included.

i. Cod Family. — The Common Cod (Gadus morrkua) (fig.
i) is the most important representative of a genus distinguished
by the possession of three dorsal and two anal fins, while in
this and some of the other species of the genus there is a filament
or barbel attached to the lower jaw. The Cod abounds on
both sides of the northern part of the North Atlantic, the most
famous fishery being on the banks of Newfoundland. The
Haddock (Gadus czglefinus] is another important species with a

FISHES 279

similar range. It can easily be distinguished from the Cod by
the black colour of the lateral line, and the presence of a rounded
black patch just behind the gill cover. A third allied but smaller
species is the Whiting (Gadus merlangus\ the range of which
is restricted to the seas of Northern Europe. It has a dark
patch at the root of each pectoral fin and lacks the barbel of the
two preceding species.

A small number of the species included in the Cod Family
are fresh-water, and the best known of these is the Burbot or
Eel-pout (Lota vulgaris), abundant in many of the rivers of
North America and North and Central Europe. In England
it is found in some of the rivers which flow into the North Sea.
There are two dorsal fins, of which the second is very long and
low, while on the under side of the body there is an anal fin of
similar character. A barbel is present as in the Cod.

2. Fiat-Fish Family. — The familiar food-fishes which make
up this family are distinguished by a remarkable want of sym-
metry, which has no parallel among Vertebrates. One might at
first sight imagine that the dark and light surfaces of the body
were respectively upper and lower. A little closer inspection,
however, would show that a long dorsal fin ran along one edge
of the body and a long anal fin along the other; and further,
that each surface had a lateral line running along it, and possessed
both gill-cover and paired fins, thus conclusively proving the
broad surfaces to be the sides. In some cases the dark upper
side is the right one and in others the left. A flat-fish starts
life with the same kind of symmetry as an ordinary fish, one
eye being on the right and the other on the left, while the
body is maintained in the usual position; but as development
proceeds, and the body gets more flattened, one side becomes
pigmented, and the eye of the contrary side is displaced so as
to be near its fellow. We may take as well-known examples,
Turbot, Plaice, and Sole.

The Turbot (Rhombus maximus) (fig. 34) is a large broad
fish limited to European seas, and with the eyes on the left side.
In the Plaice (Pleuronectes platessa), which though broad is a
much smaller fish, the dark, eye-bearing surface is the right
side. It is marked by large orange spots. This species ranges
along the west coast of Europe and extends as far north as
Iceland. The Common Sole (Solea vulgaris) also has the eyes

28o CHARACTERS OF VERTEBRATE ANIMALS

on the right side, but its shape is an elongated oval. It is con-
fined to European seas.

Sub-order 5. — TUBE-BLADDERED FISHES (Physostomi)

This large group (some 2500 species) includes many im-
portant food-fishes, and includes both marine and fresh-water
forms as well as forms which, like the Salmon, live partly in
the one and partly in the other. The fins are supported by soft
jointed rays, except, in some cases, the first ray in the dorsal and
pectoral, which may be transformed into spmes. The pelvic fins
are situated far back in what must be considered as the primitive
position, judging by other orders of fishes. The air-bladder,
which, as previously stated, is an outgrowth from the gut, always
retains a connection with it by means of a tube, whence the
name of the sub-order. Only the more important families can
be mentioned here.

1. Cat-Fish Family. — The Cat-Fishes or Siluroids include
a large number of species inhabiting the fresh waters of tropical
and temperate regions, while some of them are estuarine or
even marine, though in the latter case they do not go far from
shore. The name "Cat-Fish" has reference to the presence of
long barbels, suggesting to a lively imagination the "whiskers"
of a cat. The scaleless body is sometimes protected by bony
plates. There is only one European species, the Wels (Silurus
glanis) (fig. 165), and this is limited to the rivers east of the

Rhine. Excepting only the Giant Sturgeon, it is the largest
of European fresh-water fishes, attaining a maximum length of

13 feet and weight of 400 pounds, though average specimens
are very much smaller. In this species the skin is soft, and there
are six barbels, two very long ones above and four much smaller
ones below.

2. Salmon Family. — The members of this group are in the
main either purely fresh - water in habit, or ascend rivers to
spawn, and, with the exception of a New Zealand genus, are
confined to the northern hemisphere. There are, however, a
few purely marine species. The body is of the « typical fish-form,
and covered with scales, except in the head region. A fairly-
large first dorsal is situated about the middle of the back, and
much farther back there is a small second dorsal, which, on

FISHES

281

282 CHARACTERS OF VERTEBRATE ANIMALS

account of its fatty texture, is generally called the adipose fin.
An anal fin is placed opposite or nearly opposite this.

The Salmon (Salmo salar) (fig. 165) ranges right round the
northern hemisphere as far south as latitude 41° in the New
World and 43° in the old. It is well known as one of the fishes
that live partly in the sea and partly in fresh water, while the
Common Trout (Salmo fario) is limited to the latter.

The Common Smelt (Osmerus eferlanus) is an example of
the smaller members of the Salmon family. It is found in the
seas of both Northern and Central Europe, as well as in some,
of the lakes and rivers of the same regions.

3. Pike Family. — This is a small group of predatory fresh-
water fishes, including only seven species, of which six are
confined to the United States, while the Common Pike (Esox
lucius) has a wide distribution through the temperate regions of
the northern hemisphere. There is no fatty fin, and the single
dorsal is placed far back near the tail. The shape of the
head, with its flattened snout and projecting lower jaw, is very
characteristic.

4. Carp Family. — This is a large group of fresh -water
fishes distributed through all the great land masses except
South America and Australia. There is no fatty fin, and the
mouth is entirely devoid of teeth, though these structures are
present in the pharynx, attached to the bones which support the
gill -arches. The Common Carp (Cyprinus carpio] is a stoutly-
built fish with a long dorsal fin and two pairs of small barbels.
It is a native of Asia, but, being much esteemed as food, was
introduced into Europe at an early date, and is supposed to
have been naturalized in this country by the monks during the
Middle Ages. The well-known Gold- Fish (Carassius auratus)
is a domesticated species of a genus of carps in which barbels
are absent. It is a native of China and Japan, and the bright
colouring is the result of artificial surroundings. Remarkable
varieties of form have also been produced, as in other animals
which have come under the influence of man.

Among native members of the family may be mentioned
the Gudgeon (Gobio fluviatilis\ a small fish with a pair of short
barbels; the much larger Barbel (Barbus vulgaris], with four
barbels ; the Common Bream (Abramis brama) ; the Bleak
(Alburnus lucidus], Roach (Leuciscus rutilus], Chubb (L*

GROUP OF FRESHWATER FISH

i. The Carp (Cyprinus carpto) inhabits ponds, lakes, and
sluggish streams, living upon worms, insect larvae, &c., and vege-
table matter, to which food its thick-lipped mouth, provided with
four sensitive barbels, is well adapted. It is very tenacious of life,
and exceedingly long-lived (100 years or more). 2. The Crucian
Carp (Carassius vtilgaris) is a small fish common in the lakes and
ponds of Central and Northern Europe. It is often kept in
captivity, together with its near relative, the Gold-Fish, which is
a domesticated variety of the Golden Carp (C. auratus). 3. The
Tench (Tinea vulgaris] is a well-known ground-fish of the carp
kind, and its food is of the same mixed nature. It lives in stagnant
water, which may be so foul that no other fishes can maintain
themselves in it. 4. The Pike (Esox lucius] is an exceedingly
voracious fish, widely distributed through temperate North America,
Europe, and Asia. It attains a very considerable size.

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FISHES 283

cephalus), Dace (L. vulgaris}, and Minnow (L. phoxinus] ; Tench
( Tinea vulgaris) ; and the Common Loach (Nemachilus barba-
tulus], a small elongated slimy fish, with mouth on the under
side of the head and a fringe of six barbels round the edge
of the upper jaw.

5. Herring Family. — The fishes of this family are mostly
marine, and are found in the shallower parts of the sea all the
world over in tropical and temperate regions. Although not
distinguished by number of species, they are unsurpassed among
fishes in the number of individuals, these often swimming
together in vast shoals, which partly accounts for their very great
commercial value. The body is of the typical fish-shape, and
much laterally flattened, especially on the under side, which
may form a sharp edge, often with a saw-like margin and some-
times supported by small bony plates. Fatty fin and barbels
are absent. The head is naked, but the rest of the body is
covered by thin glittering scales, which are easily detached.

The Herring (Clupea karengus} abounds in the North
Atlantic, North Sea, and Baltic, and is also found in the Black
Sea. The Pilchard (C. pilchardus] ranges from the Mediter-
ranean round to the English Channel, and when young is the
familiar Sardine. The Sprat (C. sprattus) abounds on the west
coast of Europe and extends into the Baltic and part of the
Mediterranean.

6. Eel Family. — The snake-shaped fishes belonging to this
family have a wide distribution through both the seas and fresh
waters of tropical and temperate regions. The pelvic and
sometimes the pectoral fins also are absent, and the dorsal,
caudal, and anal fins all form a continuous fringe. The skin
is either entirely devoid of scales, or numerous very minute
scales are imbedded in it.

The Common Eel (Anguilla vulgaris) (fig. 165) has pectoral
fins and minute scales. It has a wide distribution throughout
Europe, the countries bordering the Mediterranean, and North
America. The large marine eel known as the Conger (Conger
vulgaris) is found in almost all parts of the world, and is distin-
guished from the Common Eel by the entire absence of scales,
and its large mouth armed with formidable teeth. The maximum
length appears to be about 8 feet. The Mediterranean Muraena
{Murcena helena) is the typical representative of a widely-distri-

284 CHARACTERS OF VERTEBRATE ANIMALS

buted section of marine eels. It attains the size of the Conger
and was well known to the ancient Romans, who not only
esteemed it as a delicacy but kept it as a pet. The skin is
scaleless, as in the Conger, and is brilliantly coloured. The
front end of the body is very thick, and the large mouth is
armed with powerful teeth. Pectoral fins are entirely absent.

SUB-CLASS III.— SHARKS AND RAYS (ELASMOBRANCHII)

The Spotted Dog- Fish (Scy Ilium canwila) is a good type of
this sub-class, and the description already given of it (pp. 257-264)
will serve to give an idea of the essential features in the structure
of the group. The most important of those features are the
following: — The unsymmetrical tail, and position of the mouth
and nasal openings on the under surface of the head. The
possession of spiracular clefts and at least five pairs of gill-
pouches, the external openings of which are not protected by a
gill-cover. A cartilaginous skeleton, with comparatively simple
skull and well-developed visceral skeleton, and paired-fin skeleton
on the Dog- Fish type. Numerous rows of teeth on the margins
of the jaws, constantly being renewed during life. A spiral valve
in the intestine, and a cloaca. Well-developed arterial cone in
the heart. A thick skin with placoid scales. No swim-bladder,
or at most a small tubular outgrowth from the upper side of the
gullet to represent it. Eggs large, containing much food-yolk.

It is convenient to divide these fishes into two orders, one
containing the Sharks and Dog-Fishes (Selachoidei), the other
Skates and Rays (Batoidei).

Order i. — SHARKS AND DOG-FISHES (Selachoidei) (fig. 166)

The shark-like fishes of this order comprise about 1 50 species,
distinguished by spindle-shaped bodies gradually tapering to the
tail-end, which is sharply bent up. The gill-slits open on the
side of the body, and the eyes possess lids. There are nine
families, of which only seven need be mentioned.

i. Blue Sharks. — The type of this family is the Blue Shark
(Carcharias glaucus), which often reaches the length of 15 feet.
Though not uncommon in British seas during the warmer part
of the year they are specially abundant in the tropics, like most

FISHES

285

of the larger sharks. The Common Tope (Galeus canis) is a
small shark with a very wide distribution, and is a well-known
British species. It may attain a length of 6 feet or more, and
in colour is dark grey above and white below. The body is
slender and the snout prolonged and pointed. A somewhat

A.

PECTORAL FIN

Fig. 166. — Sharks, reduced to various scales

A, Spotted Dog-Fish (Scyllium canicula). B, Blue Shark (Carcharias glaucus).
c, Hammer-headed Shark (Zygcena malleus}.

smaller form, common in British seas and widely distributed over
the world, is the Smooth Hound (Mustelus lcevis\ which can
easily be distinguished from the Tope by its blunter snout, and
the presence of whitish spots on the back. The most remarkable-
looking member of this family is undoubtedly the Hammer-headed
Shark (Zygcena malleus], in which, as the name indicates, the head
is broadened out like a T by the presence of projections upon
which the eyes are situated. It has occasionally been taken in

286 CHARACTERS OF VERTEBRATE ANIMALS

British seas, but cannot be considered as a native species, its
habitat being the warmer parts of the ocean in almost all parts
of the world.

2. Porbeagles. — This includes sharks, which are most abun-
dant in the open ocean and are often of very large size. The
pointed teeth are large and strong, and the gill-slits are usually
very wide, while the spiracle is small or even absent. The
Porbeagle (Lamna cornubicd] is a North Atlantic form and fairly
common on British coasts. It is about 10 feet long. The
Thresher or Fox- Shark (Alopecias vulpes] is distinguished by the
remarkable length of the upper lobe of the tail. It is abundant
in the Atlantic and is the commonest large shark occurring on
our coasts, but is also well-known in the Mediterranean, its range
also including the coasts of California and New Zealand. The
Basking- Shark (Selache maxima) of the North Atlantic may
reach a length of over 30 feet. It is not uncommon on our
western coasts. Unlike the last-named species, which does not
attack man, the huge Rondeletian Shark (Carcharodon Rondeletii\
which may attain the length of 40 feet, is universally dreaded. It is
found in the warmer parts of the open sea in all parts of the world.

3. Dog-Fishes. — Of this the Spotted Dog- Fish, selected as
our type (pp. 257-264), is a typical member. All are coast-
fishes, found in most tropical and temperate seas. Another British
species, closely resembling the preceding but of much larger size,
is the Nurse Hound (Scy Ilium catulus). The Zebra Shark
(Stegostoma tigrinum), common in the Indian Ocean, is a member
of this family. It may be as much as 15 feet long, and receives
its name from the characters of its markings, which consist of dark
transverse bands on a yellowish ground.

4. Spiny Dog-Fishes. — The species belonging here are mostly
of small size and are devoid of an anal fin. The gill-slits are
small and a spiracle is present. The Piked Dog- Fish (Acanthias
vulgaris] is common on the British coasts, and is characterized
by the presence of a sharp spine in front of each dorsal fin. A
much larger species is the Greenland Shark (Lcemargus borealis\
which inhabits the seas of the Arctic region, and is one of the
worst enemies of the Greenland Whale, from the tail of which
it bites large pieces. It sometimes strays as far south as Britain,
and, though it may be as much as 1 5 feet long, is harmless to man.

5. Angel-Fishes. — This contains a single species, the Angel- or

FISHES 287

Monk- Fish (Rhina squatina), found all over the world in tropical
and temperate seas, and not uncommon on our western coasts.
Its chief interest lies in its flattened shape, in which respect it
may be regarded as an intermediate stage between the sharks
and skates, and in the extremely large pectoral fins (the " wings "
of the imaginary angel) which also afford a point of resemblance
to the latter group.

6. Port Jackson Shark Family. — This is a small group of
comparatively small forms, including only one genus of four
species, of which two are found on either side of the Pacific.
The best known is the Port Jackson Shark (Cestracion Philippi)
which ranges from Japan south to Australia and New Zealand.
The head is short and blunt, and there is a sharp spine in front
of each dorsal fin; but the chief peculiarity is found in the teeth.
Those in front are sharp-pointed, but the rest have blunt rounded
crowns, and, being arranged in several closely adjacent rows, form
a surface well adapted to crush the molluscs upon which the shark
feeds.

7. Comb-toothed Sharks. - - This is a small group of tropical
and sub-tropical species possessing certain primitive characters.
There is only one dorsal fin, and the gill-slits, instead of being
five in number on each side, as in other members of the sub-
class, are either six or seven. Four of the five known species
belong to one genus (Notidanus), and one of them, the Grey
Six-gilled Shark (N. griseus], an Atlantic and Mediterranean
form, is sometimes taken in British waters. The teeth are
elongated, and each of them consists of a series of slanting cusps
diminishing in size from one end of the tooth to the other. The
other three species of this genus have seven gill-slits on each
side. The Japanese Frill-gill ed Shark (Chlamydoselache anguineus),
which inhabits very deep water, resembles an eel in shape, and
its mouth is at the front end of the body instead of upon its under
surface. There are six gill-slits on either side, and each of them
is protected by a backwardly-directed fold of skin, pleated in a
frill-like manner.

Order 2. — SKATES AND RAYS (Batoidei) (fig. 167)

The Skates and Rays which constitute this group are much
flattened from above downwards, just the reverse of what obtains

288 CHARACTERS OF VERTEBRATE ANIMALS

in an ordinary flat -fish, the breadth being increased by the
enormous development of the pectoral fins, which form great
wings extending from the head to the pelvic fins. As a result
of this depressed shape, the gill-slits, of which the typical five
pairs are present, are situated on the under side. The tail is
narrow and forms a mere appendage, upon the upper side of
which the small dorsal fins are placed, while the anal fin is absent.
There are six families, of which five may be specially mentioned.

1. True Rays. — The numerous species of the rhomboidal-
shaped fishes belonging to this family have a wide distribution,
but are chiefly temperate forms more abundant in the northern
than in the southern hemisphere. The numerous small teeth
are closely packed in a considerable number of rows, arranged
so as to form two roller-like surfaces which bite against one
another. There are about twelve British species, of which the
commonest are the Skate (Raia batis)y in which the skin is com-
paratively smooth, and the Thornback (Raia clavata), which has
the dorsal surface irregularly studded with large placoid scales
of curious shape, each consisting of a circular disc from which a
thorn-like spine projects, while a row of similar scales runs down
the middle of the back.

2. Saw-Fishes. — This includes five species of fishes especially
characteristic of tropical regions, and one of which (Pristis
antiquorum) is common in the Atlantic and Mediterranean. The
body is not broadened out to the same extent as in Rays, and
in this respect is, so to speak, half-way between them and the
Sharks. A remarkable peculiarity is found in the prolongation
of the snout into a long flattened rostrum, in the sides of which
are imbedded sharp teeth. These fishes may exceed 20 feet in
length, as much as six of this being taken up by the "saw".

3. Eagle -Rays. — These are found both in tropical and tem-
perate zones, one species, the Whip -Ray (Myliobatis aquila]
being occasionally taken in British seas. The extremely slender
tail is armed above with a strong spine, while the teeth, instead
of being pointed, have flattened crowns, and are in contact with
one another at the edges, so as to constitute a very perfect
crushing surface, shaped like a roller.

4. Sting- Rays. — This group includes much -modified Rays,
commonest in the tropics. The body is excessively broad, and
the pectoral fins run forwards so as to surround the front of the

A.

B.

DORSAL FINS

c.

Fig. 167. — Monk-Fish and Rays, reduced to various scales
A, Monk-Fish (Rhino, squatina). B, Thornback (Raia davata). c, Under Side of young Skate (Raia batis)

Electric Ray ( Torpedo nobiliana).
VOL. I. 289 19

290

CHARACTERS OF VERTEBRATE ANIMALS

head. The slender tail is usually armed with a formidable saw-
edged spine, which is the so-called " sting". One widely distri-
buted species, the Common Sting- Ray (Trygon pastinaca), is
sometimes caught off the south of England.

5. Electric Rays. — In these the broad smooth body has a
rounded outline, and on each side of the head there is an electric
organ, capable of giving severe shocks. The most familiar genus,
Torpedo, is represented by species in the Atlantic, Mediterranean,
and Indian Oceans. A well-known Mediterranean form is the
Marbled Torpedo (Torpedo marmorata), and an allied species is
taken from time to time in British seas.*

SUB-CLASS IV.— CHIMERAS (HOLOCEPHALI) (fig. 168)

This small sub-class, though related in many ways to the
preceding one, is distinguished by a number of peculiarities.
It includes only three genera of deep-water fishes. The best-

Fig. 168. — Chimseroids, reduced to various scales
A, Sea-Cat (Chimcera monstrosa], male. B, Bottle-nosed Chimaera (Callorhynchus antarcticus}. C. Harriotta.

known form is the Sea-Cat or " King of the Herrings " (Chimara
monstrosa), the distribution of which includes the North
Atlantic, Mediterranean, Cape of Good Hope, and Japan. In
this animal the large head is rounded and the tail tapers to a
mere thread. There is a powerful spine in front of the first
dorsal fin, while in the male there is a peculiar tentacle-like

FISHES

291

structure on the top of the head, armed with curved spines and
capable of being drawn back into a pit. The eyes are very
large and the skin is smooth. Only four gill-slits are present on
each side, and they are covered over by a membranous flap or
gill-cover. There is no spiracle. Another species of Chimaera
is taken off the coast of Portugal, and the third is found on the
Pacific coast of North America.

The Bottle - nosed Chimaera (Cal'lorhynchus antarcticus) of
the southern seas is more shark-like in form, and its name is
derived from the curious thickened form of the snout. A third
genus (Harriotta), recently discovered in very deep water both
in the Atlantic and Pacific, is comparatively small and is dis-
tinguished by its slender-pointed snoqt.

SUB-CLASS V.— ROUND-MOUTHS (CYCLOSTOMATA)

The Lampreys and Hags which make up this sub-class are
so unlike other fishes in many respects that many zoologists
place them in a class of their own. They have a wide distribution

Fig. 169. — Lampern (Petromyzon fluviatilis)

in the temperate regions of both hemispheres, and the lampreys
occur in fresh as well as in salt water. There are four British
species, three of these being lampreys, any one of which will
serve to illustrate the chief characters of the sub-class. They
are the Sea- Lamprey (Petromyzon marinus], the River- Lamprey
or Lampern (P. fluviatilis) (fig. 169), and the Small Lamprey
(P. branchialis}. The eel-like body is bordered by three narrow
unpaired fins, two dorsals and a caudal, but all traces of paired
fins are absent. Instead of possessing a slit-like mouth bounded
by jaws, as in all the backboned forms so far considered, a
rounded sucker-like concavity is present on the under surface of
the head, and the small mouth opens within this. The scientific

292 CHARACTERS OF VERTEBRATE ANIMALS

name of the group is derived from this peculiarity (Gk. kuklos,
a circle ; stoma, a mouth). The skin is devoid of scales, but
there are numerous little horny structures lining the mouth-
sucker, and a muscular tongue armed with similar bodies can
be protruded from the mouth and used as a rasping organ. A
single nostril opens on the top of the head, behind this is the
eye, and then come seven small round holes, which are the
external openings of as many gill-pouches. The egg of the
Lamprey develops into a larva which is so unlike the adult that
it was formerly thought to be a distinct kind of fish and received
the name of Ammocates.

The Common Hag- Fish (Myxine glutinosa) is shaped like
a Lamprey, but the dorsal fin is absent, and the imperfect eyes
are covered by the skin. The suctorial mouth is margined by
eight barbels. At first sight there appear to be no gill-openings,
but there are in reality six pairs of gill-pouches, the outer ends
of which are drawn out below the skin into backwardly directed
tubes, those on the same side uniting together and having a
common opening pretty far back on the under surface of the body.
The skin is very glandular, possessing the power of producing
vast quantities of slime which sets into a firm jelly.

PRIMITIVE VERTEBRATES (PROTOCHORDATA)

We now come to a number of very primitive forms, most
of which were classified with the backboneless animals till com-
paratively recently. The characters of the backboned animals
or Vertebrates have been enumerated and explained in an earlier
part of this volume (pp. 60-63), to which reference must be
made for details. It may, however, be remarked here that there
are three chief characters by which a Vertebrate, or, to use a
wider and better term, a Chordate, may be distinguished.

i. The possession at some period of life of a firm gelatinous
rod, the notochord, running longitudinally below the central
nervous system. Such a supporting rod can be made out in the
embryos of all the forms hitherto described, but in most of them
it is sooner or later squeezed out of existence, partly or entirely,
by the development of a vertebral column. In some few instances.

PRIMITIVE VERTEBRATES 293

however, as Lung- Fishes, Chimaera, Lamprey, and Hag, it per-
sists throughout life, though invested in a firm sheath and supple-
mented by the development of cartilage. Ordinary bony fishes
are a good instance of partial persistence of the notochord.
Almost everyone must have noticed, in the pursuit of breakfast-
table anatomy, that a gelatinous substance occupies the spaces
between the doubly-cupped ends of the vertebrae in such fish
as salmon and cod. This substance represents the part of the
notochord which has not been nipped out of existence by the
ingrowth of hard material to make up the joints of the backbone.
In a Protochordate there never is a vertebral column or a back-
bone in the proper sense of the word, but only a more or less
developed notochord.

2. It is scarcely less distinctive of Chordate animals that in
them the pharynx should be perforated by visceral clefts in the
embryo if not in the adult.

3. It is typical for a Chordate to possess a central nervous
system (brain and spinal cord) situated dorsally above the noto-
chord, and having the nature of a thick- walled tube.

We will apply these tests to the three recognized orders
of Protochordates, i.e. : i. Lancelets (Cephalochorda) ; 2. Sea-
Squirts (Urochorda); and 3. certain still simpler forms grouped
provisionally as Hemichorda. Very great theoretical interest
attaches to the study of these forms, and much attention has
been bestowed upon them, largely with a view of finding out
how far they throw light upon the obscure problem having for
its goal a determination of the characters of the simplest and
earliest Vertebrates which appeared upon the globe.

SUB-CLASS i. — LANCELETS (CEPHALOCHORDA)

Lancelets are small fish -like creatures widely distributed
round the coasts of the globe where the two conditions of sand
and shallow water are combined. There are about 8 species,
popularly referred to one genus Amphioxus (Gk. amphi, both;
oxus, sharp), which, like the ordinary name, has reference to the
fact that the flattened body is pointed at both ends. The common
European species (Amphioxus lanceolatus) (fig. 17°) '1S found
on the Mediterranean and both Atlantic coasts, including our
own islands. It is particularly abundant at Naples, where the

294

CHARACTERS OF VERTEBRATE ANIMALS

fishermen use it as bait. The translucent body varies in length
from about \y2 inch to rather more than double that size,
and at first glance one might be puzzled to say which was the
front end of the body, as no distinct head is present. On closer
inspection, however, a little hood-like structure, fringed by slender

KIDNEY TUBULES

SPINAL CORD

*OUTH
WITH CIRRI

LIVER ATRIOPORE

EPIDERMIS
SPINAL CORD

DORSAL ARTERY
DORSAL GROOVE
CCELOM

ATRIAL CA~VITY

END08TYLE

Fig. 170. — Lancelet (Amphioxris lanceolatus]

A, Side View, with internal organs seen by transparency. Semidiagrammatic. u, Transverse section.
Much enlarged and Semidiagrammatic.

tentacles, can be seen near one extremity. This marks the under
side of the head end, and, since it leads to the mouth, has been
called the oral hood, which has a certain resemblance to the
mouth sucker of a Lamprey. Here as there jaws are absent,
and the Lancelet is further devoid of the tooth-like projections
and powerful rasping tongue which distinguish the Lamprey.
Running along the upper margin of the body is a narrow dorsal
Jin, passing behind into a slightly expanded caudal Jin, which again

PRIMITIVE VERTEBRATES 295

is in one piece with a narrow anal fin running forward for a short
distance. In front of this the under side of the body is broad,
convex, marked by longitudinal pleats, and bounded on each
side by a prominent fold which runs forwards to the oral hood.
These two lateral folds converge behind and end where the
anal fin begins. In the description of the Dog- Fish it was stated
(p. 258) that the various unpaired fins are probably expanded
surviving fragments of a continuous fin which, in ancestral
forms, ran along the middle of the back, round the tail, and
forwards for some distance along the under surface. Such a
condition is actually represented by the Lancelet, for, as
just stated, its unpaired fins constitute a continuous fringe to
the body. But further, it is by no means improbable that the
pectoral and pelvic fins of an ordinary fish represent the front
and back ends of a continuous lateral Jin which once existed
on either side. There are no paired fins in the Lancelet, but
the lateral ridges in front of the anal fin are perhaps equivalent
to such continuous lateral fins.

The Lancelet is obviously segmented, i.e. divided into a
number of similar successive parts from before backwards, and
this segmentation is well seen in the muscles which make up
a great part of the side of the body, these being divided up
into > -shaped sections. At first view the animal looks as if it
were bilaterally symmetrical, but this is not the case, for the
muscle -segments do not correspond on the two sides of the
body; and further, the external opening of the intestine (there
is no cloaca) is placed on the left side of the body near the
base of the tail-fin. There is also a lack of symmetry in other
respects which need not be mentioned here.

A well-developed notochord is present, and like many of the
other internal organs can be made out without dissection by
examination of small specimens mounted whole as microscopic
objects. There is, however, one peculiarity about it. Instead
of stopping short about the middle of the brain, as it does in
the higher Vertebrates, it runs to the extreme front, and the
scientific name of the sub-class alludes to this (Gk. cephalon,
head; chorde, string).

No gill-slits are visible on the exterior, but dissection shows
that a very large number are present as oblique openings in the
wall of the large pharynx. They do not, however, open directly

296 CHARACTERS OF VERTEBRATE ANIMALS

to the outside, but into a large atrial cavity which surrounds the
pharynx, and which itself opens to the exterior by a rounded
hole, the atriopore, on the under side of the body just in front of
the anal fin. The state of things may be more clearly under-
stood by reference to the development of the tadpole (p. 254),
where the gill-slits at first open directly to the exterior but are

Fig. 171.— Two Simple Ascidians. i, Ciona, intestinalis. 2, Phallusi'a mammillata

later on covered over by the backward growth of a fold of the
body-wall, the upshot of this being the formation of a branchial
chamber into which the gill -slits open, and which opens to
the exterior itself by a small hole on the left-hand side of the
body. If this hole were in the middle line below, instead of on
the left side, it might be compared to the Lancelet's atriopore,
while the branchial cavity has much the same relations as the
atrial cavity. It would probably be incorrect to consider the
two cavities as closely equivalent, for they develop in very
different manners.

The Lancelet will also stand the third test of a Vertebrate,
for it possesses a tubular nerve-cord, situated above the notochord,

PRIMITIVE VERTEBRATES

297

but not extending so far forwards. There is, however, no distinct
brain.

It is usual to find this animal buried in the sand in a vertical
position, with the head end projecting; but it can also swim, and
is able to burrow in the sand with great rapidity.

ENDOSTYLE

SUB-CLASS II.— SEA-SQUIRTS (UROCHORDA)

Among the objects cast up by the tide on the sea-shore, or
found attached to rocks which are uncovered at low -water, are
certain leathery-looking objects which when touched emit a jet
of water, a habit which has
earned for them the name of Sea-
Squirts. On account of the firm
cover or tunic with which they
are invested the name of Tuni-
cates has also been widely used.
They are the first fixed or seden-
tary animals with which we have
had to deal, and this mode of life
has had a profound influence
upon their structure. A com-
mon British species, Ascidia
mentulay may be taken as an
example.

The plump rounded body is
attached by one end to some
foreign object, while at the other

end may be Seen tWO OrificeS Fig. 172.— Diagram to explain structure of a simple

/ r\£-\ £\ T'TT^ h Ascidian. The animal is seen from right side with dorsal

^bCt p. 2LJO, ^£3' ^7 /> CclLll surface to left and front end above. The arrows indi-

-nlarprl nn P nmiprtinn QO PQ tn cate course of water-currents which take food and oxygen

a projection, s into mouth> pass through perforations in pharynx into

give a distant resemblance to atrial cavity> and carry waste Products to exterior

through atriopore.

the skin bottles used in the East,

and which is embodied in the name of Ascidians (Gk. askos, a
wine-skin ; eidos, like) often applied to these forms. One of the
openings is situated at the extreme end and the other somewhat
on one side. The former is the mouth and the latter the atriopore,
and observation of a living specimen placed under water will
show that currents set into one and out of the other. The
protective tunic or test, which is thick and gristly in texture,

298 CHARACTERS OF VERTEBRATE ANIMALS

is interesting on account of its chemical composition, as it is
largely made up of a substance, cellulose, which is almost
entirely confined to plants. Dissection (fig. 172) fails to show
the presence of a notochord, and the central nervous system
merely consists of an elongated thickening or ganglion, placed
about half-way between the two apertures, but the pharynx is
perforated for breathing purposes as in ordinary Vertebrates.
The mouth, in fact, leads into a large branchial sac or pharynxy
perforated by innumerable small holes and suspended in an
atrial cavity which opens by the atriopore already mentioned.
There is therefore a general resemblance to the Lancelet in this
respect, but in this case the intestine also opens into the atrial
cavity.

Were we to rely only upon the anatomy of the adult we
should hesitate before placing the Sea- Squirt among the Verte-
brates, seeing that it conforms to only one of the three chief
tests; but the matter is set definitely at rest by a study of the
development. The egg of Ascidia becomes a tadpole -shaped
larva in which a notochord is present, though it is confined to
the tail, and for that reason is often called a urochord (Gk.
oura, tail; chorde, string), which gives the name Urochorda,
adopted at the head of this section as the name for the group
of Sea -Squirts generally. And further, the ascidian tadpole
is possessed of a hollow brain and spinal cord situated on the
dorsal side of the body, besides which it may be noted that
the perforations in the pharynx are at first of simple character,
consisting of paired openings suggestive of the gill -clefts of
fishes, &c.

After leading a free life for some time the tadpole attaches
itself by means of adhesive projections situated at the head end,
the tail with its urochord gets smaller and smaller and ultimately
disappears, while the central nervous system is simplified into
a single solid ganglion. We have, therefore, the remarkable
phenomenon of an animal which, when young, possesses the
distinctive vertebrate characters, but loses most of them in the
adult condition, becoming, so to speak, of lower grade. This
is a good example of biological degeneration.

It is a singularly interesting fact that one or two small free-
swimming Ascidians, of which the best known {Appendicularia)
(fig. 173) occurs in British seas, retain throughout life the tadpole

PRIMITIVE VERTEBRATES

299

form and the typical Vertebrate characters. It is possible that
these are primitive forms which retain the features distinctive
of the ancestral Ascidians, but it is also possible that we have
a case of animals which have dropped the adult stage out of
their life-history, just as the Mexican Axolotls appear to be
doing (see p. 249).

Ascidians may be divided into Fixed and Free-swimming
forms, each of which groups can be again split up into Simple

EAR

BRAIN

NOTOCHORD

INTESTINE.

GILL CLEFT

HEART-

NERVE CORD-

STOMACH'

Fig. 173. — Diagrammatic drawing of Appendicularia (much enlarged), as seen from left side, with
dorsal surface to right. D T, Dorsal tubercle.

and Colonial species. The last expression needs explanation, as
it involves a phenomenon of which no instance is furnished by
the animals previously considered. All these are propagated
solely by means of eggs, but in Ascidians and many of the
lower Invertebrates there may be increase by means of out-
growth of buds (gemmation) or by the bodily splitting (fission)
of individuals to form others. As this suggests similar processes
among plants, it is commonly known as vegetative propagation.
A collection of animals which have been formed in this way, and
which remain united together, constitute a colony, or are said to
be colonial. In Ascidians colonies may be formed by means
of budding.

3oo CHARACTERS OF VERTEBRATE ANIMALS

The individuals of Fixed Ascidians are of comparatively
large size in the Simple forms (fig. 171), of which the example
taken is a typical one. The Colonial forms are made up of
smaller individuals, and the colonies produced may be of the

most varying form and size, while the
degree of union between the members
of the colony is more or less complete.
In such a native genus, for instance, as
Clavellina, we find a creeping stalk-like
structure from which a number of indi-
viduals grow »up, each of them being
Fig. ^-Botryihis essentially similar to Ascidia. A good

To left, a small colony, showing groups 1 r • r i

of individuals. TO right, two groups of example oi more intimate union is found

in Botryllus (fig. 174), which can often

mouths at outer ends. In centre is an J^ founc[ at Jow ^Q as a bluish £ela-

atriopore common to all of them.

tinous crust upon stones and brown

sea- weed. Imbedded in this are star-shaped groups of small
individuals.

Among Free-swimming Ascidians the only Simple forms are
Appendicularia (fig. 173) and its allies. The Colonial members
include the remarkable phosphorescent Pyrosoma, which is
shaped like a hollow cylinder closed at one end, and the Salps,
of which more will be said elsewhere. Both are common in the
Mediterranean.

SUB-CLASS III.— WORM-LIKE PROTOCHORDATES

(HEMICHORDA)

Here are massed together a small number of forms about
which there has been endless discussion, and of which the one
with most certain tenure of Chordate rank is a worm-like
creature which has no common name, but which may perhaps
be called the Acorn-headed Worm (Balanoglossus] (fig. 175).
It is found at low-tide mark in many parts of the world, living
in mud or sand which it glues together into a sort of temporary
tube by means of a slimy fluid poured out from the skin. One
species is found in the Channel Islands. The front of the body
is made up of a swollen proboscis, yellow or orange in colour,
and capable of altering its shape to a very great degree. It is
attached behind by a narrow stalk, and the general outline in

PRIMITIVE VERTEBRATES

301

some species suggests the term " acorn-headed ". The mouth
is situated on the under side, at the base of this proboscis. Next
comes a comparatively short region, named from its appearance
the collar, while the rest, and by far the longest part, of the
animal may be termed the trunk. On the upper side of the
trunk, behind the collar, are a con-
siderable number of gill-slits ar-
ranged in pairs, and forming the
external apertures of gill-pouches
which communicate internally with
the digestive tube. They resemble
in many respects the corresponding
structures in the Lancelet. A small
notochord has also been identified,
but here we have the opposite
extreme from what is found in an
Ascidian tadpole, for the structure
in question is a small rod which
projects into and supports the base
of the proboscis. It is in reality
a thickened forward outgrowth
from the digestive tube, and has
a peculiar microscopic structure
which is distinctive of notochords
wherever they are found. The fact
that it grows out of the gut is also
a point in support of its notochordal

nature, for in more typical cases, as, e.g., Lancelet or Frog, the
notochord arises as a thickening in the wall of the digestive
tube. The remaining test of a Vertebrate is also answered in
a fairly satisfactory way, for what may be described as the
central nervous system in this creature is a more or less hollow
thickening running along the dorsal part of the body in the
collar region.

There are certain other more doubtful claimants to a place
in the Hemichorda, but a discussion of their characters would be
out of place in a preliminary sketch of the animal kingdom.

Fig. 175. — Acorn-headed Worm (Balanoglossits)

CHAPTER VII

BACKBONELESS ANIMALS (INVERTEBRATA). STRUCTURE
AND CLASSIFICATION OF NEMERTINES AND MOLLUSCS

A brief account of the Backboned or Vertebrate animals has
now been given, and in accordance with the usual custom from
the time of Aristotle downwards all the remaining forms, far
more numerous than they, may be conveniently lumped together
as Backboneless animals or Invertebrates, divisible into a number
of great groups or phyla, each of which is on a footing with the
phylum Vertebrata. The lower Invertebrates are so unlike the
Vertebrates that close comparison is not possible, but there are
certain features which broadly serve to mark off a higher In-
vertebrate from a typical backboned animal. These are, to a
large extent, implied in the summary given previously (pp. 60-63)
of the chief Vertebrate characters, but it may be useful at this
point to take such a form as a Cray- Fish or Lobster and point
out the distinctive features in question (fig. 176).

The body of a Lobster has the same two-sided or bilateral
symmetry as that of a Vertebrate, and there is a clear distinction
between front (anterior) and back (posterior) ends, upper (dorsal)
and lower (ventral) surfaces, and right and left sides. The body,
too, is segmented, or divided into a number of similar parts from
before backwards, as in, say, a Lancelet. This is evident in the
Lobster's tail. It must not, however, be hastily assumed that
a segment of a Lobster is the exact equivalent either of a Verte-
brate segment or a segment in an Invertebrate from another
group.

Now come a number of important differences. A large
number of jointed limbs are present, arranged in pairs, while a
Vertebrate has at most two pairs of limbs, though these may
differ in nature in different animals, and in the simplest case,
that of fishes, are unjointed fins. The limbs are modified for

302

CHARACTERS OF INVERTEBRATE ANIMALS

303

various purposes. The most obvious are those which end in
the large pincers, behind which four large pairs of walking -legs
are apparent. Under the head are a number of overlapping
limbs, turned somewhat forwards, which guard the mouth and
act as jaws. In a Vertebrate the jaws are part of the bony
framework of the head, helping to bound the mouth-cavity, and
the lower jaw works up and down. But the limb-jaws of the
Lobster are outside the opening of the mouth, and from the nature
of the case work against one another from side to side. To realize
this, raise your hands to your mouth and "clap" them together,
which will give an idea of the way in which one pair of the
Lobster's jaws are worked. Six pairs, however, are present
In all.

By combining the knowledge obtained by dissecting one
Lobster from the side and making a cross-section through another,
the following distinctive characters of higher Invertebrates can be
easily verified: —

1. There is a protective external skeleton (exoskeleton) but
no internal skeleton (endoskeleton), i.e. nothing can be discovered
equivalent to the skull, backbone, &c., of, say, a Perch, or to the
notochord of a Lancelet. The absence of endoskeleton modifies
the structure in many ways, as, e.g., in regard to the attachment
of muscles. In such a limb as the human arm the numerous
muscles are attached to the bones, but in a Lobster's leg they
are attached to the firm

exo-skeleton.

2. The body is not a
double but a single tube in
structure.

3. The side-walls of the
digestive tube are not per-
forated by gill-slits.

4. The heart is situated
dor sally, the exact opposite
of the Vertebrate condition.

5. The nervous system consists of a ring surrounding trie
gullet, and continued backwards into a ventral nerve-cord. The
dorsal side of the ring is thickened into a double brain or cerebral
ganglion. A very large number of Invertebrates possess such
a nerve-ring and ventral cord, while many more have the ring

LOW NERVE TUBE
NEURAL TUBE
NOTOCHORD

B

NERVE-CORD

Fig. 176. — Diagrammatic Cross-sections through, A, a Verte-
brate; B, a higher Invertebrate

3o4 CHARACTERS OF INVERTEBRATE ANIMALS

though not the cord. In no Invertebrate does the central nervous
system consist, as in Vertebrates, of a hollow cord running along
the dorsal side of the body.

The contrasts between a Vertebrate and a higher Invertebrate
are illustrated by the accompanying diagrams (fig. 176).

The following groups or phyla of the Invertebrata are recog-
nized. It must not be imagined, however, that they are anything
like of equal size, for some are exceedingly large, while others are
relatively small.

I. Nemertines (NEMERTEA). — Worm-like, marine forms, which in some
respects approach the Chordata in structure.

II. Molluscs (MOLLUSCA), including such forms as Cuttle-Fishes, Snails,.
Slugs, Oysters, and Mussels.

III. Jointed-limbed Animals (ARTHROPODA), the largest group of the
animal kingdom, comprising such creatures as Insects, Scorpions, Spiders,,
and Mites; Centipedes and Millipedes; Shrimps, Lobsters, and Crabs.

IV. Segmented Worms (ANNELIDA). — A large group of forms, in-
cluding innumerable Marine Worms (free- living and tube- inhabiting),.
Earth- Worms, Fresh-water Worms, and Leeches.

V. Siphon-Worms (GEPHYREA).

VI. Wheel Animalcules (ROTIFERA).

VII. Moss-Polypes and Lamp-Shells (MOLLUSCOIDA). — The great
bulk of these are fixed marine animals, and those belonging to the first
group are nearly all colonial.

VIII. Flat- Worms (PLATYHELMIA). — The most familiar of the forms
grouped here are the Flukes and Tape-Worms, which are found as parasites
within the bodies of other animals.

IX. Thread-Worms (NEMATHELMIA). — The name of the group indi-
cates the shape of these creatures, most of which are parasites, either in
plants or else within the bodies of other animals.

X. Echinoderms (ECHINODERMATA). — This phylum is constituted by
such marine forms as Star-Fishes, Sea-Urchins, Sea-Lilies, and Sea-
Cucumbers.

XL Zoophytes (CCELENTERATA). — Mostly marine animals, which may
be either simple or colonial, fixed or free-swimming. Familiar examples
are Jelly-Fishes, Sea-Anemones, and Corals.

XII. Sponges (PORIFERA). — Mostly marine, colonial, and fixed.

XIII. Animalcules (PROTOZOA). — This lowest phylum includes an
immense number of simply-constructed animals, which are nearly always
very small or microscopic in size. They are found almost everywhere, but
are unfamiliar to those who are not in the habit of using the compound
microscope.

NEMERTINES

305

A brief survey will now be made of these thirteen phyla, but
many particulars regarding them will be found in other parts of
this work.

NEMERTINES (NEMERTEA)

Although the worm-like forms which belong here have a
wide distribution, and are particularly common between tide-
marks on almost all coasts, they are nevertheless practically
unknown except to the professed naturalist, and have received
no common names. There are some forty British species. The
vast majority are marine, and either shore or shallow- water forms,
but they are also represented in fresh water and even on land.
They have been given here the first place among Invertebrates,
in deference to the views of many zoologists, in whose opinion they
come near to the Protochordates.

The body of a typical Nemertine (fig. 177) is cylindrical, and
presents no trace of segmentation. It may be only a small
fraction of an inch in length, or in other cases many yards long.
A common British form (Linens marinus) is one of the species
which are extremely elongated, and it may not infrequently be
found under stones, with its slimy black body twisted up into a
complicated coil. Other species may be more or less brightly
coloured.

The mouth is a small oval opening on the under side of the
head end, while the aperture of the intestine is at the extreme
tip of the tail. Close examination will show that above the
mouth on the front end of the body there is a small pore, and
in a living specimen a narrow thread may sometimes be seen
to shoot out from this pore, through which it can again be
drawn back into the body. This thread is known as the
proboscis, and, as described elsewhere, it is used as a means of
killing or paralysing the marine worms upon which a Nemertine
chiefly feeds. When within the body it is enclosed in a special
sheath which overlies the digestive tube. The proboscis is
hollow, and the way in which it is protruded and again drawn
back may be understood by taking the somewhat hackneyed
illustration of a glove with one finger. If this finger be pulled
back into the main glove by turning it outside inwards, we shall
have a rough model of the proboscis when lying within the body.

VOL. I. 20

306

CHARACTERS OF INVERTEBRATE ANIMALS

If now the finger be pushed out, it will represent the extended
proboscis. The pulling in is effected by means of a muscle band
which runs along the interior of the thread and is attached to
its tip, while the pushing out is the result of fluid being squeezed
into the thread from its sheath. This kind of principle is utilized
elsewhere in the animal kingdom, and a very familiar example

A.

PROBOSCIS PORE

LATERALNERVE

GUT

Fig. 177. — Structure of a Nemertine (diagrammatic)

A, Side view, internal organs seen by transparency. B, View from above of front end; position of mouth
and beginning of gut indicated by the dotted line, c, Cross-section.

is found in the " horns " of the common Snail, which can either
be stretched out (as immortalized in the nursery rhyme wherein
the adventures of certain snail-hunting tailors are set forth) or
withdrawn into the body at will.

Reasons have been adduced for thinking that the proboscis-
sheath is comparable to a notochord, and the proboscis to a
curious little structure attached to the under side of the brain in
Vertebrates, and known as the pituitary body.

The central nervous system of a Nemertine is interesting in
many ways. It consists of a nerve-ring, which encircles the front
end of the proboscis, and not the digestive tube as is usual among

MOLLUSCS 307

Invertebrates. Each side of the ring is thickened into a ganglion,
from which a lateral nerve-cord runs along the corresponding side
of the body, while there is a much more slender nerve running
back in the middle line above from the upper side of the ring.
This dorsal nerve has attracted a great deal of attention, for it
has been compared to the spinal cord of a Vertebrate, though,
unlike this, it is solid.

MOLLUSCS (MOLLUSCA)

Molluscs include such familiar shell -fish as Periwinkles,
Oysters, Cockles, and Mussels, as well as soft-bodied animals
like Cuttle- Fishes, to which the term Mollusca (Lat. mo His, soft)
was originally applied.

Examination of such a typical form as the Ormer or Sea-
Ear (Haliotis tuber culatd), which is common in the Channel Isles,
will give some idea of the characters of Mollusca in general,
and of the special subdivision to which this particular sea-snail
belongs (fig. 178).

External Characters. - - The most obvious feature is the
presence of a large external shell covering the upper side of the
body, from which it cannot be detached without cutting through
a large rounded fleshy mass, the shell-muscle. The shell is
not symmetrical, for a row of holes can be seen running along
near its left margin, and on the right side at the back a spiral
twist can be made out. Turning the animal over, a huge fleshy
mass with a flattened surface is seen projecting from the under
side of the body. By means of this foot the Ormer is able to
adhere to rocks like a Limpet, and to crawl about like an
ordinary Snail. A foot in some form or other is characteristic of
all Mollusca, and it must be understood that the word is here
employed with a special meaning of its own. Projecting in front
from above the foot a short head can be seen bearing a blunt
snout, at the end of which the mouth is placed, and a pair of
pointed feelers or tentacles, which are solid, and cannot therefore,
like those of a common Snail, be withdrawn into the body. Seen
from below, the body of the Ormer is bilaterally symmetrical, and
the twisted condition of its upper part is a special condition
characteristic of snail-like forms in general.

On removal of the shell by cutting through the shell-

308 CHARACTERS OF INVERTEBRATE ANIMALS

muscle the soft upper part of the body in which a large part of
the viscera are contained will be exposed, and it will be noticed
that this visceral hump, as it has been called, is twisted behind
in correspondence with the twist in the shell. Skirting the
visceral hump is a flap, produced by a pulling out, so to speak,
of the body wall, and known as the mantle. In the Ormer it
is narrow for most of its extent, but is very well developed in
the part underlying the row of holes in the shell, where it roofs
in a large mantle -cavity, which has a long slit-like aperture above
and a wide opening in front about the head. That this mantle-
cavity should freely communicate with the exterior is very
necessary, for not only does it contain the breathing organs, but
the intestine and the kidneys open into it. A very small amount
of dissection reveals the presence of the breathing organs in
the form of two plume-like gills attached along their sides, and
having their tips pointing forwards. The projecting end of the
intestine will also be seen, and right at the back of the cavity
two small holes by which the kidneys open.

Just behind the mantle-cavity the heart is situated, consisting
of a central ventricle, which pumps purified blood from the gills
through arteries which come off from it fore and aft, and of a
thin-walled auricle on either side. The ventricle is folded round
the intestine, a noteworthy peculiarity, though one not known
to have any physiological meaning. A heart like this, which
contains pure blood only, is said to be systemic, and it should be
noted how markedly it differs from the heart of an ordinary fish,
which contains impure blood only. The complex heart of a Bird
or Mammal is physiologically equivalent to both these varieties
of heart, for its right half receives impure blood and pumps it
to the breathing organs, while its left half is concerned with the
reception of pure blood from those organs, and the distribution
of the same to the general system.

The digestive organs of the Ormer consist of a long digestive
tube with large glands opening into it, and including pharynx,
gullet, stomach, and intestine, the last, as already noted, ending
in the mantle-cavity. Particular interest attaches to the pharynx,
or buccal mass, which is partly modified into a complex rasping
organ (odontophore), characteristic of two great groups of the
Mollusca. It essentially consists of a rounded cushion rising from
the floor of the pharynx, over which is stretched from front to

A.

TENTACLES

MANTLE
NERVE

Fig. 178. — Structure of the Ormer (Haliotis]

A, Shell from above. B, Semidiagrammatic view from above after removal of shell. The roof of the mantle-cavity
has been cut away and the heart exposed. The small left kidney (unshaded) and hinder part of large right kidney
(shaded) shown by transparency, c, Digestive organs. D, Diagram of nervous system.

3io CHARACTERS OF INVERTEBRATE ANIMALS

back a horny ribbon, the radula, beset with transverse rows of
flinty teeth. This ribbon, often called the " tongue" or " palate",
has often been compared to a finger-nail, and as worn away it
constantly grows forwards from a projection (radula sac) at the
back of the pharynx, just as the finger-nail does from its root.
A fuller account of this organ will be given in another place.

The kidneys of the Ormer are two irregular brown bodies,,
opening as described. The left one is very small, and would
seem not to act as a kidney at all.

There are certain characteristic features of the nervous system
which require notice. It consists of a mrve-ring surrounding the
beginning of the digestive tube, and of other connected parts.
The upper part of the ring consists of a transverse band con-
necting two swellings, the brain (or cerebral) ganglia, and from
each of these two cords run downwards to constitute one side
of the ring. The outer cords end below in a pair of lateral
(or pleural) ganglia, and the inner cords in a pair of Joot (or
pedal) ganglia, which are united together in the middle line so
as to complete the ring below, while each lateral ganglion is
also connected with the adjoining foot ganglion. From the
brain ganglia, nerves run off to the sensitive parts of the head
including the tentacles, and strong nerves to the foot run back-
wards through the substance of that organ from the foot ganglia.
There still remains to be described a nerve-loop, which connects
the two lateral ganglia and gives off nerves to some of the
internal organs. It is a nerve -cord which, starting from one
lateral ganglion, runs obliquely backwards, and, turning round in
a curve at the level of the hinder end of the mantle - cavity,
sweeps forwards again, and takes an oblique course to the other
lateral ganglion, taking altogether a course which may be compared
to the figure 8. Three ganglia are seen as swellings upon this
loop, one close to each gill, and the third at the back end of
the 8. The curious course of the loop is one result of the
twisting of the body which has affected both the visceral hump
and shell.

The most important organs of sense are the tentacles, which
have to do with touch, so-called organs of hearing, consisting
of a pair of little rounded sacs attached to the foot ganglia,
and two small cup-shaped eyes, one at the base of each tentacle.
There is also a special sense-organ connected with each gill,

HEAD-FOOTED MOLLUSCS 311

which is generally considered a kind of organ of smell, entrusted
with the duty of testing the quality of the water which enters
the pallial cavity. It is termed the osphradium.

The preceding account of the Ormer illustrates the most
prominent characters of Mollusca generally, which are: (i) the
absence of segmentation, (2) the presence of a mantle, (3) the
muscular foot, (4) the systemic heart, (5) plume-like gills, and
(6) a nerve-ring surrounding the first part of the digestive tube.
The vast majority of Molluscs either possess all these characters
or else a sufficient number of them to leave no doubt as to
how they should be classified. Other very common, though by
no means universal, characters of the group are the presence
of a shell and development of a rasping organ (odontophore).
Large numbers of Molluscs are also distinguished by the
bilateral symmetry of their bodies, and though the Ormer is
not one of these, it is, as already pointed out, symmetrical so
far as the lower half of the body is concerned.

Five classes are recognized among Mollusca, as follows: —

1. Head -footed Molluscs (CEPHALOPODA), including the Pearly
Nautilus, Cuttle-Fishes, Squids, and Octopi.

2. Snails and Slugs (GASTROPODA).

3. Bivalve Molluscs (LAMELLIBRANCHIA), including forms with the
shell in two pieces, e.g. Oyster, Mussel, and Cockle.

4. Tusk-shells (SCAPHOPODA).

5. Proto- Molluscs (AMPHINEURA), a small group of which the only
common member is Chiton, distinguished by the possession of eight over-
lapping shelly plates on the upper surface of the body.

CLASS i.— HEAD-FOOTED MOLLUSCS (CEPHALOPODA)

As a good type for description we may select the Common
Cuttle-Fish (Sepia officinalis], one of our native species, which
preys upon fishes and Crustacea in shallow water, and is a free-
swimming form (fig. 179).

External Characters. — The body is bilaterally symmetrical,
and at one end of it the mouth may be seen, provided with a
pair of horny jaws resembling those of a parrot, and surrounded
by ten arms or tentacles, of which two are very long and can be
drawn back into special pouches. The inner sides of the eight
short arms are studded with adhesive suckers, and each long
arm swells at its end into an oval pad, one side of which is

312

CHARACTERS OF INVERTEBRATE ANIMALS

similarly provided. Outside the circlet of arms a large eye can
be seen on either side, covered by a circular eyelid perforated
by a small hole. The presence of mouth and eyes shows that
we are dealing with the head end of the animal. In comparing
the body with that of the Ormer we must place this end down-
wards and slant the rest of the animal, which is mostly visceral

VISCERAL HUMP

MANTLE CAVITY

POSTERIOR
,ORTA

.VENTRICLE
AURICLE

VENTRAL
GANGLIA
OUTH CAVITY
RADULA

SHORT

Xi— - V

XBEAKS

Fig. 179.— The Cuttle-Fish (Sepia officinalis] reduced
A, View from left side. B, Side-dissection. Arrows show course of water into and out of mantle cavity.

hump, upwards and backwards as shown in the diagram (A). It
will then be clear that the long axis of the body is pretty nearly
at right angles to the direction corresponding to the long axis
in the Ormer. The long visceral hump will possess two gently-
curved sides facing respectively forwards and backwards, and
two sharp edges placed right and left and margined by a fin-like
expansion. The next point will be to find mantle and foot. The
former is readily made out running round the edge of the large
visceral hump at its lower end, just above a narrowed "neck"
indicating the boundary of the head. Most of it is at the back,
where it constitutes the hinder wall of a large mantle -cavity,
into which a large slit-like opening leads. The foot has grown

HEAD-FOOTED MOLLUSCS 313

round the head, and chiefly consists of the ten arms or tentacles.
It is from this peculiarity the name of the class is derived
(Gk. kephalon, a head; pous, a foot). Just above the back of the
head, and projecting from the opening into the mantle-cavity,
will be noticed a muscular conical tube, the funnel, by the
ejection of water through which the Cuttle- Fish is able to swim
rapidly backwards. It will be observed that the mantle-cavity
is here at the back, while in the Ormer it is in front, one result
of the twisting which the visceral hump of that animal has under-
gone. In its natural position, whether resting on the sea-bottom
or swimming, the front side of the head and visceral hump is
directed upwards, and this side is much darker than the other.
It may be taken as a general rule that the surface of an
animal habitually facing in this direction is the darkest part
of the body, though its actual nature varies in different animals.
Here it is the front side, but in a Dog- Fish it is the true upper
or dorsal side, and in a flat-fish either the right or left side
according to the species (see p. 279). One peculiarity of the
Cuttle- Fish's skin is especially noteworthy. If a living specimen
is watched, it will be seen that beautiful purplish flushes of colour
sweep over the body from time to time, leaving it comparatively
pale in the interval. The cause of this is to be sought in the
presence of innumerable little rounded colour-bodies (chromato-
phores), which, under the control of the nervous system, vary
in size. When reduced to their smallest dimensions the skin
is pale, but when fully expanded it is dark. A similar pheno-
menon has been described for the Frog (p. 251), where, however,
the colour changes are comparatively slow.

Cutting open the mantle-cavity, we shall find similar parts
and openings to those described for the Ormer (p. 308). In
the middle line there is the projecting end of the intestine, and
on either side of this a kidney aperture, while a plume -like
gill is to be seen on either side. As is well known, the Cuttle-
Fish and many of its allies are able to eject an inky substance
into the surrounding water as a means of protection. This ink
is formed within a rounded ink-bag, and carried off through a
slender tube which has a common external opening with the
intestine.

The front side of the visceral hump has imbedded in it a
broad " cuttle-bone ", composed of overlapping layers of cal-

3i4 CHARACTERS OF INVERTEBRATE ANIMALS

careous material. This is to be regarded as a shell, and is not
internal in the same sense as the internal skeleton of a Verte-
brate, for it is inclosed in a pouch of the skin which has lost the.
opening to the exterior probably possessed by ancestral forms.
Some Molluscs still exist in which the shell is almost but not
quite covered by folds of the skin which have grown over it.

As regards the internal structure of the Cuttle- Fish, it need
only be remarked that there is a very large rasping organ-
(odontophore) ; a systemic heart, consisting of a ventricle and
two auricles; and a nerve-ring, swollen into very large ganglia
and protected by a cartilaginous case. •

The eggs are enclosed in oval cases which are united together
into masses, that have been compared to bunches of grapes in
appearance, and which are among the common objects cast up
on the sea-shore.

Cephalopods are divided into two sub-classes, named, accord-
ing to the number of gills: i. Dibranchiata (Gk. dis, twice;
branchia, gills), of which the Cuttle- Fish is an example; and
2. Tetrabranchiata (Gk. tetra, four; branchia), of which the only
living representative is the Pearly Nautilus.

Sub-class i.— CUTTLE-FISHES (Dibranchiata)

This sub-class is again divided into two groups, Decapoda with
ten arms, and Octopoda with eight. The former includes the
Cuttle-Fishes, of which Sepia is a type, the Squids,, and Spirula.
Squids, or, as they are sometimes termed, Calamaries, have an
even wider distribution than Cuttle-Fishes, for they are not
only found in coastal waters, but are also pelagic, i.e. living in
the open sea, where shoals of them are met with. A common
Atlantic and Mediterranean species which abounds on our shores
is the Common Squid (Loligo vulgaris). This animal is of more
slender build than the Cuttle- Fish, and a large triangular fin
projects from each side. The shell or "pen " is a narrow horny
structure, shaped like a lance-head. The old name Calamary
for creatures of the kind was given in allusion to this "pen5*
(L. calamus, a quill), its shape being compared to a short quill.

Some of the Squid family attain gigantic dimensions,
and there can be little doubt that specimens of the kind are
largely responsible for the numerous tales and legends which

HEAD-FOOTED MOLLUSCS 315

exist regarding a supposed marine monster, the Great Sea
Serpent, or Kraken. Actual measurements which have been
made from time to time of bodies or portions of the bodies of
such creatures leave no doubt that a total length of over 50 feet
may be attained, the greater part of this, however, being taken
up by the long arms. Gigantic Squids of the
kind are sometimes cast ashore on the western
coasts of Britain among other localities.

Spirula (Fig. 180) is a small animal in which
the shell is spiral and divided into numerous
chambers by transverse partitions. It is partly
enclosed in folds of the skin. The animal itself
is but rarely met with, though its shells are
common on Pacific shores and may be seen in
most museums.

The Octopods, or 8-armed Cephalopods, differ
from the Cuttle- Fishes and Squids in the absence
of the two long arms, besides which they are
entirely devoid of an internal shell. The visceral
hump is short and rounded, and the suckers on the arms are
unstalked. The group includes the Octopi and their allies, and
the Paper Nautilus or Argonaut.

The Common Octopus (Octopus vulgaris) is common on
rocky shores on the margins of both Atlantic and Mediterranean,
lurking in crevices, crawling by means of its sucker -studded
arms, or swimming swiftly backwards like the Squids. Each
arm is provided with two rows of suckers, while in an Octopod,
common on British coasts, Eledone moschata, there is only one.
The specific name of this particular species, which is eaten by
the Italians, has reference to the strong musky odour of the
animal.

Some Octopods attain a very large size, though they are
inferior to Squids in this respect. Large specimens are reputed
to be common on the shores of the island of Sark in the
Channel group, and a well-known description of an imaginary
combat with one of these is given in Victor Hugo's Toilers of
the Sea.

The Argonaut or Paper Nautilus (Argonauta argd) is a pelagic
form, common in the Mediterranean, in which the female is pro-
vided with a thin cap-shaped shell, which is symmetrical, and

3i6

CHARACTERS OF INVERTEBRATE ANIMALS

large enough to contain the entire body of the animal. It is
mainly secreted by the inner surfaces of two of the arms, which
are dilated at their ends into large lappets. By means of these
the animal holds on to the shell, which is not attached to it by
any muscular or fibrous tissue.

Sub-class 2 — PEARLY NAUTILUS (Tetrabranchiata)

The only living representative of this is the Pearly Nautilus,
of which the best -known species ( Nautilus pompilius] (fig. 181)
has a wide distribution in the Indian -and Pacific Oceans. The

animal is enclosed
in a large spiral
shell, of which the
coiled -up portion
projects forwards
towards the ani-
mal's front end.
The body does not
occupy all the shell,
for a considerable
part of this is di-
vided into a series
of gas -containing
chambers by means
of curved parti-
tions, concave to-
wards the external
aperture. The last and broadest part forms a body-chamber in
which the animal is contained, the rounded end of its visceral
hump resting against the concave surface of the last partition.
The shell grows in size by successive additions to its aperture
or mouth in accordance with the growth of the animal, and at
the same time the older part of it is from time to time cut
off by formation of a new partition, the body slipping forwards,
as it were, so as to permit of this. The old chambered part
of the shell is not, however, entirely devoid of soft parts, for
each partition is perforated in the middle by a hole continuous
with a short tube directed away from the body-chamber, and in
this way a hollow structure know as the siphuncle is constituted,

Fig. 181.— Pearly Nautilus (Nautilus pompilius}. The left half of its
shell has been removed

SNAILS AND SLUGS 317

which is traversed by a fleshy cord, continuous almost like a
tail with the rounded end of the visceral hump. This is also
connected with either side of the body-chamber by means of a
broad shell-muscle. The shell consists of an external membrane
exhibiting brown and white marking, a white porcelain-like layer,
and an internal layer possessing a beautiful pearly lustre, the
appearance of which has suggested the ordinary name of the
animal.

The foot is not constituted by long tentacles or arms as in
the Cuttle- Fishes, &c., but consists of a number of lobes upon
which are borne a large number of slender adhesive tentacles,
the tips of which can be drawn back into sheaths. A funnel
is present as before, but instead of being a complete tube it is
made up of two halves which are rolled upon each other. Within
the mantle-cavity, which occupies the same relative position as
in a Cuttle- Fish, there are four instead of two plume-like gills, and
in correspondence with this the heart has four auricles, one for
receiving the purified blood from each gill, and there are four
instead of two kidneys. The eye is of extremely simple structure,
and has been compared to a pin-hole camera, consisting as it
does of a deep cup, which would be closed externally were it not
for the presence of an extremely small rounded aperture like a
pin-hole.

CLASS 2.— SNAILS AND SLUGS (GASTROPODA)

The Ormer already described (pp. 307-3 1 1 ) belongs to this
class, that includes a very large number of species, of which the
vast majority are distinguished by the presence of a head bearing
tentacles, a flattened creeping foot, and a shell which consists of
only one valve or piece, and is therefore said to be univalve.
The class is split up into smaller divisions as follows: —

Sub-class i. — Streptoneura (Prosobranchia).

Order (i). Comb-gilled Snails (Ctenobranchia).
Order (2). Shield-gilled Snails (Aspidobranchia).

Sub-class 2. — Euthyneura.

Order (i). Hind-gilled Snails (Opisthobranchia).
Order (2). Lung Snails and Slugs (Pulmonata).

3i8 CHARACTERS OF INVERTEBRATE ANIMALS

Sub-class i.— STREPTONEURA (Prosobranchia)

This subdivision of Gastropods is partly founded on the
course taken by the nerve-loop which is attached to the nerve-
ring. It is here twisted, as, e.g., in the Ormer (see p. 310), into
a shape resembling the figure 8. Another important feature is
afforded by the gill or gills which, when present, are in front of
the heart, as again in the Ormer (see p. 308). These Molluscs
may therefore be termed " fore-gilled " or prosobranch (Gk. pro,
in front of; branchia, gills). They include most of the marine
snails which are to be found on the »sea-shore. On the shape
of the gills, among other characters, the two orders of the sub-
class are marked off from one another, i.e. (i) Comb-gilled Snails,
with a single gill consisting of an axis bearing a series of small
flattened plates, comparable to the teeth of a comb; and (2) Shield-
gilled Snails, in which there are two series of such plates, one on
each side of the gill-axis. In some members of the second order
two gills are present.

Order (i). Comb-gilled Snails (Ctenobranchia). — This order is
divided into no less than fifty-nine families, so that space will
prevent more than a brief notice of a few common forms.

Probably no sea-snail is more familiar than the Periwinkle
(Littorina littored), common on the rocks between tide-marks,
and illustrating a number of points in which the members of
the order differ from the Ormer and related forms. The thick
rounded shell is obviously spiral, and the visceral hump it covers
is of the same shape. The spiral, as in most snails, is a right-
handed one, i.e. with its turns running in the same direction as
an ordinary screw or corkscrew, so that if the shell be placed on
end with apex above, its turns or whorls will be seen to slope
up from left to right. The most primitive Molluscs known are
bilaterally symmetrical, devoid of a prominent visceral hump,
and with a posterior mantle-cavity into which the intestine, &c.,
open. Such spirally-twisted forms as Periwinkle have apparently
arisen from simple forms of the kind by development of a visceral
hump, together with a strong shell to cover it, and also to serve
as a shelter into which the animal might withdraw itself. At
the same time twisting took place, perhaps as a result of the
weight of the parts, and the result has been that mantle-cavity,
end of intestine, gills, heart, and kidneys have been brought

SNAILS AND SLUGS

round to the front (fig. 182). It is at any rate pretty clear that a
spiral hump and shell are more compact and convenient than a
much elongated hump covered by an extinguisher-shaped shell.
In most cases the twisting, as viewed from above, has taken place
in a direction opposed to the hands of a watch, but in some few
snails the opposite has been
the case, so that the spiral shell
is left-handed.

A little observation at the
sea-side will show that Peri-
winkles are in the habit of
•creeping about on the rocks,
feeding on sea -weed, from
which they are able to scrape
small pieces by means of the
rasping organ. The part of
such an animal which protrudes
from the shell will be seen to
be bilaterally symmetrical, and
to mainly consist of a foot much
smaller than that of the Ormer
(see p. 307), and a head pro-
vided with a prominent snout
and two tentacles, each of which
bears a small eye at its base in
the form of a black spot. The
projecting hind-end of the foot
bears upon its upper side a
horny plate, the operculum,
which when the animal is com-
pletely withdrawn into the shell
by means of the shell -muscle
stops up the aperture, thus
guarding the only weak point in the defences. The operculum
corresponds in shape with the aperture or mouth of the shell, which
here, as in vegetarian snails generally, possesses a continuous
margin devoid of any notch.

Examination of the mantle - cavity and the related organs
will show several important points of difference from the Ormer
(see p. 308). As before, the last part of the intestine can be

Fig. 182. — Diagram of a Comb-gilled Snail, seen from above.

The roof of mantle-cavity and overlying shell supposed

transparent.

i, Mouth; 2, brain-ganglion; zb, nerve-cord connecting
side-ganglion (above) with foot-ganglion (below) ; 3, one of
the three ganglia on the twisted nerve-loop; 4, gill; 46, os-
phradium ; 5, opening of intestine ; 6, heart in pericardium ;
8, a gland (purple-gland in Purpura); 9, siphon; 10, 10, foot;
u, operculum.

320 CHARACTERS OF INVERTEBRATE ANIMALS

seen, lying, however, well over on the right side, while on the
left there is a single gill with the comb-like shape characteristic
of the order, and running alongside it a projecting ridge, the
water-testing organ (osphradium). The heart is placed imme-
diately behind the gill, and has but one auricle, placed in front
of the ventricle, which is not folded round the intestine as in
the Ormer. It is indeed exceptional for the intestine of a snail
to run through the heart, though it is characteristic of bivalve
molluscs. There is but one kidney in the Periwinkle, opening
into the back of the mantle-cavity, on the left-hand side. The
suppression of one gill, auricle, and kichiey is believed to be one
result of the twisting of the body, though exactly why is uncertain.
They have perhaps been subjected to pressure, and so to speak
squeezed out of existence. The twisting of the visceral loop in
the nervous system is another result of the coiling of the body, and
this is easily understood.

Two species closely related to the Periwinkle are common
on British coasts. In one (Littorina rudis] the coiled apex or
spire of the shell is very short. The other (L. obtusata) is a
small form, varying in colour from greenish-brown to orange-
yellow, and entirely devoid of a projecting spire, the apex of the
shell being rounded off so as to make the general outline of the
shell spheroidal. It is common on the brown sea- weed (Fucus)
with which 'tween-tide rocks are often thickly covered.

The River-Snail (Paludina), common in the streams of this
country, is something like the Periwinkle in general shape, but
it is a good deal larger and the shell is much thinner.

Living side by side with the Periwinkle on our rocks will
be found the Purple Snail (Purpura lapillus), with a dense angular
white shell extremely unlike the rounded covering of the former
species. It belongs to a different family, and is a good example
of a carnivorous sea-snail. The mouth of the shell is notched
at its front end, i.e. the end away from the spire, for the trans-
mission of the siphon, a spout-like prolongation of the mantle
by means of which water enters the mantle-cavity. The Purple
is one of the forms in which the pharynx with its rasping organ
is situated in the end of a long proboscis, that is retracted when
not in use. The typical genus of this particular family is Murex,
many of the tropical species of which possess extremely beautiful
shells, covered with long spines and having the front angle of

SNAILS AND SLUGS

321

the mouth drawn out into a long canal for the reception of the
siphon. Tyrian purple was obtained from species of Murex
and Purpura, the organ yielding it being a gland in the roof of

Fig. 183. — Whelk (Buccinuni). a a, Tentacles; b, siphon; c, operculum

the mantle-cavity, of which the juice turns purple on exposure
to sunlight.

The Common Whelk (Buccinum undatum) (fig. 183), inhabit-
ing both shallow and deep water around our coasts, closely re-
sembles the Purple in structure,
but is very much larger.

As examples of other families
may be mentioned: — Mitre-Shells
(Mitra), Volutes (Valuta), Olive-
Shells (Olivd) (fig. 184), Harp-
Shells (Harpa), Cone -Shells
(Conus\ Turret-Shells (Turritella\ Wing-Shells (Strombus)
(fig. 185), Helmet-Shells (Cassis), and Cowries (Cypraa) (fig. 186).
In many of these the shells are extremely handsome, and occupy
a prominent place in museums and private collections.

Special mention must be made of the Heteropods, a group
of comb-gilled snails which swim freely in the open sea. The
body in these pelagic forms is transparent, and the foot is a
laterally-flattened fin-like structure, by means of which the animal
swims back downwards. The shell may be spiral (Atlanta) or
cap-shaped (Carinaria), but in some cases (Pterotr ached) is
entirely absent.

Fig. 184.— Olive (OKva)

VOL. I.

21

322

CHARACTERS OF INVERTEBRATE ANIMALS

Order (2). Shield-gilled Snails (Aspidobranchia). — As already
mentioned, these forms possess a gill or gills in which the axis
has a series of plates on either side. The primitive bilateral

Fig. 185. — Strombus
a, Proboscis; 3, notch in shell-mouth; c, eye-bearing tentacles; d, foot; e, operculum.

symmetry of the body has not been disturbed to the same extent,
for both right and left gills, auricles, and kidneys may be present.

There are fifteen families, of which one, the Trochidcz, repre-
sented by twenty British species, may be taken as representing

forms with well-coiled shell,
the colours and markings of

o

which are often of extreme
beauty. Two auricles and
kidneys are present, but
only one gill.

The Ormer (Haliotis
tuberculatd), already de-
scribed (pp. 307-311), is
the type of another family,
and, as we have seen, it
possesses two auricles, gills,
and kidneys. There is good reason to believe that it is descended
from forms possessing a well-coiled visceral hump, covered by a
shell of corresponding shape, and large enough to serve as a
retreat into which the animal could withdraw itself at the approach

Fig. 186.— Cowry (Cypraa)

SNAILS AND SLUGS 323

of danger. Although the hump and shell still retain a certain
amount of twisting they have been flattened out to a large extent,
and the shell no longer serves as a refuge. This, however, is
made up for by the immense size of the foot, by which the animal
can adhere firmly to the rock, at the same time pulling the shell
down so as to cover the exposed parts.

In the much smaller Key-hole Limpet (Fissure Ha Grceca) of
the Mediterranean the visceral hump is completely flattened out,
and the shell is conical, with a hole at the apex communicating
with the mantle-cavity. It possesses two gills, &c., like the
Ormer.

In John Knox's Limpet (Acmcea testudinalis], not uncommon
on certain parts of the British coast, still further changes have
taken place, for there is only one auricle and a single gill. Nor
does the intestine pass through the heart, as is the case in the
shield - gilled forms so far mentioned. The Common Limpet
(Patella vulgata) agrees with this species in most respects, but
has lost both the gills, at least as functional breathing -organs.
If the small mantle-cavity lying above the neck be cut open the
end of the intestine will be seen projecting into it, and on each
side of this the opening of a kidney. On the floor of the cavity
are two little orange-coloured projections, examination of which
as to structure and nerve-supply shows that each represents the
vestige of a gill covered by its water-testing organ (osphradium).
The Limpet, however, does possess gills, though of another kind,
which are seen as a large number of flattened plates running right
round the body well above the foot and overhung by the mantle
skirt, which is a well- developed continuous flap. Since these gills
are not the equivalents of the ordinary plume-like gills characteristic
of Molluscs they are termed secondary gills. This use for the
word secondary is a common one in zoology.

At first sight a Limpet, with its simple conical shell, might
be taken for a very primitive animal. If it were so, however,
the mantle-cavity, with its related organs, would be at the hind
end of the body instead of in front, and the nerve-loop of the
nervous system would not be, as it is, 8-shaped. These con-
siderations, and comparison with other forms, would lead to the
conclusion that the apparent simplicity is secondary, and that
the Limpet's ancestors were forms with spirally coiled visceral
hump and shell. A very interesting confirmation of this con-

324

CHARACTERS OF INVERTEBRATE ANIMALS

elusion is afforded by the life-history, for at an early period of
its existence, when it is a free-swimming larva, it actually does
possess spirally twisted visceral hump and shell. This is another
example of the law of recapitulation previously exemplified (see
p. 14).

Sub-class 2. — EUTHYNEURA

These are forms in which the nerve-loop of the nervous system
is not twisted (except in one family), but it would appear that
this is not, as might at first sight be supposed, a primitive feature,
but the result of an untwisting process1. A further character is
the possession of two pairs of tentacles by the head. There are
two orders : i. Hind-gilled Snails (Opisthobranchia), with the
auricle of the heart behind the ventricle, and the gill in a
corresponding situation ; and 2. Lung Snails (Pulmonata), in
which the gills are entirely absent and the mantle-cavity has
been converted into a lung.

Order I.— HlND-GILLED SNAILS (Opisthobranchia)

A very great variety of marine forms are placed in this order,
some snail-like in appearance, others slug-like, and others again
of modified shape and adapted for swimming in the open sea.
A distinction is drawn between species in which
there is typically a gill sheltered in the mantle-
cavity (Tectibranchs), and the Sea- Slugs (Nudi-
branchs), devoid of mantle and shell.

Among the Tectibranchs the Bubble- Shells
possess a thin translucent spiral
shell (fig. 187), overlapped by a
body- fold (epipodium) each side.

Another related family is exem-
plified by a small white mollusc,
Philine aperta, very common in
shallow water round the British
coast The shell is something
like that of a bubble-shell, but
is not visible externally, as folds of the mantle have grown
completely over it. In the Sea- Hare (Aplysia) the shell is still
further reduced, being a thin oval plate situated on the upper

"-of

Fig. 187.— Bubble-shell (Bulla]

a, Head-lappets; b, right epipodium; c, shell;

d, mantle-lobe; e, hind-end of foot;/", shell.

SNAILS AND SLUGS

325

10

side of the body and almost entirely covered over. A large
gill is present, protected in a mantle-cavity which opens on the
right-hand side (see fig. 188). The external opening of the
single kidney is at the root of the gill, and the intestine terminates
still further back outside the mantle-cavity altogether. The Sea-
Hare was at one time pointed to as a good example of a form
in which the twisting process had begun, carrying the mantle-
cavity with its organs on to the
right side. If this were so, how-
ever, we should expect to find two
auricles to the heart, two gills,
and two kidneys, which is not
the case. A more adequate
explanation is that the Sea- Hare
is descended from forms in which
coiled visceral hump and shell
were present, and which had lost
an auricle, a gill, and a kidney;
forms, in fact, resembling such a
species as the Periwinkle in struc-
ture (see p. 318). We must sup-
pose that in these the visceral
hump gradually flattened out and
the shell gradually became re-
duced, while at the same time a
certain amount of untwisting took
place, bringing back the mantle-

ravitv tn tVlP ricrVlt Vmnrl qiHe of intestine; 6, heart in pericardium ; io,io«, right epipo-

cavity to tn< dium. io^ left epipodium foided over back,

the body. In this way a secon-
dary or spurious simplicity has been acquired. The non-twisted
nerve-loop (fig. 188) is characteristic of Euthyneura generally.

Eight out of the twenty families embraced by the Tecti-
branchs are collectively known as the Wing-footed Snails or
Pteropods (Gk. pteron, a wing; pous, a foot), formerly regarded
as a distinct class of the Mollusca. They are small pelagic
creatures, vast shoals of which are to be found swimming in
the open sea. Some of them possess a mantle-cavity and a
transparent shell either spiral or conical in shape. In these the
foot is transformed into a pair of fin-like structures. Others
again have .no mantle and shell, and though they possess fins,

Fig. 188.— Diagram of aTectibranch Snail, seen
from above

i, Mouth ; 2, nerve-ring with ganglia; 3, one of the
two ganglia on the untwisted nerve-loop; 4, gill, just in
front of which is seen the osphradium ; 5, opening of

326 CHARACTERS OF INVERTEBRATE ANIMALS

these are not formed from the foot, but from the region of the
body immediately above it, and equivalent to flaps found in this
position in the Sea- Hare (epipodia) or to the halves of the funnel
in a Pearly Nautilus (see p. 317).

Sea-Slugs (Nudibranchs). — These are beautifully coloured
creatures with a large creeping foot. Their symmetrical form
is not a primitive character, but due to the untwisting process
just described for the Sea- Hare. Here, however, modification
has gone a stage further, for not only is there no shell, but
both mantle-cavity and the typical plume-like gill are absent.
The intestine opens in the middle line* on the posterior part of
the body.

A common British genus is Doris, in which the absence
of plume-gills is made up for by the presence of a circlet of

branched secondary
gills situated on the
upper side of the
body around the
opening of the in-
testine. If the ex-
Fig. i89.-Eolis

panded gills of a

living specimen be touched they are immediately drawn in, being
sheltered in a ring -like groove when so retracted. Another
common genus is Eolis (fig. 189), in which the back is studded
with numerous slender club-like processes.

A very interesting little Nudibranch is the free-swimming
Mediterranean form Phyllirhoe, which possess a transparent
laterally-flattened body and is devoid of foot. There are numerous
little phosphorescent bodies in the skin.

Order 2. — LUNG SNAILS (Pulmonata)

The seventeen families of this order are mostly inhabitants
of the land or of fresh-water, and familiar examples are furnished
by the land-snails and slugs. The common Garden Snail (Helix
aspersa) may be taken as an illustrative type (fig. 190).

The part of the body which is protruded from the shell when
the animal crawls is symmetrical, and its under part is made
up of the well-developed foot, which has a rounded front end and
ends in a point behind. The head is fairly distinct, and bears

SNAILS AND SLUGS

327

two pairs of tentacles, which are hollow and can be drawn back
into the body. The front pair are short, while the others are
long, and each of them bears an eye at its tip. There is a well-
coiled visceral hump covered by a shell of corresponding shape,
into which the animal can be withdrawn, though there is no
operculum to guard the entrance. During the winter the snail

TENTACLE

BUCCAL GANGLIA

LUNG VEIN

AURICLE "J
VENTRICLE j

PERICARDIUM
RTA

Fig. 190. — Structure of Garden Snail (Helix aspersa)

A, General dissection, from upper side; roof of lung spread out to right. B, Buccal mass, right half removed.

c, Nerve-ring, from back.

remains in a torpid condition within its shell, or in other words
hibernates, under which condition the want of operculum is com-
pensated for by the formation of a limy partition across the
mouth of the shell, leaving, however, a small aperture for
breathing purposes.

As would be expected in a form so twisted, the mantle-cavity
is in front, but no longer has a wide opening to the exterior, as
this would lead to its delicate lining being dried up. There is,
instead, a small aperture on the right-hand side, easily seen just
within the margin of the shell. When the mantle-cavity is opened

328 CHARACTERS OF INVERTEBRATE ANIMALS

no trace of a gill can be seen, but the thin mantle which forms
its roof is raised up into a net-work of ridges traversed by blood-
vessels and acting as a lung. The intestine runs down the right
side of the mantle-cavity to its termination close by the lung-
opening, and by its side runs the slender tube which carries off
the waste matter from the single kidney, abutting against which is
the pericardium containing a two-chambered heart.

The nerve-ring encircles the beginning of the gullet just
behind the pharynx, and the nerve-loop is exceedingly short, and
fused with it in such a way as to be exceedingly difficult to re-
cognize. About 6000 species are inducted in the Pulmonata, and
of these some 3500 belong to the same genus (Helix] as the
Garden Snail. A much larger species than this is the Roman
Snail (Helix pomatia), common abroad, and on the chalk downs
of Kent and Surrey. It is said to have been introduced in
Roman times for culinary purposes.

Land- Slugs may be regarded as derived from snail-like
forms which have been more or less flattened out, and in which
the shell is reduced or, it may be, absent altogether. Two
common British species may be mentioned as examples, the
small grey Field -Slug (Limax agrestis] with a reduced internal
shell, and the much larger Black Slug (Arion ater] in which the
shell is entirely absent.

The Pulmonate forms so far mentioned all agree in the pos-
session of four tentacles, with eyes situated on the tips of the
larger posterior ones. But there are still other forms in which
only one pair of tentacles is present, at the bases of which the
eyes are placed. Among these may be mentioned the Pond-
Snail (Limnaa stagnalis] with a thin, pointed shell, and the
Trumpet -Shell (Planorbis corneus), also an inhabitant of fresh
water, and possessing a flat spiral shell.

CLASS 3.— BIVALVE MOLLUSCS (LAMELLIBRANCHIA)

The shell of a Gastropod, when it possesses one, always
consists of one piece or valve, i.e. is univalve p; but in the class
now to be considered there is a bivalve shell consisting of a right
and a left valve. The Fresh-water Mussels, abundant in many
of our streams, canals, and ponds, furnish a convenient type.
They belong to two genera, Anodon and Unio, which agree in

BIVALVE MOLLUSCS

329

Fig. 191. — Shells of Fresh-water Mussel (Unto], seen from within

ab, Impressions of anterior and posterior adductors; df, hinge- teeth;

e, ligament; h, pallial line.

their main features, though there are certain differences, especially
as regards the shell.

External Characters. — The animal is completely invested by
the two elongated oval pieces which make up the shell, and are
right and left respec-
tively (fig. 191). They
are united together
above, along what is
called the hinge -line,
and can either be
brought close together
elsewhere so as entirely
to cover the soft parts,
or may be separated so
as to "gape" more or
less, which is always the
case in dead specimens.
A series of closely-set
lines following the curve of the shell can be seen on the out-
side, and it will be found that these "lines of growth", which mark
successive additions of material, have as their centre a pointed
projection near the straight upper edge of the valve, known as
the beak or umbo.
This clearly marks
the oldest part,
and, as in bivalves,
usually projects
forwards, and is
nearer the front
than the back end.
When the shell is
removed from the
soft parts by cut-
ting through cer-
tain muscles, it will be found that each valve covers a soft flap
which lines it during life, and is really half the mantle (fig. 192),
here clearly divided into right and left halves, which may be
compared to the flaps of a man's coat, if the further assumption
be made that the coat is a part of the body and its flaps are big
enough to entirely cover the occupant.

fO.ST.ERTO.R- AORT.A

AITRIEC0

ANTERIOR AORTA

Kl

ANTEfcJOTJ RETRACTOR

Fig. 192. — Structure of Fresh-water Mussel (Anodonta}

Gi-Gs, ganglia: Go, opening of oviduct; Ki and K.2, internal and external
kidney-openings

330 CHARACTERS OF INVERTEBRATE ANIMALS

Structure and Formation of the Shell. — The shell is a horny
structure largely impregnated by salts of lime. It is produced
by the underlying epidermis, from which a sticky substance
exudes which afterwards becomes hard. In all classes of Mol-
luscs the mantle has a great deal to do with the formation of
the shell, but even here, where the mantle is very well de-
veloped, it does not line the part of the shell near the hinge-
line, and yet that part can be repaired if broken. In a Garden
Snail the mantle is of comparatively small extent, the chief part
of it being the roof of the lung, yet all parts of the snail's
shell can be repaired. The shell of tfte Mussel, and the same
thing is true for a land- or sea-snail, consists of three layers: (i)
a greenish external skin, often called incorrectly the epidermis;
(2) a middle prismatic layer in which the calcareous material
consists of oblique prisms; and (3) an internal pearly layer,
made up of numerous thin lamellae, the edges of which form a
series of minute wavy ridges on the inner side of the shell, which
are the agents to which the rainbow tints of the pearly layer are
due. The edge of the mantle is thickened, and, both here and
in most Mollusca, plays a very important part in the formation of
the shell, the two outer layers originating from it alone. Repairs,
therefore, except at the edge of the shell, are carried out in
pearly material only, though further observations are wanted on
the subject.

The two valves are united together in the region behind the
umbo by a horny elastic band known as the ligament, which is
kept on the stretch when the shell is closed. In the shell of Unio
(fig. 191) there are projecting teeth along the inner side of the
hinge-line in each valve, which fit into corresponding sockets in
the other valve. Such teeth and sockets are entirely absent in
Anodon, whence its name (Gk. an, without; odous, a tooth). The
inner side presents a number of markings due to the attachment
of muscles, and therefore called muscular impressions. The two
largest of these are oval, and situated respectively near the front
and back ends. They correspond (see fig. 192) to a couple of
large muscles, the fibres of which run transversely across from
valve to valve, and which, since their contraction serves to adduct
or pull the valves together, are termed the anterior and posterior
adductor muscles (L. adduco, I lead to). When they cease to
contract, the elasticity of the stretched ligament comes into play

BIVALVE MOLLUSCS 331

and pulls the shell open. Running from one adductor scar to
the other is a curved pallial line, marking the attachment of the
pallium or mantle to the shell. In a case like this, where the
line is unbroken by any indentation, it is integropalliate, a point
which will be referred to later when other bivalves are considered.

Both shell and contained animal are bilaterally symmetrical, a
fact which is expressed as regarding the former by using the term
equivalve. Each valve is in fact a mirror-image of the other, but
in itself does not exhibit bilateral symmetry, i.e. is inequilateral.

The Mussel is in the habit of remaining obliquely buried in
the mud with its hinder end projecting, and examination of an
aquarium specimen in this position will show two openings, one
above the other, between the mantle lobes (fig. 192). Water-
currents continually set into the lower or inhalent aperture,
serving the double purpose of carrying food to the mouth and
oxygen to the breathing-organs, while other currents as con-
stantly flow out of the upper or exhalent aperture, taking with
them the various forms of waste matter. The inhalent aperture
is fringed with sensitive tentacles, and if these are touched the
shell at once closes, an arrangement which is obviously protec-
tive. The Mussel, therefore, is able to feed, breathe, and get rid
of waste, with most of its body concealed from observation.

After removal of the shell (fig. 192) it will be found that the
mantle-lobes are not united together except between the two
apertures just described. If one of them be turned back other
parts come into view, and the first thing to determine is which
is front and which back end. A distinct head will be looked
for in vain, and its absence is one of the characters of this class,
which sometimes receive the name of " headless " Molluscs
(Acephala, from Gk. a, without ; kephalon, a head). This
cannot be regarded as a primitive feature, and there is good
reason to believe that the bivalves are descended from forms
which possessed a distinct head, the dwindling of which has
been brought about by a sluggish mode of life and dependence
as regards food upon minute organisms brought to the mouth
by water currents. The mouth will be seen in the Mussel as a
wide slit just behind one of the adductor muscles, at the end
further from the inhalent and exhalent apertures, which thus mark
the hinder end of the animal. There is a complete absence of
anything in the way of jaws, but a pair of soft leaf-shaped bodies,

332 CHARACTERS OF INVERTEBRATE ANIMALS

the labial palps, may be seen on either side of the mouth. The
body hangs down between the mantle-lobes, and its ventral part
is modified into the orange-coloured foot, which, instead of pre-
senting a creeping under surface, as in a snail, is flattened from
side to side and projects forwards as a muscular body which has
been variously compared, as regards its shape, to an axe or
ploughshare. It can be protruded from between the valves of
the shell, and serves as a pushing-organ, by which the animal can
slowly plough its way through the mud with its front end first.

Very conspicuous are the plate-like gills, which have suggested
the scientific name of the class (Lat. lamella, a plate; Gk. branchia,
gills), and which are not only breathing-organs but are also largely
concerned with setting up the currents of water which play such
an important part in the life of the animal, being largely aided,
however, in both these duties by the lobes of the mantle. The
water-currents are a result of ciliary action (see p. 49). Each
gill consists of an outer and an inner plate, and, despite its special-
ized form, has been produced by the modification of a gill-plume
similar in kind to those found in the Ormer (see p. 308). The
stem of the gill runs fairly parallel to the long axis of the body,
and is attached to the body- wall above. The mantle -cavity is
here the huge space between the mantle-lobes into which the
gills and lower part of the body hang down, and, by the attach-
ment of the former to adjacent parts, it is divided into a large
lower section into which the inhalent aperture leads and a much
narrower upper portion, above the gills, and communicating with
the exterior by the exhalent aperture. This is most clearly seen
behind the posterior adductor muscle, where the inner plate of
one gill is seen to be fused along the middle line with the corre-
sponding plate of the other gill, thus forming a partition between
the upper and lower sections of the mantle-cavity.

Digestive Organs (fig. 192). — The most striking feature is a
negative one, consisting in the entire absence of the characteristic
rasping organ (odontophore) possessed by the other molluscan
classes. It is believed that bivalves are descended from ancestors
which were provided with this structure, which has been lost as
a result of the same conditions which led to the dwindling of
the head, and which have already been alluded to. The mouth
leads into a short gullet, which opens into a stomach, that again
continues into a coiled intestine, the last part of which runs up

BIVALVE MOLLUSCS 333

to the dorsal side, traverses the heart (see below), and runs back
over the posterior adductor to its termination in the upper section
of the mantle-cavity.

Circulatory Organs (fig. 192). — The heart, situated in a
pericardial cavity, has the dorsal situation characteristic of Inverte-
brates, and is essentially similar to the heart of the Ormer (see
p. 308), consisting as it does of a central ventricle to which a
thin-walled auricle is attached on either side. Purified blood is
received by the auricles from the mantle-lobes and gills, and then
passes into the ventricle, which distributes it to the body.

Respiratory and Excretory Organs (fig. 192). — As already
mentioned, the function of breathing is carried out by mantle-
lobes and gills. As to excretion of nitrogenous waste, this is
effected by two elongated brown kidneys underlying the peri-
cardium, with which they communicate on the one hand, while
they open to the exterior on the other,

Nervous System and Sense Organs (fig. 192). — The central
nervous system consists, as in the Ormer (see p. 310), of a nerve-
ring and a nerve-loop. The former presents a ganglion on each
side of the mouth (equivalent to one of the brain ganglia of the
Ormer with a lateral ganglion fused with it) connected with
one another above and with a pair of foot-ganglia embedded in
the body close to the muscular foot. The nerve-loop is connected
in front with the upper ganglia of the ring, and its posterior end
lies just below the posterior adductor, where it is thickened into
a pair of visceral ganglia. The three pairs of ganglia send out
nerves to the parts of the body in their neighbourhood.

The Mussel undoubtedly possesses the sense of touch, espe-
cially as regards the edge of the mantle. It is also probable,
though not absolutely certain, that it is endowed with smell and
taste, and water -testing organs (osphradia) can be recognized
near the visceral ganglia, though there is some doubt about
their function, for they lie in the upper section of the mantle-
cavity in the course of the outgoing currents of water, which is
not in accordance with their supposed function. So-called organs
of hearing are present, as in the Ormer (see p. 310), in the form
of two little vesicles connected with the foot-ganglia and containing
particles of carbonate of lime. They are probably concerned
with the sense of equilibrium. Eyes are altogether absent.

Lamellibranchs are divided into five orders based on the

334 CHARACTERS OF INVERTEBRATE ANIMALS

characters of the gills, and embrace between them some forty
families. It will be enough for our present purpose to mention
a few common species in illustration of the range of characters
found within the class.

The Cockle Family includes a large number of shallow-water
forms found along coasts and estuaries in most parts of the
world. They are especially characteristic of places where sand
is abundant. The best-known British form is the Edible Cockle
(Cardium edule) of the Atlantic and Mediterranean areas. The
shell is rounded in outline, and marked by prominent ribs which
radiate from the beak. The edges of the mantle - lobes are
much more extensively united than is the case with the Fresh-
water Mussel, but an orifice is left in front through which the
foot can be protruded, and at the back inhalent and exhalent
apertures are seen as before, a striking difference being that they
are here placed at the ends of two short tubes or siphons, both
fringed by tentacles. The narrow bent foot is able not only to
push the animal through the sand, but also by its sudden con-
traction to bring about springing movements in the water. The
siphons can be drawn back within the shell by means of a special
retractor muscle, and the attachment of this to the shell causes
the pallial line to be indented at its posterior end, just as, to use
a somewhat fanciful comparison that has been employed, a coast-
line is broken by a bay or, to use the Latin
word, sinus. By examination of the shell only
we are therefore able to say in a given case
(fig. 193) whether siphons of any size were
present, the extent of the bay being roughly
proportional to their development. This sinu-
palliate condition is contrasted with the integro-
palliate one described for the Mussel (p. 331),
but it must not be forgotten that small siphons may be present
devoid of muscles sufficiently powerful to indent the pallial line
perceptibly.

The Gaper Family presents a certain amount of resemblance
to the preceding as regards habit, and a common British form,
the Sand Gaper (My a arenaria) (fig. 194), is found both in mud
and sand off many parts of our coast, and also on the opposite
side of the North Atlantic. The thick oval shell is not ribbed
like the Cockle, and the name " Gaper " has reference to the

of 9f'Iinupadiiia°ef she'

BIVALVE MOLLUSCS

335

fact that the valves cannot be brought together behind owing
to the presence of enormous siphons that can only be drawn back
into the shell to a limited extent. The two siphons are united
into a single fleshy
mass with two orifices
at its tip, and pro-
tected by a brown

wrinkled layer con- rig. i94.— sand-Gaper (ji/y«

tinuous with the outer

horny layer of the shell. The protective arrangement suggested in
the Mussel (see p. 331) is here carried to a much greater extent,
for when the animal is buried in the mud, with only the tip of the
siphon- tube projecting, it is singularly inconspicuous, though feed-
ing and breathing can go
on without interruption.
The foot of the Gaper is
small, and is of no use for

Fig. 195. — Razor-Shell (Solen}. Foot seen on left, siphons on right

springing.

A closely-related family is that of the Razor-shells (fig. 195),
of which two British species are very common, i.e. Solen siliqua
and Solen ensis. In both cases the shell is very long and narrow,
and gapes at both ends, but in the former species it is straight and
in the latter curved.

Some very interesting boring molluscs come fairly close in
affinity to the Gapers. The Rock-borers (Saxicava) and Piddocks
(Pkolas) (fig. 196), both of which can excavate burrows in hard
rock, include a number of British species belonging to two families,
while a third family is represented in the Atlantic and Mediter-
ranean by the so-called " Ship-worm " (Teredo navalis] that often
completely riddles timber with its burrows, which are lined by a
smooth shelly layer secreted by the surface of the long siphonal
tubes.

All the preceding families belong to one, and that the largest,
of the five orders of bivalves. We may take the Sea Mussel
Family as representing another order, and of the forms included
by far the most familiar, largely on account of its economic import-
ance, is the Edible Mussel (Mytilus edulis] (fig. 197), of which
vast numbers are found together attached to stones, wooden piles,
or other firm objects, by means of strong blackish threads consti-
tuting the byssus. The dark bluish shell is somewhat wedge-

336

CHARACTERS OF INVERTEBRATE ANIMALS

shaped, and the sharp beaks are placed at the extreme front end.
There are no siphons, but merely two apertures, as in the Fresh-
water Mussel, the inhalent one being very large and fringed.
The small dark foot, though capable of being used as a locomotor
organ, is not in constant employment as in free bivalves, which
probably accounts for its relatively small size. The byssus arises
from a deep pit behind the foot, and though it is commonly found

Fig. 196. — Piddocks (Pholas dactylus] in their Burrows

mooring the animal, it can be cast off if necessary, enabling the
animal to creep away to some more desirable spot. The anterior
adductor muscle is much smaller than the other, and this point
is of special interest, because it foreshadows cases where only the
posterior adductor is present in the adult. This is a tendency
to specialization, but the gills on the other hand are simpler in
structure than those of a Fresh-water Mussel, for though they
consist of two plates on each side, yet each of these can easily
be broken up into distinct filaments. The gill is, in fact, a some-
what modified plume-gill in which the separate side-branches of
the plume have not yet firmly united into plates, as in the fresh-
water mussel.

The Ark-Shells constitute a family belonging to the same

BIVALVE MOLLUSCS

337

order as the Marine Mussels, and agree with them in the character
of the gills. The shell is somewhat rectangular in form, with a
long hinge-line possessing many small similar teeth. The foot
has a flattened under-surface, an exception to the general rule
among bivalves. The group is represented by species in all parts

of the world, and some of

1 It ifv JT^to the individual species have

a very wide distribution, as
in the case of the British
species Area lactea.

In the Scallop Family
we have represented a third
order of bivalves which pos-
sess gills more complicated
than those of Ark- Shells and
Edible Mussels, but less so
than in the Fresh -water
Mussel and associated
families. There are several

Fig. 197.— Edible Mussel (Mytiltts edulis)

Fig. 198.— Pilgrim Scallop (Pecten Jacobeeus)

species of British Scallops belonging to the genus Pecten, in which
the shells are fan-shaped. The Pilgrim Scallop (Pecten Jacob&us)
(fig. 198) is interesting as the source of the " cockle-shell " which
the mediaeval pilgrim to the Holy Land wore in his hat. Only
one adductor muscle is present, the equivalent of the posterior
one in the families so far mentioned. In some of the species the
valves of the shell are equal according to the general rule, and
in these the animal is able to swim by flapping them. In other
cases, of which the edible Scallop commonly seen in fishmongers'
shops furnishes an example, the animal is sedentary, and during
life rests with its right valve below, this being well curved, while
the upper or left valve forms a flat lid. A byssus is present,

VOL. I.

22

338 CHARACTERS OF INVERTEBRATE ANIMALS

serving to attach the creature to some firm object. There is in
this genus an interesting variation on the mode of opening the
shell described for the Fresh- water Mussel (see p. 330). There
is no external ligament but what is called an internal ligament
or cartilage, placed in a deep pit at the hinge and kept compressed
when the shell is shut. When therefore the adductor muscle
ceases to contract, the elasticity of this body will come into play,
causing the shell to gape, just as a door might be made to fly
open by the expansion of a piece of india-rubber shut into its
hinge and thereby strongly compressed. The mantle - lobes of
Pecten are quite free from one another, so that not only are
siphons absent but also special inhalent and exhalent openings.
The long plate-like gills follow the curve of the body, and water
has ready access to them through the wide cleft between the
mantle-lobes exposed when the animal opens its shell. The edge
of the mantle is fringed by long tentacles, and bears quite a
number of spherical eyes of a beautiful green colour. These have
a very complicated structure, approaching in some respects to the
eyes of Vertebrates.

The Oyster Family is closely allied to the preceding, but its
members are still more modified. The shell is very irregular,
and the animal is attached by the substance of the left valve,
which in the Common Oyster (Ostrea edulis] of British seas is
hollowed out while the right valve is lid-like, just the opposite
to what is the case in a Scallop. The foot is entirely absent,
and though the mantle-edges bear short tentacles, they are devoid
of eyes.

Brief mention may be made of the Nucula Family as repre-
senting a comparatively small order of bivalves in which the gills
present primitive characters. In the type genus Nucula, for
example, the gill on each side is small and obviously like one of
the gill-plumes of the Ormer, and a further primitive character
is found in the possession of a foot with flattened creeping surface.

CLASS 4.— TUSK-SHELLS (SCAPHOPODA)

This small class includes the typical genus Dentalium and its
allies. A British form, the Common Tusk- Shell (Dentalium
vulgare) (fig. 199), may be taken as a type. It is found burrowing
in the sandy parts of the sea-floor. The curved body is bilaterally

TUSK-SHELLS. PROTO-MOLLUSCS

339

symmetrical, with the upper side concave, and it is covered by
a tubular shell shaped something like an elephant's tusk, whence
its name. This shell has an aperture at each end, the larger
one being in front, and its formation may be understood if we
suppose the presence of two long
mantle -flaps which have fused
together in the middle line below,
and that shelly matter has been
secreted continuously all round.
This view is justified by the develop-
ment. So far there would appear
to be affinity with the bivalve mol-
luscs, and this is confirmed by the
shape of the/00/ — which, however,
has a lobe each side as well as a
central portion — and by the charac-
ter of the nervous system. In other
respects there is an approach to
Gastropods, for though the head is
much reduced it is not entirely
absent, the mouth being placed on
the end of a short non- re tractile proboscis, behind which is a
pharynx provided with the typical rasping- organ.

Springing from the base of the proboscis are two bunches
of long filaments dilated at their ends, and capable of protrusion
for some distance from the mouth of the shell. These are sup-
posed to be of use in capturing small organisms as food, and it
is possible that they represent the gills of other forms, though this
is by no means certain.

Fig. 199.— Tusk-shell (Dentalium}. Shell
partly removed

CLASS 5.— PROTO-MOLLUSCS (AMPHINEURA)

The different classes of Molluscs are all supposed to have
sprung from bilaterally symmetrical forms with fairly distinct
head, a rasping-organ, and a creeping foot. There was probably
a continuous mantle-flap sheltering in the hinder-part of the body
a pair of plume-like gills, near which would open the paired
kidneys, one on each side of the intestinal aperture. The heart
would be dorsal and posterior in position, and would most likely
consist of a muscular ventricle with a thin-walled auricle on each

340

CHARACTERS OF INVERTEBRATE ANIMALS

side. From creatures of this kind we can imagine Cephalopods,
Gastropods, Lamellibranchs, and Scaphopods derived by special-
ization along different lines, the nature of which has been already
indicated in the description of these classes. The small number
of forms which make up the present class have probably retained
to a higher degree than any other living forms the characters

B.

Fig. 200. — Proto-molluscs

A, Mail-Shell (Chiton], seen from above and below.
B, Proneomenia, right side and under surface, c, Chaetoderma.

of these hypothetical ancestors, though they also have undergone
modifications of their own, and it is often a difficult problem to
determine which of their characteristics are primitive and which
not.

The most abundant and familiar of these animals are the
Mail-Shells or Chitons (fig. 200), most of which live under stones
near low-water mark. A common British form is Chiton mar-
ginatus. In the bilateral symmetry, the presence of a fairly well-
marked head, a broad, creeping foot, and a continuous mantle-flap,
the Chitons are probably primitive. The same thing may be
said regarding the dorsal heart, which is placed posteriorly and
possesses the three typical chambers, and the kidneys, which are
paired and open far back. The rasping-organ, however, is highly
complex, and the gills are arranged in a row on each side, instead
of being two in number. Quite possibly, however, the many-

PROTO-MOLLUSCS 341

gilled is more primitive than the two-gilled condition, and there
are some Chitons which have a limited number of gills far back
on each side. As to the shell, we find eight overlapping plates
situated on the dorsal side.

The remaining Proto-molluscs (fig. 200) are unfamiliar marine
forms inhabiting moderately or very deep water. They possess
no shell, but the skin is beset with calcareous spicules, and they
are more or less worm-like in appearance. The foot is either
a narrow ridge placed in a longitudinal groove (Neomenia, Pro-
neomenia), or may be entirely absent (Chcetoderma). There is a
small posterior mantle-cavity into which the intestine and paired
kidneys open, while it may shelter a pair of plume-like gills
(Chcetoderma}, or these may be represented by a tuft of filaments
(Neomenia) or merely by folds (Proneomenia). Though at first
sight the view is tempting that these genera are more primitive
than Chiton, and give an indication of how Molluscs might arise
from worm-like forms, it is more probable that they are simplified
animals which have lost some of the typical molluscan characters,
and are, so to speak, going downhill rather than uphill.

CHAPTER VIII

STRUCTURE AND CLASSIFICATION OF JOINTED-LIMBED
ANIMALS (ARTHROPODA)

This is by far the largest of the great groups of the animal
kingdom, including, as it does, Insects; Scorpions, Spiders, and
Mites; Centipedes and Millipedes; Lobsters, Crabs, Shrimps, and
a host of other Crustacea.

The Lobster, a typical member of the phylum, has already
been briefly described in illustration of the characters of the
higher Invertebrates (see pp. 302-304). It may be convenient
to point out the respects in which it is typical of the group
Arthropoda. These are: bilateral symmetry , the division of the
body into a series of segments grouped into regions and bearing
a series of paired jointed limbs, and the presence of a central
nervous system consisting of a nerve-ring and a ventral cord.
It may also be noted that the so-called body -cavity consists of a
set of blood - containing spaces situated between the internal
organs and the wall of the body. They form, therefore, a part
of the blood-system, while the body-cavity of a Vertebrate (see
p. 42) belongs to the lymph - system. Molluscs agree with
Arthropods in this respect, except that in them the pericardial
cavity does not contain blood, but is comparable to the corre-
sponding cavity in a Vertebrate so far as that particular feature
is concerned. In Arthropods, however, the heart is situated in
a blood-containing space from which blood passes into it.

The phylum is divided into the following classes, which will be
considered in the same order: —

A. — Air-breathing Forms (Tracheata).

Class I. Insects (INSECTA).

Class 2. Spider-like Animals (ARACHNIDA). — Scorpions, Spiders, Mites

Class 3. Centipedes and Millipedes (MYRIAPODA).

Class 4. Peripatus (PROTOTRACHEATA).

342

INSECTS 343

B. — Aquatic Forms (Branchiata).

Class 5. Crustaceans (CRUSTACEA). — Lobsters, Crabs, Shrimps, &c.
Class 6. King-Crabs (XlPHOSURA).
Class 7. Sea-Spiders (PYCNOGONIDA).

A.— AIR-BREATHING ARTHROPODS (TRACHEATA)
CLASS i.— INSECTS (INSECTA)

This class embraces an astonishing number of species, more
numerous, in fact, than those of all other groups of land animals
put together. Yet, in spite of this, they do not exhibit so wide a
range of characters as might be expected, so that the study of a
carefully-chosen type forms an intelligible key to the entire class.
The too -familiar Cockroach or Black ''Beetle" (Periplaneta
orientalis^] furnishes just such a type, though the American
species (P. Americana] obtainable in many seaports, is decidedly
better on account of its larger size.

External Characters (fig. 201). — The body is obviously
divided into three regions — head, thorax, and abdomen — the dis-
tinction between the first two being emphasized by the presence
of a narrow neck. In such an insect as a wasp the demarcation
between thorax and abdomen is equally sharp, while in many
insects, on the other hand, the three regions are bounded by a
continuous curved outline unbroken by constrictions.

Each one of these three regions of the body is made up of rings
or segments, which differ very much among themselves in char-
acter, and in some places are so closely fused together that their
exact number cannot be definitely made out. Arthropods, in
fact, or any large group of them, furnish innumerable instances of
the phenomena described elsewhere (see p. 195) in reference to
the skeleton of the limbs, where all sorts of modification of a
common plan may arise by unequal development of parts, fusion,
and reduction. One of the lower Arthropods, for example, such
as a Centipede or Peripatus, consists of a large number of similar
segments which have only undergone great specialization at the
head end, while in such highly specialized forms as Insects the
segments have been much reduced in number, and are grouped
into regions which, in correspondence with special uses, have
acquired special characters.

B

ANTENN/C

TROCHANTER

'TARSUS

Fig. 201. — External Characters of Cockroach (Periplaneta orientalis]

A, B, c, Side views of male and female, and top view of male; I, 11, in, segments of thorax; i-io, segments of
abdomen. D, A leg (enlarged). E, Jaws (much enlarged). M^, First maxilla; Mx2, second maxilla.

344

INSECTS 345

Among Insects the Cockroach is a fairly central type, and
the least modified part of it is the abdomen, in which ten seg-
ments can clearly be distinguished. A pair of flattened jointed
rods, the cerci, spring from the last segment, and are probably
to be regarded as limbs, which are otherwise absent in this region,
if certain doubtful structures be excepted. This practically
limbless condition of the abdomen is characteristic of insects, and
has been brought about by reduction. The thorax is commonly
regarded as made up of three segments, and each of these bears
a pair of jointed limbs or appendages in the shape of walking-
legs. Each leg is composed of several pieces (10) movably jointed
together, a character which is common to all Arthropod limbs, and
has given the name to the phylum (Gk. arthros, jointed; pous,
a foot). The parts of the leg differ much among themselves
in respect of size and shape. The thorax also bears a pair of
flattened expansions, forming the wings, attached to its second
and third segments, which in the male of the common Cock-
roach, and both sexes in the American species, extend far back-
wards and overlap the abdomen. The fore-wings are horny
structures, and may be termed the wing-covers, since they cover
and protect the delicate membranous hind-wings, or wings proper.
In the female of the ordinary Cockroach the fore- wings are very
small and the hind-wings absent, reduction having taken place in
both cases.

The head consists of a number of segments which have fused
so intimately together that the boundaries between them cannot
be made out. As, however, in the higher Arthropods the presence
of pairs of jointed limbs is taken to indicate the existence of a
corresponding number of segments, a certain clue is afforded,
though, as all segments do not bear limbs, such evidence is
only partial. In this case, as in Insects generally, there are
four pairs of appendages, so that the head possesses at least
four segments. The first of these are two slender jointed feelers
or antennce, serving as organs of touch, and probably also having
to do with other senses. The remaining appendages are three
pairs of jaws, named, from before backwards, mandibles, first
maxillae, and second maxillae. As in all Invertebrates they are
quite outside the opening of the mouth, and those of them which
are used for biting work from side to side. Each mandible con-
sists of a single broad horny piece, toothed on its inner edge,

346 CHARACTERS OF INVERTEBRATE ANIMALS

where it bites against its fellow. The first maxilla on either
side has a two-jointed stalk, upon the end of which are borne a
slender outer and a shorter inner branch. The former is made
up of a number of narrow joints, and is known as a palp, while
the inner branch consists of two parts, a cutting blade next the
middle line and a soft piece adjacent to the palp. The smaller
second maxillce are built on the same lines, but are fused together
to form what is commonly called the " under lip" or labium. It
is the united stalks which have so fused, and though these par-
ticular appendages are considered as jaws, the union has of course
taken away any power of biting against ^ach other. Projecting
from the front of the mouth is a broad horny plate, the "upper
lip " or labrum, and in the narrow space between this and the
labium the mandibles and cutting parts of the first maxillae work
against one another. The Cockroach is clearly an example of
Insects with biting mouth-parts, but in other cases we find
the corresponding appendages adapted to very different uses,
furnishing one of the best examples known of modifications of
a common type.

A large kidney-shaped eye will be seen on each side of
the head, and a good lens will show that each of these possesses
a large number of six-sided facets. An eye like this is generally
known as a " compound eye ", each of the facets having been
formerly regarded as equivalent to an independent simple eye.
Some of the other external characters are mentioned elsewhere.

Skin and Exoskeleton. — The body is invested in a strong
horny covering secreted by the underlying epidermis. Move-
ment is rendered possible by the presence of softer areas between
the firmer tracts, so that the head is not immovably fixed, the
segments of the abdomen can move one upon the other, and
the joints of the limbs can be bent in various directions. In
fact the same problem has had to be solved as that involved
in the construction of a suit of armour, i.e. the combination of
efficient protecting power without undue sacrifice of flexibility.
It is of especial importance to notice that the narrow side of
the body is for the most part provided with a softer investment
than the broad upper and lower surfaces.

Digestive Organs (fig. 202). — The head of the Cockroach
is bent downwards at right angles to the long axis of the body,
so that the mouth-opening at its end faces downwards. It leads

SALIVARY
GLANDS

ABDOMINAL
GANGLIA

6TIGMATA*',

Fig. 202.— Structure of Cockroach (Periplaneta orientalis] (enlarged)

A, Digestive organs and nervous system. B, Air-tubes (tracheae), c, Heart and muscles, i-in, Segments of

thorax; i-io, segments of abdomen.

347

348 CHARACTERS OF INVERTEBRATE ANIMALS

into a small mouth-cavity, from the back of which projects a
pointed tongue, and which passes into a narrow gullet continuous
with a large thin-walled crop, and that again with a smaller thick-
walled gizzard, from which a fairly long intestine runs to its
termination between the cerci. The gizzard is provided with
an internal chewing arrangement, such as is not uncommon
among Arthropods. It consists of a firm lining raised into
horny teeth and bristly ridges. Digestive fluids are poured
into the digestive tube by a pair of branching salivary glands
which open into the mouth-cavity, and a number of club-shaped
"liver' tubes (digestive caeca) which encirtle the beginning of the
intestine.

Circulatory Organs (fig. 202). — This is not very highly special-
ized, for a reason to be mentioned later. The whole of the body
is traversed by irregular blood-spaces of various sizes, but the
only definite tubular structure is the heart, a long narrow blood-
vessel running through the thorax and abdomen close to the
upper surface. Along its sides are numerous pairs of valvular
openings, through which blood passes in from the surrounding
blood-space (pericardial sinus) to be pumped forwards. The heart
is systemic, as it contains pure blood.

Respiratory Organs (fig. 202). — These organs present a very
interesting and remarkable arrangement, for instead of the impure
blood being sent to a localized lung for purification, air is con-
veyed to all parts of the body by means of branching tracheal
tubes. These communicate with the exterior by means of ten
pairs of small openings or stigmata on the sides of the body,
all but the first two pairs being in the abdomen. Observation
of a living Cockroach will show that the abdomen alternately
dilates and contracts, the result being that air is drawn into and
expelled from the stigmata. It is therefore possible to choke
an insect by smearing the sides of the body with oil or some
other substance which will block up these openings. The tracheal
tubes appear silvery under the microscope, owing to the air which
they contain, and they are lined by an elastic membrane thickened
into a continuous spiral thread, so as to render them very flexible
and non-collapsible. Just within each spiracle is a kind of valve,
formed as a projection of the lining membrane and helping to
prevent the intrusion of foreign particles.

As a result of this very thorough system of aeration, the blood

INSECTS

349

never has the opportunity of remaining impure, a fact which ac-
counts to some extent for the imperfect condition of the circulatory
organs. The restless activity of insects, and their great muscular
powers, are also no doubt related to the unusually perfect state of
the breathing apparatus.

Excretory Organs. — These also are very unlike anything so
far described in the animal groups reviewed. They consist of
a large number of very slender Malpighian tubes, which open
into the intestine not far from its commencement.

Nervous System and Sense Organs (fig. 202). — The nervous
system consists of a narrow and much- thickened nerve-ring closely
encircling the gullet, and of a ventral double nerve-cord. The
upper part of the ring is formed by a pair of large brain-ganglia,
sending nerves to the eyes and antennae, while its lower part is
made up of another pair of ganglia, supplying the upper lip and
three pairs of jaws. The ventral cord dilates within the thorax
into three pairs of ganglia which innervate the three thoracic
segments, and within the abdomen into six smaller pairs of
ganglia, of which the first five belong to the corresponding
abdominal segments, while the last and largest pair provide for
the nerve-supply of the last five segments. Segmentation of the
body as exhibited by Vertebrates, Arthropods, and some other
phyla of the animal kingdom, means the existence of a number
of successive rings or segments, containing sections of the various
internal organs. Where the segmentation is very well marked,
many structures are affected by it, and the more primitive the
animal the more closely do the segments resemble one another.
As regards the nervous system of Arthropods, there can be no
doubt that the simplest condition is found in the presence of a
distinct pair of ganglia for each segment, but Insects especially
present considerable modifications of this primitive arrangement.
Where a segment is relatively large, its ganglia will be so too,
as seen in the thorax of the Cockroach, while fusion of segments
will be accompanied by fusion of ganglia, as in the case of the
jaw-bearing segments. The ganglia, further, may be among the
first parts to fuse together, as seen in the last pair of the abdominal
chain in the Cockroach, which represent the ganglia for the last
five segments all united together, though the segments themselves
are still more or less distinct.

The chief sense organs are those of touch — including the

350 CHARACTERS OF INVERTEBRATE ANIMALS

antennae, maxillary palps, labial palps, and cerci — and those of
vision, as represented by the large compound eyes. There is
good reason to believe that the antennae are organs of smell as
well as of touch, and it is probable that some of the mouth parts,
perhaps the second maxillae, have to do with taste. It may be
further remarked that great difficulty attaches to the interpretation
of many of the sense organs possessed by animals (see p. 264),
and Insects, among others, are undoubtedly endowed with special
senses of which it is difficult, if not impossible, for us to form
any idea, our only definite standards being our own sense organs,
which are those of much specialized land animals. In a subsequent
chapter this interesting subject will be more fully discussed.

Life-history. — An insect such as a Butterfly or Moth goes
through widely different stages in its life - history. Hatching
from the egg as a caterpillar, it grows to a considerable size,
and then becomes a motionless chrysalis, from which the adult
insect later on escapes, its body having been formed by a series
of revolutionary changes from the substance of the chrysalis
body. This life-history, familiar to all who have kept silkworms,
and furnishing a useful metaphor to moralists of all centuries, is
one which exhibits complete change of form, or metamorphosis,
to use the technical expression. The life-history of the Frog
(see p. 254) is an example of the same kind of thing, the
essence of which consists in considerable modification in form and
structure after the animal has been hatched out and has started
a free independent existence. Insects differ very much among
themselves as to the amount of metamorphosis, and the Cockroach
exemplifies an order in which it is so slight as to be practically
absent. The eggs are laid in horny capsules, each containing
sixteen, and the just-hatched Cockroach closely resembles the
adult, from which it chiefly differs in its smaller size and the
absence of wings. As growth proceeds, the firm exoskeleton is
not increased in size to suit the enlarging body as in a Mollusc,
but is bodily thrown off or "moulted", a very common phenomenon
among Arthropods. After some seven moults the adult size is
attained.

Classification of Insects. — Considering that some 250,000
distinct species have been described, and that at least ten times
this number are believed to exist, it will be realized that the
classification of insects is likely to present some difficulties, espe-

INSECTS 351

cially as there are not the same obvious distinctions as are found
in some other groups, e.g. Molluscs. Various subdivisions have
been proposed, some of the modern ones being complex, but it
will be the simplest plan to adopt here the comparatively old-
fashioned grouping into nine orders, primarily based upon the
characters of the wings, and which is mainly due to Linnaeus.
It will be enough for the present purpose if some of the commonest
insects are mentioned, and sufficient detail given to enable the
reader to refer an insect to its proper order.

Order I. Bugs (HEMIPTERA). — Four wings, of which the front pair are
often only membranous at the tips.

Order 2. Fringe-winged Insects (THYSANOPTERA). — Four wings, all
very narrow and fringed.

Order 3. Flies (DlPTERA). — A single pair of membranous wings equiva-
lent to the front pair of other insects.

Order 4. Moths and Butterflies (LEPIDOPTERA). — Four wings present,
covered with scales.

Order 5. Beetles (COLEOPTERA). — Four wings, of which the front ones
are converted into hard wing-cases, covering the membranous hind-wings,
which during repose are folded transversely.

Order 6. Membrane-winged Insects (HYMENOPTERA). — Four mem-
branous wings possessing but few nervures, and which cannot be folded up.
Fore-wing larger than the hind.

Order 7. Net -winged Insects (NEUROPTERA). — Four membranous
wings, with an elaborate net-work of nervures.

Order 8. Straight- winged Insects (ORTHOPTERA). — Four wings, of
which the front pair are horny wing-cases, while the others are membranous
and fold up in a fan-like manner when at rest

Order 9. Wingless Insects (APTERA). — Primitive wingless insects.
Insects belonging to other orders may have lost their wings by a process of
reduction affecting one or both sexes.

Order i. — BUGS (Hemiptera)

The English name given to the order is somewhat libellous,
for though a number of repulsive forms are included here, others
again are beautifully coloured and attractive objects. A typical
species is outlined in fig. 203. There is considerable variation
in the matter of wings, as both pairs may be much alike, or the
fore-wings may be wing-cases, or again both pairs may be absent.
In all cases the mouth-parts are converted into piercing and
sucking organs, which, though much unlike the corresponding

352

CHARACTERS OF INVERTEBRATE ANIMALS

parts of the Cockroach both in appearance and use, can be
compared with them part for part. The narrow upper lip
is pointed, and below it are the mandibles, but these, instead of
being cutting-jaws, are long piercing stylets, grooved along their
inner sides in such a way as, when applied together, to form a
double tube down the lower half of which saliva can pass to the
object pierced, while juices can travel along its upper half to the
mouth. Outside, and close to the mandibles, are the first maxillce

in the form of two more
stylets devoid of palps.
The four stylets to-

Fig. 203.— A typical Bug (Capsus)
a, b, First two segments of thorax ; c, d, horny
part of wing-covers; e,/, membranous part of
wing-covers; g, antennae; h,k,h, legs; i, eyes;
sc h, top of head.

Fig. 204. — Cicada, seen
from below

gether constitute an efficient piercing organ which can be pro-
truded or drawn back at will by the action of special muscles,
and it also forms a channel along which fluids can be conducted
to the mouth, as just explained. The second maxillce, including
their palps, are fused together into a sheath for the piercing mouth-
parts. In most cases the adult condition is attained without any
large amount of metamorphosis.

The order is conveniently divided into two sub-orders; i.e.

1. Homoptera, in which the two pairs of wings are similar; and

2. Heteroptera, in which they are unlike.

i. Homoptera. — These all live upon the juices of plants,
and their fore-wings are of uniform texture. Cicadas (fig. 204)
are large, broad insects writh membranous wings of considerable
size, which may be opaque and brightly coloured. The antennae
are very small, and the head not only bears large lateral eyes,
but also three simple eyes or ocelli placed in front and often gem-

INSECTS 353

like in appearance. The males are provided with musical organs
on the under-side of the thorax, by which they are enabled to
make a chirping sound, the well-known " song of the Cicada ".
The eggs are deposited in the branches of trees by means of a
piercing organ with which the tail of the female is provided, and
from them are hatched wingless larvae, which dig into the ground
by means of their fore-legs and subsist on the juices of roots,
ultimately making their way up again and becoming adult. The
larvae may remain as such for a long period, in one North
American species for seventeen years. Cicadas, of which there
are very numerous species, inhabit the warmer parts of the earth,
some of them being natives of South Europe.

Lantern- Flies and their allies make up a closely-related family,
many members of which are beautifully coloured, but the species
found in Europe (including Britain) are not highly endowed in
this respect. The name " Ian tern-fly" has not been so far justified.
Most persons have seen, during the summer months, those
frothy masses on plants to which the name "cuckoo spit" is
vulgarly given. These en-
close the larvae of small insects
belonging to this sub-order,
and known as Frog-Hoppers
on account of their leaping
powers.

Plant- Lice, or Green-Flies,
furnish other examples (fig.
205). The best-known forms Fig. 2o5.-cabbage Aphis (Aphis *«

are the little Preen aphideS I)2> Male (natural size and enlarged); 3,4, female (natural
" size and enlarged)

common on geraniums and

other garden plants, and which, like their immediate allies, are
distinguished by a complex life-history. The Vine -Louse (Phyl-
loxera vastatrix] is a creature of the kind which does immense
damage in vineyards.

The Cochineal Insect (Coccus cacti], on the other hand, is of
economic importance. It is a native of Mexico and Central
America, where it feeds upon cacti.

2. Heteroptera. - - To the insects of this sub-order the name
11 bug " is properly applied. When wings are present, the front
pair are transformed into wing-covers, the tips of which, however,
remain membranous. Many of the species are distinguished by

VOL. I. 23

354 CHARACTERS OF INVERTEBRATE ANIMALS

a disagreeable odour, due to the secretion of certain glands which
open on the under-side of the third segment of the thorax. Some
are terrestrial and others aquatic. Many of the land bugs are
not among the personal enemies of man, but feed on the juices
of plants. These forms mostly escape the observation of those
who are not entomologists, but the wingless blood-sucking bed-
bug (Cimex lectularius) has an effective way of compelling
attention that has rendered it the most widely known among its
order. Even more odious are the small wingless forms known
as lice which infest the bodies of human beings and many other
animals, multiplying with astonishing rapidity. They are regarded
as bugs which have become degenerate as a result of their parasitic
habit. A typical example is the Head- Louse (Pediculus capitis],
which feeds on the blood of the scalp, and lays its eggs on the
hairs, to which they are firmly attached.

The water-bugs comprise a number of forms familiar to the
student of fresh-water life, and to some extent known even
to the casual observer. The skaters, which are often seen
gliding about on the surface of the water, are of this kind, or, to
speak more accurately, constitute a family allied both to land-
and water-bugs. The Needle-Bug (Limnobates stagnorum), with
long attenuated body and deliberate movements, is conspicuous
among them. The Pond- Skater (Gerris paludum) has a stouter
body and moves very rapidly. One genus (Halobatcs) is especially
interesting in being a marine form living upon the surface of the
sea even at great distances from land. This is remarkable, for
though insects are exceedingly dominant as terrestrial forms, and
are far from uncommon in fresh water, they are almost unrepre-
sented in the marine fauna.

Water-bugs proper are inconspicuous insects distinguishable
from their terrestrial allies by their extremely-reduced antennae,
and by the fact that they actually live in the water and not merely
on it like the skaters. The Water- Scorpions are rapacious narrow-
headed forms, in which the fore-legs are used for seizing their
prey. The narrow tail-like prolongation which has suggested
the common name is an arrangement connected with breathing,
as will elsewhere be described. In some species there is a broad
flat body (e.g. Nepa cinerea), while in others it is long and narrow
(Ranatra linearis\ Equally common are the broad - bodied
Water-Boatmen (Notonecta), which, by means of their long hair-

INSECTS

355

fringed hind-legs, row themselves along upon their backs. Like
the forms already mentioned, they prey upon small flies and the
like.

Order 2.— FRINGE-WINGED INSECTS (Thysanoptera)

This is a very restricted order, including minute insects with
suctorial mouth, long narrow body, well-developed slender an-
tennae, and four narrow wings
with a fringe of hairs. The
male is wingless. There

s

also a peculiarity about the
feet, which end in bladder-like
lobes.

Some of the species infest
flowers, e.g. those of Elder;
and the Corn Thrips ( Thrips
cerealiitm] (fig. 206) does

/ V o / tig. 200. — Lorn I hnps (Tnrtps cereahum); i, 2, female walk-

deal of damaP~e tO *n£; 3' 4' fema^e flying- Potato Thrips (Thrips minutissima};

5, 6, larva; 7, 8, female flying; 2, 3, 5, 7, natural size; i, 4, 6, 8,

enlarged.

Order 3. — FLIES (Diptera)

This is one of the largest orders of Insects, and includes not
only innumerable Flies, but also those modified forms which are
known as Fleas. As the scientific name indicates (Gk. dis, twice;
pteron, a wing), only two wings are present (fig. 207). These
are membranous, with comparatively few nervures, most of which
run longitudinally, and they correspond to the fore-wings of other
Insects. The hind- wings, however, have not entirely disappeared,
but are represented by two club-shaped vestiges, which from their
function are known as balancers (halteres\

The mouth-parts (fig. 207) are adapted for sucking, and often
for piercing as well, but there are considerable differences from
the arrangements which serve similar purposes in the Hemiptera.
These structures are best developed in the females of such forms
as Gnats, Mosquitoes, and Gad- Flies, where mandibles, maxillae,
and tongue are in the form of five piercing stylets, ensheathed
in an imperfect tube formed by the labium and partly covered
over by the long sharp labrum. Maxillary palps can be seen
at the base of the proboscis. In the males of these forms, and
both sexes of some members of the group, e.g. House- Fly (Musca

356

CHARACTERS OF INVERTEBRATE ANIMALS

domestica), the piercing parts are much reduced and the mouth
is purely suctorial.

Diptera undergo a complete metamorphosis. From the egg
a limbless larva (maggot) is hatched, which becomes a pupa,

Fig. 207. — Structure of Flies (Diptera}, enlarged to various scales

i, Parts of body: i, head; 0,0, eyes;/, f, antennae; II and in, thorax; c, c, wings; e, e, balancers ; d,d,d,\egs;
iv, abdomen, with segments indicated. 2, Head (front view): a, antennae; b, b, eyes; c, forehead; d, d, upper
lip; e, e, palps; f, body of proboscis; g,g, suctorial flaps of proboscis. 3, 4, Head (side view); a, antennas;

b, eyes; d, lower part of face; e, palp; f, proboscis; gt suctorial flaps of proboscis; h, crown of head. 5, Pro-
boscis (front view); a, a, palps; b,b, suctorial flaps; c, c, stalk. 6, Proboscis (side view): a, palps; b, suctorial
flaps; c, stalk; d, mandibles. 7-10, Antennas of various flies. n, Foot: a, a, claws; b, b, adhesive lappets;

c, bristles.

that may or may not possess the power of movement, and from
which the adult insect or imago is developed. As the adult does

not grow in size, the various -sized Flies
which are often to be seen on windows and
elsewhere are different species, and not, as
often supposed, different stages in the growth
of the same species.

The Common Gnat (Culex pipiens) (fig.
208) is a representative of a very large
family in which the females are distinguished
by their blood -sucking propensities, while
the males are supposed to live upon the
juices of plants. The body is slender,
the legs long, and the antennae are well-
developed, those of the male being beau-
tiful plume-like objects. The eggs are laid on the surface of
ponds, &c., a considerable number being agglutinated together

Fig. 208.— Common Gnat (Culex
), much enlarged

INSECTS 357

into a sort of raft. The larvte which hatch out from them
(fig. 209) look something like little red worms with large heads,
and they possess a special breathing arrangement in the form of
two tubes, each with a stigma at its tip, projecting from the hinder
end of the body. The favourite position of the larva is suspended
head downwards close to the surface, with the ends of these
tubes just projecting above water, and so enabling breathing to

Fig. 209. — Larva of Common Gnat (Culex pipiens), greatly enlarged

be carried on. After several moults, the larva becomes a pupa,
at the front end of which the wings, legs, and antennae of the
adult can be dimly made out under the skin. From each side
of the thorax a breathing-tube projects, and these are used like
those of the larva, except that owing to their position the animal
is suspended tail downwards. The pupa is active, and swims
by jerking its tail. Examination from time to time of almost
any old rain-water butt will enable one to trace the successive
stages of the life-history of the Gnat, the final one of which is
reached by the splitting of the pupal skin down the middle of
the back, making a rent from which the adult Gnat makes
its exit, being floated up meanwhile by the buoyancy of its
investments.

Midges resemble Gnats in many particulars, and pass through
a somewhat similar life-history; but they are much smaller, and
it is only the females of some species which possess mouth-parts
adapted for blood-sucking. The small black Midge (Ceratopogon)
which is so troublesome in the summer months is one of these
forms. The Plumed Midge (Chironomus plumosus\ of which
hosts may be seen dancing together any summer evening, is a
well-known species, distinguished by the great beauty of the
antennae in the male.

358 CHARACTERS OF INVERTEBRATE ANIMALS

The Crane-Flies are much larger insects, of which the Daddy-
Long- Legs (Tipula oleracea and other species) is known to all.
The peculiarly long legs are especially useful in enabling the
animal to progress rapidly through grass. The eggs are laid
in meadows, and the larvae feed upon the roots of grasses.

All the preceding are distinguished by comparatively long
antennae, but in numerous families these appendages are very
short. Well-known examples are the Horse- Stinger or Cleg
(Tabanus bovis\ a large speckled insect, the bite of whose
female has been experienced by most of us in walking through
woods; the notorious cattle pests called * Bot- Flies (Hypoderma
bovis, ox-bot; CEstrus ovis, sheep-bot; Gastrophilus equi, horse-
bot); and the Tsetse - Fly (Glossina morsitans). Here, too,
belong the Blue- Bottle (Musca vomitoria), House-Fly (Musca
domestica], and their allies, constituting a family in which the
piercing mouth-parts, as found in many of the preceding, have
been reduced to mere vestiges, while a complex sucking proboscis
is present, formed chiefly by the labium. The Fly's tongue is one
of the commonest, most beautiful, and at the same time most
complicated, of microscopic objects.

The Fleas are doubtfully associated with the Diptera as degraded
forms living parasitically on the bodies of warm-blooded animals.
The mouth-parts are modified for piercing and sucking, but pre-
sent considerable differences from the arrangement described for
the Gnats, &c. (p. 355). Wings are absent, and so are compound
eyes, the organs of vision consisting only of a simple eye on each
side. The long strong hind-legs are associated with great powers
of leaping. The Common Flea (Pulex irritans] is not, as often
imagined, the same species as those infesting cats, dogs, and
other domestic animals. It appears, indeed, that there are very
numerous sorts of Fleas associated with different Mammalia, even
Bats being attended by their own peculiar species.

Order 4. — MOTHS AND BUTTERFLIES (Lepidoptera)

The insects of this order are, in the vast majority of cases,
so characteristic-looking that they can be recognized at first sight
mainly because they possess two large pairs of wings covered with
minute variously-shaped scales (fig. 210), easily rubbed off as what
is popularly called the " dust " of the wing. The presence of

INSECTS

359

Fig. 210.— Scales from Wings of various Butterflies
(greatly enlarged)

these scales gives rises to all sorts and combinations of colours,
often of extreme beauty. The male and female of the same
species are often very different in appearance.

The head, which is well-marked off from the thorax, bears
long antennae and prominent compound eyes, while its mouth-
parts are converted into a
\ongproboscis, carried rolled
up into a spiral when not in
use (fig. 2 1 1 ). This organ
is used to suck up the nectar
of flowers or, in some cases,
liquid matter of a less sa-
voury kind, but differs en-
tirely in structure from the
corresponding organ of a
Fly. The upper lip is incon-
spicuous, and the mandibles
have disappeared altogether,
while the second maxilla are fused into a small plate bearing two
conspicuous palps. The sucking part is formed entirely from the
much -elongated first maxillcz, each of which is a long jointed
structure grooved deeply on its inner side, so that when approxi-
mated to its fellow a
tube is formed, the
firmness of which is
often enhanced by a
series of interlocking
hooks.

The thorax bears
not only the wings,
but three pairs of
weak legs, and is
fairly well marked
off from the abdomen, which is generally elongated, but may either
be very slender or else broad according to the species.

The life-history of a Moth or Butterfly furnishes a good
example of complete metamorphosis. The eggs are laid upon
some special food-plant, and the larvae which hatch out from
them are ravenous caterpillars which feed upon vegetable matter
or, more rarely, other substances. The head is provided with

Fig. an.— Heads and Proboscides of various Butterflies (enlarged)

360 CHARACTERS OF INVERTEBRATE ANIMALS

an extremely short pair of antennae, a group of simple eyes on
each side, and biting mouth -parts. The cylindrical trunk is
composed of eleven segments, the first three of which bear
jointed legs corresponding to those of the adult; and besides
this, from one to five of the other segments possess stumpy pro-
legs which have sucker-like ends. Those at the posterior end
of the body are often modified in various ways.

Caterpillars vary much in size, colour, and other characters,
among which may be mentioned presence or absence of hairs,
some being smooth and hairless, and others so hirsute as to
have earned the popular name of " woolly bears ". After leading
a life entirely devoted to continuous feeding for a period varying
from weeks to years, during which time the skin is frequently
moulted as the body increases in size, the caterpillar becomes
lethargic and passes into the quiescent pupa stage, constituting
what is generally called a chrysalis. A continuous horny covering
invests body and limbs alike, beneath which the parts of the
perfect insect can be dimly traced. Innumerable methods of
concealment and protection ward off to some extent the attacks
of enemies during this helpless period. In some cases the
chrysalis is to be found above-ground suspended by a silken cord,
or fastened to some object by a girdle of the same material, in
which cases its colour commonly harmonizes with the surround-
ings and renders it inconspicuous. Other chrysalides are sheltered
underground, and others again are to be found within cocoons, of
which the silkworm is the most familiar illustration.

The last stage is reached when the perfect insect or imago
issues from the chrysalis at a time dependent upon favourable
conditions of temperature and various other factors, such, e.g.,
as a suitable state of the particular food-plant upon which the
eggs are laid.

The most convenient way of subdividing Lepidoptera is into
the two groups of (i) Butterflies (Rhopalocera) and (2) Moths
(Heterocera), of which the latter include a very much greater
number of species, there being in Britain, for example, over
1900 sorts of Moths as against between sixty and seventy species
of Butterflies.

i. Butterflies (Rhopalocera). — Butterflies for the most part
are active in the daytime, especially during sunny weather.
They can readily be distinguished from Moths by the antennae,

INSECTS 361

which are club-shaped, and in most cases by their habit of
shutting the wings together over the back when they alight.
As the wings are far less brilliantly coloured on their under
sides, which are then the only surfaces visible, a very efficient
means of protection is afforded. The body is comparatively
slender, and the demarcation between thorax and abdomen well-
marked. The eggs are often sculptured in a way which renders
them extremely beautiful microscopic objects, and the caterpillars
may be naked, slightly hairy, or covered with branching spines.
They posses five pairs of pro-legs. The angular chrysalis is
sometimes simply suspended by the tail, or it may be fixed head
upwards both by the tail and by a silken girdle round the thoracic
region. It is frequently marked with metallic patches, to which
the name chrysalis is due (Gk. chrysos, gold).

Butterflies are found in all parts of the world, and some
common examples may here be mentioned.

The Fritillary Family is at once the largest and most widely
distributed group of Butterflies. They are distinguished by the
remarkable fact that their fore-legs are so much reduced as to
be useless for walking purposes.
Among British species are the
Great Tortoiseshell ( Vanessa
polychloros] (fig. 212), the Small
Tortoiseshell (V. urtic<z\ the
Peacock Butterfly ( V. Io\ and
the Red Admiral ( V. Atlanta],
in all of which the larvae feed
on nettles: the Painted Lady
{ V. cardui], an almost cosmo-
politan species, of which the
larvae feed on thistles; and the
Purple Emperor (Apatnra Iris), a much rarer and finer insect,
with a predilection for carrion. Closely related to these is the
Resplendent Ptolemy (Morpho Neoptolemus], a gorgeous tropical
Butterfly. Morpho cypris is brilliant blue, streaked with white.

The family of Whites includes several British species which
are perhaps more familiar than any other, partly on account of
their conspicuous colour and partly owing to the ravages which
their larvae perpetrate in the kitchen -garden. The abdomen is
enveloped by the basal part of the hind-wings. This family is

362 CHARACTERS OF INVERTEBRATE ANIMALS

almost universally distributed, and British species are: — the
Black - veined White (Aporia cratcegi\ the Large White or
Cabbage Butterfly (Pieris brassiccz], the Small White or Garden
White (P. rap<z\ and the less common Green -veined White
(P. napi)\ the beautiful little Orange Tip (Anthocharis carda-
mines), so named from the markings on the wings of the male;
and the handsome Brimstone Butterfly (Gonopteryx rhamni), with
angular outlines to the wings.

The Swallow -tailed Butterflies, which rival the Whites in
their range of distribution, include numerous gorgeous tropical
species, and the common name has reference to the slender "tail'*
into which the posterior margin of each hind-wing is often pro-
duced. The type genus, Papilio, comprises over 300 species,
including our largest British form (Papilo mackaon), now limited
to the fen district. The huge Bird-winged Butterflies (Ornithop-
terd] of the Malay region belong to the same family, though they
are without tails.

The family of Blues and Coppers is represented in all parts
of the world, many of the tropical species being exceedingly
beautiful. Two common British species of small size may be
mentioned in illustration: the Common Blue (Polyommatus alexis)
and the Small Copper (Chrysophamis phlceas}.

2. Moths (Heterocera). — Moths are mostly nocturnal forms,
differing from Butterflies in many particulars, among which may-
be mentioned the absence of knobs on the ends of the antennae,
and the habit of reposing with the wings spread out horizontally
or folded round the body. The body is often much thickened,
and the demarcation between thorax and abdomen is not well
marked. In most cases there is a special " hook-and-eye "
arrangement for uniting the wings during flight, and consisting
of one or more bristles at the root of the hind-wings, which fit into
a socket on the under side of the adjoining part of the fore-wings.
The larvae are either naked or more or less hairy, and may possess
as many pro-legs as those of Butterflies, or fewer. The pupae
are rounded in outline, usually enclosed in a cocoon, and in many
cases concealed underground.

The group may be divided into (I) Large Moths, including
Hawk-Moths, Clear- Wings, Spinners, Owlets, and Loopers; and
(II) Small Moths, embracing Leaf- Rollers, Leaf-Miners, Plume-
Moths, and many others.

INSECTS 363

I. Large Moths, as the name indicates, are usually of con-
siderable size, and agree with the Butterflies in the following
characters: — The wings are elaborately veined, and the larvae,
which feed upon leaves, have downwardly-directed heads, and,
usually, a curved line of horny bristles near the tip of each pro-leg.
The abdomen of the pupa is devoid of transverse rows of spines.

Hawk-Moths are large swiftly-flying insects with a very long
proboscis, suited for draining honey from flowers (such as honey-
suckle) possessing a very long tubular spur. The caterpillars
have smooth brightly-coloured skins, and are provided with five
pairs of pro-legs, while a horn-like projection is found on the
upper surface, near the hinder end of the body. The pupae are
found underground, enclosed in cocoons of earth. British ex-
amples are the Death's- Head Moth (Acherontia Atropos}, with
markings on the upper side of the thorax looking like a skull and
cross-bones, and the Privet Moth (Sphinx ligustri). The larva
of a common European species, the Pine Hawk- Moth (Sphinx
pinastri), is very destructive to pine-trees.

The Clear- Wings are remarkable from the absence of scales on
the wings, owing to which, and the nature of their markings, they
resemble wasps, &c., this being no doubt a protective arrange-
ment. The Hornet Clear- Wing ( Trochilium apiforme) is a typical
native species.

Spinners are large clumsy moths, clothed with abundant hair,
and provided with a short proboscis. The colours are dull, and
the two sexes differ considerably in appearance, e.g. in the character
of the antennae, which are plume-like in the male, and thread-like
in the female. The caterpillars are more or less hairy, and before
becoming pupae spin cocoons, which may be entirely of silk, or
contain a number of cast hairs in addition. One of the most
beautiful British species belongs here, the Emperor Moth (Saturnia
carpini\ with an eye-like marking on each wing, and the cater-
pillar, abundant on heather, of emerald-green with pink tubercles.
The spread of wing may be as much as 3 inches, but this
appears small by comparison with exotic species, especially the
Atlas Moth (Attacus atlas), in which the wings measure about a
foot across. Among other native species may be mentioned
the large Goat Moth (Cossus ligniperda), with wood-boring larva;
the Tiger Moth (Arctia caja)\ the Puss Moth (Cerura vinula)y
with an extraordinary-looking caterpillar; the Buff-Tip (Pygcera

364

CHARACTERS OF INVERTEBRATE ANIMALS

Fig. 213. — Cabbage Moth (Mamestra brassicce)
i, Caterpillar; 2, chrysalis; 3, adult.

bucephald) *, the Pale Tussock- Moth (Dasychira piidibunda) ; the
Lackey (Clisiocampa neustria)', and the Oak Eggar (Lasiocampa
quercus}. Spinner Moths, however, are best known from the
Silkworm Moth (Bombyx mort), a native of China introduced into
many countries for the sake of its silk.

Owlets form the largest group of the Lepidoptera, and include
dull-coloured species, with comparatively small fore-wings, each

of which is characteristically
marked with a couple of spots,
one round and the other some-
what kidney - shaped. The
larvae of many species, as
"surface caterpillars ", do much
harm to crops, and the pupae
are found underground, en-
closed in earthen cocoons.
Common British species are:
the Common Wainscot Moth
(Leucania pallens], the Cab-
bage Moth (Mamestra brassicce] (fig. 213), the Heart-and-Dart
Moth (Agrotis exclamationis], and the Silver Y (Plusia gamma}.
Loopers are moths of slender build, with broad thin wings and
small antennae. The caterpillar has but two pairs of pro-legs,
placed right at the posterior end of the body, and this necessitates
a curious mode of locomotion, which gives the name to the group.
The body is stretched out to its full length, and then, while holding
firmly by means of its three ordinary legs, the pro-legs are brought
up and fixed close behind them, the body being thus thrown into
a loop. Now, holding firmly by the pro-legs, the body is stretched
forwards, and the ordinary legs attach themselves again. By
repeating these manoeuvres the caterpillar can move rapidly along
in a curious looping fashion. When at rest the larva has a curious
habit of attaching itself by the pro-legs, and stretching out the
body at an angle to the surface of attachment, at the same time
stiffening itself. This position can be retained for hours, and
the caterpillar looks so like a short bare twig that it is rendered
extremely inconspicuous to its enemies. Common British species
are: the Brimstone Moth (Rumia cratczgata), the Magpie or Cur-
rant Moth (Abraxas grossulariata), the Winter Moth (Cheimatobia
brumata), and the Chimney Sweeper (Tanagra chcerophyllatd).

INSECTS

365

II. The Small Moths, as their name indicates, differ from
the members of the other group in size, and usually possess long
slender antennae. The caterpillars, which burrow in vegetable
substances or conceal themselves by rolling leaves together, have
their heads forwardly directed, and a circlet of spines near the
tip of each pro-leg, of which there are five pairs. The pupae
are generally distinguished by the presence of transverse rows of
spines on the upper side of the abdomen. Among the groups
may be mentioned Leaf- Rollers, Leaf- Miners, and Plume- Moths.

Leaf -Rollers are so named from the habit many of the larvae
have of feeding either between leaves which they have glued to-
gether with silk, or else inside individual leaves which have been
rolled up and fixed in a similar way. Common British species
are: the Green Oak Moth (Tortrix viridana), and the Codlin
Moth (Carpocapsa porno
nella], of which the cater-
pillar tunnels within the
fruit of apples and pears.

The Leaf -Miners con-
stitute a large group of
small and very small moths

Fig. 214.— Adult stage of a Clothes
Moth (enlarged)

Fig. 215.— Larvae of a Clothes Moth

(enlarged)

with narrow hair-fringed wings. One of the prettiest species is
the Little Ermine- Moth (Hyponomeuta padella], the larvae of
which keep together in companies as hatched out, and spin a
considerable amount of web. They devastate the leaves of
hawthorn, sloe, &c. Another beautiful but extremely small form
is the Brown Dolly (Lithocolletis corylella), the caterpillars of
which mine in hazel leaves. Unfortunately, however, the best-
known are the different species of Clothes Moth (Tinea pellionellay
Trichophaga tapetzella) (figs. 214 and 215).

The Plume-Moths are a comparatively small group of pretty
little long-legged moths with wings split up into a varying number

366

CHARACTERS OF INVERTEBRATE ANIMALS

of plume-like portions. Common British species are : the Common
Plume- Moth (Pterophorws pterodactylus] and the Twenty- Plume
Moth (Alucita polydactyla).

Fig. 216. — Lady-Birds

i, Cluster of eggs; 2, egg (greatly magnified); 3, larva (mag-
nified); 4, actual length of same; 5, 6, pupae; 7, 8, varieties of
Two-spotted Lady- Bird (Coccinella bipunctata); 9, Seven-
spotted Lady-Bird (C. septempunctata}.

Order 5. — BEETLES (Coleoptera)

This is by far the largest order of insects, and includes forms
which are for the most part easily recognizable, though the name
of " beetle " is popularly but erroneously given to members of

other groups, e.g. to the Cock-
roach. "The wings of an aver-
age beetle are of very charac-
teristic appearance and nature,
as may be seen by examining
such a typical example as the
little Lady- Bird (Coccinella),
which is at once known by its
conspicuous colouring of black
spots on a red ground (fig.
216). At first sight such
an insect appears to have no
wings at all, but every child who has induced a lady-bird to "fly
away home ", by persuasive shoves added to an alarming story
of domestic calamity, knows better than that. The fore -wings

are not, however, organs of
flight, but horny wing-covers
or elytra (see p. 345) stretch-
ing back over the abdomen,
and abutting against one
another in the middle line.
They protect the membran-
ous hind -wings (fig. 217),
which, when not in use, are
hidden beneath them, and
are not only folded longitu-
dinally, as in a Cockroach,
but also transversely, a very
characteristic feature for
beetles. The head is large, and bears a pair of compound eyes,
and two antennae varying greatly in shape according to the

Fig. 217.— Stages of Turnip Flea-Beetle (Haltica nemorum}

i, Adult (enlarged), showing wing-covers and wings spread
out ; 2, 3, natural size of same ; 4, 5, eggs (enlarged) ; 6, 7, bur-
rows of larvae (enlarged); 8, 9, larva (natural size and enlarged);
10, n, pupa (natural size and enlarged).

INSECTS 367

species. The mouth-parts are adapted for biting, and are built
on the same plan as in the Cockroach (see p. 345), but with many
differences in detail, as seen, for example, in the more complete
fusion which has taken place between the two second maxillae, and
in the fact that these jaws are much reduced. The life- history of
a beetle (figs. 216 and 217) exhibits a complete metamorphosis,
the larva being a grub possessed, in most cases, of three pairs
of legs corresponding with those of the adult, and becoming a
pupa differing from that of a moth or butterfly as regards the
limbs and wings, which form prominent projections instead of
being merely indicated beneath the continuous
horny covering.

Only a few common species, representing
important families, can be mentioned here.

Tiger-Beetles are very active, predaceous in-
sects, including somewhere about a thousand
species, and distributed generally throughout the
world, though most abundant in the tropics. A
common British form is the Green Tiger- Beetle
(Cicindela campestris) (fig. 2 1 8), common on sandy FJ 2l8._Green T*ger-
banks, and distinguished by the beautiful golden- ^tle (Odndeia campes-
green colour of its upper surface. The larva is
provided with enormous curved mandibles, and excavates a
vertical burrow in which it lies in wait for prey.

Ground- Beetles are also predaceous, and even more widely
•distributed than the Tiger- Beetles, though much more numerous,
there being some eleven thousand species. They are least
abundant in the tropics. A large and well-known British species,
the Violet Ground - Beetle (Carabus violaceus), so named on
account of the dark violet sheen exhibited by its upper surface,
is common in fields and gardens, and may also be found in houses,
where it preys upon cockroaches and crickets.

Water-Beetles include ravenous forms which resemble ground-
beetles in many respects, but are adapted to an aquatic life. The
largest British form is the Great Water- Beetle (Dytiscus marginalis]
(fig. 219), and here may also be included the Whirligig Beetle
(Gyrinus natator], which almost everyone must have noticed
twirling round and round at the surface of the water in ponds
and ditches.

Rove-Beetles, possessing broad heads, narrow7 bodies, and short

368

CHARACTERS OF INVERTEBRATE ANIMALS

elytra, agree with the preceding families in their carnivorous
habits. Most familiar perhaps in Britain is the Devil's Coach-
Horse (Ocypus olens), which has the curious habit of turning up its

tail when molested (fig. 220).

The Scarabs rival the
ground-beetles in number of
species and include many large
and handsome forms, distin-
guished by characteristic an-
tennae, of which the last few
joints *are so broadened out
that when expanded they look
like a small fan. The adults
and larvae feed either upon

Fig. 2i9.-Great Water-Beetle (Dytiscus marginalis) Vegetable SubstanCCS Or OR

Male to left, and female to right. Part of the fore-foot of dung. Here belongs the lar-
male (enlarged) is represented in centre to show the pad and . T> V V. <-U C*.

suckers with which it is provided. gest British species, the Stag-

Beetle {Lucanus cervus}, in

which the mandibles of the male resemble antlers. The largest-
known beetles are not very distantly related, and forms com-
monly seen in museums are the Hercules - Beetle (Dynastes

hercules) from tropical Ame-
rica, and the Goliath- Beetle
(Goliathus Drurei), the male
of the former species some-
times exceeding 5 inches in
length, as against the 3.
inches of our Stag -Beetle.
One of the commonest Bri-
tish dung - beetles is the

Fig. 220.— Devil's Coach-Horse (Ocypus olens) Dumble- Dor ( GeotrUpCS SteV-

i, Standing with turned-up tail; 2, flying; 3, head enlarged, rnMriu S\ A d 1 1 am ^Vl i n QPP t
to show eyes, antenna, and jaws. COTariUS), a ,

of bluish-black colour, often

seen crawling slowly along country roads. An allied genus
includes the Sacred Scarab (Scarabceus sacer) of the Egyptians.
The Chafers constitute a large and well-known group of the
Scarab Beetles. Common British species are the Cockchafer
(Melolontha vulgaris], the green-and-brown Garden Chafer (Phyl-
lopertha horticola), and the beautiful golden-green Rose Chafer
(Cetonia aurata] (fig. 221).

INSECTS

369

Weevils are small beetles with
damage to timber, fruit, and grain
are known. A common Bri-
tish form is the Nut Weevil
(Balaninus glandium), which
lays its eggs in hazel-nuts
and acorns, upon the kernels
of which the larvae feed when
they have hatched out. Two
species of Weevil are repre-
sented in fig. 222.

Lady-Birds (fig. 216) are
among the insects which are
of great use to man, as they

long snouts which do great
Some ten thousand species

Fig. 222.— Corn-Weevil

i, Grain of wheat, showing the punctured hole; and 5, the exit
of the perfect weevil. 2, Pupa (natural size); 3, magnified.
4, Grain of Indian corn, with weevil inside. 6, 7, Corn- Weevil
(Calandra gra.na.ria], natural size and magnified; 8,9, Rice-
Weevil (C. oryzee], natural size and magnified.

Fig. 221.— Rose Chafer (Cetojiia aurata)
i, Adult; 2, larva; 3, cocoon; 4, pupa.

prey upon plant-lice, to which
their larvae are a veritable
terror. Over a thousand
species are known distributed
throughout most regions of
the globe. The commonest
British species are the Seven-
spotted Lady -Bird (Cocci-
nella septempunctata) and the
Two -spotted Lady -Bird (C.
bipunctata), distinguishable by
the number of black spots
upon the red elytra.

Order 6. — MEMBRANE-WINGED INSECTS (Hymenoptera)

This very large order contains many thousand described species,
including among many others the different kinds of bee, wasp,
and ant. Not a few live in communities of exceedingly complex
organization, and these display so much intelligence that there is
much to be said for the view that the order is the highest among
insects. Four membranous wings (fig. 223) are present, whence
the technical name (Gk. hymen, membrane; pteron, wing); they
possess comparatively few nervures, and are not folded during
repose. It will be remembered that in most Moths there is a
" hook- and - ey e " arrangement for coupling the wings together

VOL. I.

24

370

CHARACTERS OF INVERTEBRATE ANIMALS

HIND-WING

during flight. In the present order the same end is brought
about by a more perfect contrivance. On the front margin of
the hind-wing, which is smaller than the fore-wing, a series of
hooks will be found which catch on to a fold on the hinder
margin of the latter, and due to a curling up of the edge. The
complex mouth-parts are adapted for both biting and licking, as
may be seen by careful examination of a hive-bee (fig. 223).

The triangular upper lip will here be
seen to overlap a pair of powerful man-
dibles^ under which come the long Jirst
maxillce, provided with cutting -blades
and palps (in this species very small), and
lastly follow the fused second maxillce.
These are drawn out into a slender
tongue-like structure suited for licking,
and grooved for the conduction of liquid.
Female Hymenoptera are either pro-
vided with a sting at the posterior end of
the body, or else with a piercing arrange-
ment (ovipositor) for making holes in
which to lay the eggs. The metamor-
phosis is particularly well-marked. Larvae
resembling caterpillars, or it may be of
worm-like appearance, hatch out from

Fig. 223. — Structure of Hymenoptera . i • i i -11

A, Wings of a bee. e, Diagram of mouth- *C CggS, which haVC prCVlOUSly been

parts of Honey-Bee (Apis meiiifica], much deposited in plants, the bodies of other

enlarged and widely separated. A

insects, or sometimes in specially-con-
structed chambers (as in bees). The pupae are generally enclosed
in a cocoon of silk, and, as in beetles (see p. 366), their limbs
project freely.

Three sub-orders are recognized: — i. Plant- Eaters, 2. Insect-
Eaters, and 3. Stinging Hymenoptera.

i. Plant -Eating Hymenoptera include the Saw -Flies and
Wood-borers, of which some thousand species have been described.
The ovipositor of the female is adapted for boring holes in plants,
and the abdomen does not in either sex narrow to a stalk at its
base, as in a wasp or ant. The mouth-parts are not so specialized
as in the example taken above. The larvae resemble caterpillars,
for which indeed they are often mistaken, but may easily be dis-
tinguished by the presence of more than five pairs of pro-legs in

INSECTS

Fig. 224. — Turnip Saw-Fly (Athalia spinanim
i, Adult female, enlarged (natural size represented to left of it);
3, 2, Eggs (natural size and enlarged) ; 4, 5, 6, larvae ; 7, cocoon ;
8, pupa in cocoon.

addition to the three pairs of legs proper, while a caterpillar never
has more than five pairs of pro-legs, and often fewer. Nor do
these pro-legs possess, like those of most caterpillars, a curved row
or circlet of minute bristles
near their tips. Further, a
"false" caterpillar has a
rounded instead of a flat-
tened head, moves its ab-
domen vertically up and
down when disturbed, and
curls it up in a state of rest.

The "saw" of a saw-fly
consists of two curved saw-
like blades protected by
sheaths when not in use.
The blades are worked alter-
nately backwards and forwards, and the eggs slip down between
them into the incision formed. A common species is the Turnip
Saw- Fly (Athalia spinarum) (fig. 224), the larvae of which ravage
the crop after which the insect
is named.

In a Wood-Borer the blades
corresponding to the saws of a
saw-fly are fused together into
a boring spine, which is used
like an auger. A conspicuous
species, not infrequently seen
in this country, is the Large
Wood- Borer (Sir ex gigas) (fig.
225), also called Wood- Wasp
from the transverse black and
yellow bandings of its body.
The female bores holes in pine-trees in which to lay her eggs,
and from these, eyeless grubs hatch out which only possess the
three pairs of ordinary legs. These larvae burrow in the timber
by means of their strong mandibles, and later on become pupae
enclosed in cocoons formed from silk mixed with fragments of
wood.

2. The Insect - E ating Hymenoptera are so named because
the larvae are commonly parasitic within the larvae of other insects,

Fig. 225. — Large Wood- Borer or Wood-Wasp (S ire x gigas}

372 CHARACTERS OF INVERTEBRATE ANIMALS

though to this there are many exceptions. The abdomen is
stalked, and the female is provided with a piercing ovipositor by
which punctures are made for the reception of eggs. The larvae
are pale legless grubs. It is stated as probable that the sub-
order contains 20,000 species or even more, which means an
innumerable host of enemies to other insects. The Winter Moth
(Cheimatobia brumata], for example, is attacked by no less than
63 different kinds of Hymenoptera belonging to this group. Two
leading families are the Gall- Flies and Ichneumon- Flies.

Gall-Flies are so named because they puncture plants for
egg-laying purposes, with the result that trTe wounded parts give
rise to those peculiar excrescences known as "galls", of which
the spherical brown bodies called " King Charles's oak-apples ",
common on the oak, are known to everyone who has been in
the country. This tree indeed is peculiarly liable to the attacks
of different species of gall-fly, which lead to the production of
galls of totally different appearance, some resembling currants,
others looking like little cones, and others again being in the form
of circular scales (" oak spangles ") on the backs of the leaves.

We may mention, as a specific example of a gall-fly, the form
Rhodites rosce, which is responsible for the tufted red galls often
seen on wild rose-trees and known as rose-bedeguars or " old
man's beard ".

Ichneumon- Flies constitute a family of which nearly 6000 species
have been described, over 1200 of these being British. The
larvae usually attack caterpillars, in or on which the eggs were
laid by the parent. Insects of other kinds, and even spiders, are,
however, attacked by some of the species. It was till recently
thought that the parasite subsisted by devouring the non-vital
parts of its host, but it is more probable that it simply absorbs
the blood of the caterpillar through its skin.

One common ichneumon - fly (Microgaster glomeratus] lays
its eggs in the caterpillar of the common white Cabbage Butterfly,
within which they hatch out. When the unfortunate host has
reached its full term of growth, and should be ready to turn into
a chrysalis, it is too much enfeebled to do so. The unwelcome
guests now bite their way out of the caterpillar and become pupae
on their own account, and it is no uncommon thing to find a dead
caterpillar which has fallen a victim to ichneumon larvae side by
side with a little heap of pupae belonging to these foes. Another

INSECTS 373

species (Hemiteles melanarius) (fig. 226) lays its eggs in the
chrysalis of the Green-veined White.

3. Stinging Hymenoptera are generally distinguished by the
presence of a sting in the female, which takes the place of the
ovipositor found in the forms
so far mentioned. Special
cells are usually constructed
in which the helpless larvae are
reared. As in the last sub-
order, the abdomen is attached
by a stalk, which may be ex-
tremely slender. Ants, Wasps,
and Bees are here included,

Of Which live in SOCial Fi§ 226.— The Green-veined White (Pieris napi], and an

. . _ ... _ Ichneumon-Fly (Hemiteles melanarius), which lays its eggs

tieS, the pOlltlCS OI in the chrysalis of the same, i, 2, 3, 4, Adult female, eggs,

*11 U J 1 ' "U * caterpillar, and chrysalis of the butterfly; 5, adult female of the

Will be dealt With in ichneumon-fly, of which natural size is shown by 6.

another part of this book,

and which in itself would require an entire volume to do it

justice.

More than a thousand species of Ants have been described,
of which over thirty are British. Among these are the large red
Wood Ant (Formica rufa), the large "ant-hills" of which may
be seen in fir-woods, the Slave Ant (Formica fusca), the Slave-
making Ant (Polyergus rufescens), the Black Ant (Lasius niger\
the Yellow Ant (Lasius flavus), and the Solitary Ant (Mutilla
Europcea).

Sand-Wasps dig tunnels in the ground, at the end of which
their eggs are laid. Many of them make a curious provision for
their larvae in the shape of insects, caterpillars, grubs, or it may
be spiders, which they have stung in the nerve-cord so as to
render them powerless without actually killing them. Among
British forms may be mentioned the Path-Wasp (Pompilius
exaltatus), which stores up spiders; the Common Sand- Wasp
(Ammophila sabulosa), which buries caterpillars; and the Fly-
storing Sand- Wasp (Mellinus arvensis), which does the same to
flies.

Wasps are either solitary or social, the most familiar example
of the latter kind being the Common Wasp (Vespa vulgaris],
which, as is well known, constructs nests in banks and other
places. The combs which these contain are constructed of a kind

374

CHARACTERS OF INVERTEBRATE ANIMALS

of paper formed by working up bark and decayed wood to a sort
of pulp. Another and larger social wasp is the Common Hornet
( Vespa crabro], in which the front part of the body is of a reddish
colour. Of solitary species the Mud- Wasps (Odynerus parietum
and others) may be mentioned, which construct their cells of mud

in the crevices of walls, the
hollow stems of plants, and
other places.

Bees, like Wasps, include
social and solitary species, the
Hive- Bee (Apis mellificd) fur-
nishing a good example of the
former sort, that also includes
the Humble- Bee (Bombus ter-
restris] (fig. 227), which makes
its nests in the ground. In
solitary bees the labium is
much shorter than in the
Hive- Bee. They belong to
numerous genera, and include Leaf-Cutter Bees, which make their
cells from pieces of leaf, Carpenter- Bees (Xylocopa), which cut out
cells one above another in the trunks of trees, and Flower- Bees
(Anthophord), forms which look something like humble-bees.

Fig. 227. — Bees and Flowers of Broad-Bean

i, Wood-Bee (B. leucorum}. 2, 2, Holes cut by bee in bases
of flowers. 4, Humble -Bee (Bombus terrestris) extracting
nectar through one of the holes.

Order 7.— NET-WINGED INSECTS (Neuroptera) (fig. 228)

This order, which is much smaller than some of those already
dealt with, contains a great variety of forms which differ very
widely in appearance. The most typical members of the order
possess four membranous wings exhibiting an elaborate net-work
of nervures, quite unlike the simple arrangement characteristic
of Hymenoptera. The mouth-parts are usually adapted for biting.
Eleven families are recognized, arranged in five groups; i.e.—
i. Dragon -Flies, May -Flies, and Stone -Flies; 2. Flat -winged
Neuroptera; 3. Caddis -Flies; 4. White Ants and Book -Lice;
5. Biting- Lice.

i. Dragon- Flies, May -Flies, and Stone-Flies all possess the four
typical wings and pass through an aquatic larval stage. Dragon-
Flies are among the most beautiful objects to be seen in the course
of a summer walk in the country. The four large wings are of

INSECTS

375

Fig. 228. — Net-winged Insects (Neursptera)

i, la, il>, Stages of common May- Fly (Ephemera vulgatd) ; 2, another species of May- Fly (Palin-
genia horaria}', 3, 3^, adult and larva of Horse-Stinger (Libellula depressa) ; 4, 5, Common Scorpion-Fly
(Panorpa communis) ; 6, 6a, Alder-Fly (Sialis lutaria] ; 7, Large Caddis- Fly Phryganea grandis] ;
8, Diamond-spotted Caddis- Fly (Limnophilus rhombicus).

376 CHARACTERS OF INVERTEBRATE ANIMALS

about equal size, and the long body is handsomely marked and
coloured. The freely movable head is provided with very small
antennae, a pair of huge compound eyes, three simple eyes (ocelli),
and biting mouth-parts. Dragon- Flies catch other insects on the
wing, and their flight is exceedingly rapid, making them the
swallows of the insect world. The eggs are laid either in water
or attached to water-plants, and from them extremely voracious
aquatic larvae hatch out, which are distinguished by the possession
of an extremely long transversely -jointed lower lip. This is
usually known as the "mask", because when not in use it is
folded up in front of the face, from which position it can be
suddenly shot out for the capture of small animals. The action
has been compared to that of an old-fashioned carriage -step.
The larval condition is maintained for about a year or rather
less, during which time a number of moults occur, while towards
the end of the period the rudiments of wings make their appear-
ance. There is no motionless pupa stage, but the full-grown
larva climbs up the stem of some plant till it is above water,
when its skin splits longitudinally along the dorsal surface, and
the adult dragon-fly, which has been meanwhile forming within,
gradually works its way out. Over forty species of British dragon-
flies are known, of which the following may be mentioned: — The
Great Dragon- Fly (sEsckna grandis), a large reddish-brown insect,
with lighter markings; the Horse -Stinger (Libellula depressa)
(fig. 228), with broad abdomen, light brown, with yellow spots
in the female and violet in the male; and the little Demoiselle
Dragon- Fly (Agrion puella), with T-shaped head, and slender
abdomen, black in the female and banded with light-blue in the
male.

The May-Flies or Day-Flies are fragile insects in which the
hind- wings are much smaller than the others, and the abdomen
has two or three slender tails attached to it. The adult only
lives a short time, though the traditional day may in some cases
be extended to a fortnight. The life-history broadly resembles
that of the dragon-flies, and the larva of some forms appears to
live three years, an unusually long time, contrasting sharply with
the brief existence of the imago. Something like 300 species
have been described, and common British forms are the Common
May-Fly or Gray Drake (Ephemera vulgatd] (fig. 228) and the
Green Drake (E. Danica). Some of the most successful lures of

INSECTS 377

the fly-fisher are copies of various may-flies, of which the two
drakes are examples, as are also many of the " duns " and
" spinners ".

Stone-Flies are dull-coloured flattened insects with four mem-
branous wings which, when at rest, are disposed so as to overlap
the back and sides of the body. The antennae are long, and
there are usually two filaments of similar appearance attached
to the tip of the abdomen. The life-history resembles that of
the preceding two groups. Stone- Flies are widely distributed,
and include a large number of species. The best-known British
form is the Common Stone- Fly (Per la bicaudata], well known to
anglers as a good bait for trout.

2. Flat-winged Neuroptera include Alder- Flies, Snake- Flies,
Scorpion-Flies, Ant-lions, Lace-wing Flies, and other forms, in
all of which there are four similar wings, not capable of being
folded, but turned back when at rest so as to lie either flat, or
sloping like the roof of a house. The adult possesses well-
developed mandibles, contrasting in this respect with the last
group of Neuroptera, in which the mouth-parts are much reduced.
There is a terrestrial or aquatic larva, which becomes a quiescent
pupa.

The Alder- Fly (Sialis lutaria) (fig. 228), which figures on
the angler's list, is a brown insect with brownish wings, clumsy
body, and long antennae. The wings in repose cover the thorax
and abdomen in a roof-like manner. It is common on river-
banks in Britain. The cylindrical greyish eggs are deposited in
patches on the stems of grasses or rushes near the water, into
which the rapacious larvae that hatch out from them find their
way. Later on, they come out of the water and bury themselves
in the soil, where they become pupae.

Snake- Flies, of which there are several British species (e.g.
Raphidia ophiopsis), are more slenderly built insects, sometimes
found in woods. They are distinguished by the presence of a
sort of neck, and there is an ovipositor in the female. The
rapacious larva is found in rotten wood, where it passes into a
pupa stage.

Scorpion- Flies are much more abundant in this country than
Snake-Flies, and the Common Scorpion-Fly (Panorpa communis)
(fig. 228) may be taken as a type. The name is due to the fact
that the abdomen in the male ends in a pair of pincers, and its

378 CHARACTERS OF INVERTEBRATE ANIMALS

hinder part can be curled up over the back like a scorpion's tail
The wings are narrow, and when at rest are held in a horizontal
position. The head is of characteristic shape, for it is prolonged
into a downwardly-directed beak, which has been compared in
appearance to the face of a horse. Though the insect is small
(about y?, inch long), its black body, yellow legs and beak,
and wings speckled with white and brown make it a striking
and beautiful object. The larva is predatory and terrestrial,
being not unlike the false caterpillar of a saw-fly in appearance,
possessing as it does three pairs of legs proper, followed by eight
pairs of pro-legs. Later on, it passes into a quiescent pupa stage.
Ant- Lions are insects with slender body, four equal mem-
branous wings, and antennae clubbed like those of butterflies.
They do not occur in this country, but are common on the
Continent, where they have long attracted attention from the
peculiar habits of the larvae in the type-genus (Myrmeleo). It
is these to which the name ant-lion was originally applied on
account of the devastations they commit among those and other
insects, their mode of operation being to dig pit-falls in the sand,

at the bottom of which they
remain buried, with only the
enormous mandibles project-
ing.

Laceiving- Flies are repre-
sented by about fifteen native
species, a common example
being the Golden -eyed Fly

Fig. 229. -Golden-eyed Fly (Chrysopa vulgaris) (CkrySOpd VulgdriS) (fig.

i. Adult female ; 2, stalked eggs ; 3, 4, larva (natural size and an extremely fragile

enlarged); 5,6, cocoon (natural size and enlarged). J .

insect with four gauzy wings,

long slender antennae, and brilliant eyes gleaming like gold. The
eggs are attached to leaves by long stalks, and the voracious
larvae which hatch out from them are known as "aphis lions",
a name fully justified by the effective manner in which they keep
plant-lice in check.

3. Caddis- Flies (fig. 228) in the adult condition look rather
like moths, but their wings are hairy instead of scaly, and their
larvae are aquatic " caddis- worms ", remarkable for the habit of
forming protective cases from sand-grains, bits of stick, or other
foreign matters. A caddis-worm is something like a caterpillar,

INSECTS 379

devoid, however, of pro-legs, while at the end of its tail is a pair
of pincers by which it can attach itself firmly to the case. The
food is mainly of vegetable nature. After from seven to
ten months the larva closes the opening of the tube with silk
and passes into the pupa stage, which, after remaining quiescent
for two or three weeks, bites its way out of the case and swims
on its back to some plant or other object up which it can climb
out of the water. The pupal skin then splits, and the caddis-fly
emerges.

4. White Ants, more correctly called Termites, since they
have nothing whatever to do with ants proper, are but too well
known to the inhabitants of tropical countries on account of the
havoc they work with wooden furniture and the like. They are
social insects, living in communities organized in an extraordinarily
complex manner, about which particulars will be given in the
sequel. In every community both winged and wingless individuals
may be found, the former possessing four very long narrow wings
of the kind characteristic of the order, which are held flat on the
back when at rest. There is but a slight metamorphosis. Two
species are found in South Europe (Calotermes flavicollis and
Termes lucifugus), but the most remarkable forms are natives
of tropical Africa, and some of these (e.g. Termes bellicosus)
construct nests of earth which may be as much as 14 feet
high.

Book- Lice are minute forms of somewhat doubtful affinities,
and most familiarly known by the wingless species giving the
name to the group, but also including the " death watches "
(Atropos divinatoria and others), commonly reputed by the super-
stitious to herald death by a ticking noise, though the sound is
more probably produced by a wood-eating beetle (Anobium).
The winged species, which are the most numerous, are to be
found feeding on lichens, fungi, and other plants. Of the four
membranous wings, which are provided with but few nervures,
the hind ones are much the smaller. The antennae are very long.
There is but a slight metamorphosis. Among the British winged
species may be mentioned Psocus fasciatus.

5. Biting -Lice are small wingless, large -headed creatures
which live on the skins of birds and mammals, and must care-
fully be distinguished from ordinary lice (p. 354), in which the
mouth-parts are adapted for piercing and sucking. The common

38o

CHARACTERS OF INVERTEBRATE ANIMALS

fowl is infested by no less than five species (Menopon pallidum
and others), and the " dust baths " in which this bird indulges
are no doubt taken with a view of getting rid of these and other
parasites. Mammals are less troubled with attacks of the kind,
the dog being an example of animals which are thus attended.
Trichodectes latus is the name of its unwelcome guest.

Order 8. — STRAIGHT-WINGED INSECTS (Orthoptera)

The Cockroach, of which an account has already been given,
may be taken as a type of this order, wfiich also includes such
familiar forms as locusts, grasshoppers, earwigs, and crickets.
The mouth-parts are adapted for biting, and it is particularly to
be noticed that the second maxillae are not so closely fused
together to form a lower lip or labium as in most other insects.
The fore-wings are modified into leathery wing-covers, and the
large membranous hind ones are usually traversed by nervures
radiating from the point of attachment, and are thus enabled to
fold up in a fan-like manner when not in use. There can
scarcely be said to be a metamorphosis, for the young insects
when just hatched differ from the adult mainly in size and in
the absence of wings, while there is no quiescent pupa stage.
It is supposed that at least 10,000 species of recent Orthoptera
exist, including the largest known insects, but of this large
number less than forty are native to Britain. A distinction is
drawn between: i. Running Orthoptera, including Earwigs, Cock-
roaches, Soothsayers, Stick-
, Insects, and Leaf -Insects;

\ / t 2. Leaping Orthoptera, em-

f bracing Grasshoppers, Lo-

custs, and Crickets.

i. In Running Orthoptera
all three pairs of legs are
pretty much alike. The
Common Earwig (Forficula
auricularid] (fig. 230) may be
taken as a type of a family
in which the wings are folded up in a remarkably complex manner
under the short wing-covers, and the tail is provided with curved
forceps. No satisfactory explanation has been given of the curious

Fig. 230. — Common Earwig (Forficula auricularici)
Adult (line indicates actual size) and earlier stages

INSECTS 381

popular notion that these insects are in the habit of entering the
human ear, for this does not appear to be the case, and though
it is true that the hind-wings are strikingly ear-shaped, they are
so rarely seen unfolded that it may be doubted whether this has
to do with the name "earwig".

Cockroaches are widely distributed insects, especially common
in tropical regions. Only three small species appear to be indi-
genous to Britain, for the familiar " black beetle " is undoubtedly
an importation. Some of the exotic species are brightly coloured,
while others are wingless.

Soothsayers or Praying- Insects form a remarkable group de-
pendent upon a warm climate, and often assuming the most
extraordinary forms, calculated in many cases to harmonize with
the surroundings, being thus rendered inconspicuous to their
prey, which consists of other insects. The front-legs are modified
into seizing-organs, and it is the curious way in which these are
extended that has given rise to the common names, as, e.g.,
that of Praying Mantis (Mantis religiosd) applied to a French
species, the only European one found at any great distance from
the Mediterranean shores. A Mantis in such an attitude is, how-
ever, merely on the look-out for insects, and not in a prophetic
or devout frame of mind.

Stick- and Leaf-Insects assume even more remarkable forms
than the members of the preceding family, and mostly have a
close resemblance to sticks, leaves, pieces of bark, and other
parts of plants, which in this case may be looked upon as a
protective arrangement, for, unlike the Soothsayers, they affect
a vegetable diet. They are widely distributed through the
warmer parts of the globe, and some of them may be as much
as 9 inches long.

2. Leaping Orthoptera, in accordance with their habit of
springing, possess very large hind-legs. Remarkable structures
related to hearing are usually present, and the males generally
possess musical organs as well. Three families are recognized-
Locusts and Grasshoppers, Green Grasshoppers, and Crickets.

Locusts and Grasshoppers are distinguished by the shortness
of their antennae and the presence of auditory organs in the first
segment of the abdomen. The familiar little grasshoppers of
British fields represent a number of species of varying size, some of
the larger belonging to the genera Stenobothrus and Gomphocerus,

382

CHARACTERS OF INVERTEBRATE ANIMALS

while the very smallest are included in the genus Tettix. The
characteristic chirping is produced by rubbing the inner side of the
hind-legs against the outer surface of the front wings, the former
being provided with a ridge made up of small peg-like projec-
tions. The sounds audible to human ears are produced only by
the male.

What are popularly known as " locusts " are simply species
of grasshopper, which from time to time appear in swarms

Fig. 231. — Migratory Grasshopper — "Locust" (Acridium peregrinuni)

which migrate from place to place and do a vast amount of
damage. The best-known species is the European migratory
locust, which ranges from China to the Atlantic. A large species
(Acridium peregrinum) (fig. 231), common in North Africa, is
probably the locust mentioned in the book of Exodus.

Green Grasshoppers are easily distinguished from the members
of the preceding group by their extremely long and slender an-
tennae. There are generally auditory organs situated in the front
legs just below the knee. Chirping organs, when present, are
placed on the bases of the fore-wings, the left carrying a
roughened edge (file) and the latter a sharp edge. In most
species the female possesses a long egg-laying tube or ovipositor.

INSECTS

383

The most conspicuous British species is the Large Green Grass-
hopper (Locust a viridissimd) which, despite its generic name,
is not a locust at all. Other examples are the North American
Katydids.

Crickets agree in many essential particulars with the members
of the preceding group, possessing as they do long, slender an-
tennae and similarly situated musical and vocal organs, while
the female is usually provided with an ovipositor. There are,
however, differences in detail ; e.g. the tarsus is usually three-
jointed instead of four-jointed, and the musical organs involve
a larger part of the wing. There are four different kinds of
cricket in the British area, of which by far the most familiar
is the House-Cricket (Gryllus domesticus], which is one of the
animals most constantly associated with human dwellings, and
is not unconnected with superstitious ideas. Habit has ren-
dered its chirpings agreeable to our ears, and Dickens's ever-
popular story, The Cricket on the Hearth, gives it an interest
which few Orthoptera can boast. It is perhaps rather un-
romantic to add that in dra-
matic presentments of the
tale the all-important chirp is
imitated by using a glass-
stoppered bottle, the stopper
of which is twisted round so as
to produce a creaking sound.
This particular insect ranges
over a large part of the Old
World, and also occurs in
North America. Its distri-
bution has probably been
extended by human agency.
Of the two British species of
Field-Cricket, one (Nemobius
sylvestris] is small in size,
while the other (Gryllus campestris] is a good deal larger than the
Common Cricket, and usually black in colour. The Mole-Cricket
(Gryllotalpa vulgaris] (fig. 232) is a remarkable form, practically
limited to the south of England with us, though common on the
Continent. It burrows underground by means of its remarkably
modified fore-feet, and its habits will be dealt writh elsewhere.

Fig. 232. — Mole-Cricket (Gryllotalpa vulgaris]
i, Eggs; 2, 3, larvae; 4, adult.

384 CHARACTERS OF INVERTEBRATE ANIMALS

Order 9.— WINGLESS INSECTS (Aptera)

This is a group of small inconspicuous insects which are of
great interest theoretically, as they are probably to be regarded
as insects in their simplest form, i.e. are of "primitive" nature.
They never possess any traces of wings, nor do they appear to
be descendants of winged ancestors, as is the case with the
numerous wingless insects placed in other groups. The segmen-
tation of the body is more clearly marked than in other insect
orders, and the segments are not so specialized, besides which small
abdominal appendages are not uncommon.0 The horny covering
of the body is comparatively delicate. Two sub-orders are
recognized: — i. Tassel - Tails (Tkysanura) and 2. Springers
(Collembola).

1. Tkysanura. — The most marked feature of the sub-order
consists in the presence of two or three long styles projecting

from the hind end of the body. Pro-
bably the commonest British species is
the Silver- Fish (Lepisma sacckarina)
(fig. 233), found in brown sugar and old
books. It possesses three tail-filaments,
and its silvery lustre is due to the pre-
sence of peculiar scales, which make

Fig. 233 -Aptera (magnified) beautiful microscopic objects. Another

sPecies> common in the crevices of rocks
at the sea-side, is Mackilis maritima,

which is not unlike the preceding, but grey in colour instead of

silvery.

2. Collembola. — Many, but by no means all, of these possess a
curious springing apparatus in the form of two stiff bristles which
can be folded under the body and secured by a sort of catch
projecting from the third segment. When released the animal
is thrown into the air much like the "jack-jumper" children
are so fond of fabricating from the "merrythought" of a goose.
They are common under bark, dead leaves, stones, &c., and
one species (Podura aquaticd] may often be seen floating
on the surface of stagnant pools. Some kinds are abundant
in Alpine regions, on the surface of snow or ice, and among
these may be mentioned the Glacier-" Flea " (Desoria glacialis)
(fig. 233). The Collembola are not all provided with a springing

SPIDER-LIKE ANIMALS

385

apparatus, and of these special interest attaches to Anurida mari-
tima, remarkable for its habit of floating on the surface of rock-
pools along certain parts of the coast. As remarked elsewhere,
insects as a rule are very intolerant of salt water.

CLASS 2.— SPIDER-LIKE ANIMALS (ARACHNIDA)

This class is constituted by Scorpions, Spiders, Mites, and
other allied forms, though the affinities of some of these is more
than doubtful. The majority of species live upon animal matter,
and many of them pursue living prey. Arachnids are popularly
confused with insects, from which, however, they differ in many

COMB

Fig. 234.— Scorpion, seen from above (A) and below (B). i, Chelicerse; 2, pedipalps; 3-6, walking legs.

important particulars. These differences, as well as the points
of agreement, will be best understood by briefly describing a
Scorpion as a type (fig. 234). Spiders are more familiar to us
in this country, but as they are much specialized it will be better
to take them later. In dealing with the Scorpion, comparison
may well be made with the account of a typical insect given
on pp. 343-350.

The obviously segmented body is protected by a firm horny
coating, which is very thick and hard in some places, while in
others it is comparatively soft and flexible, so as to permit of a
certain amount of movement. The body is not divided, as in

VOL. I.

25

386 CHARACTERS OF INVERTEBRATE ANIMALS

an insect, into well-defined head, thorax, and abdomen, for the
parts commonly considered as equivalent to the first two of these
are fused together into one mass known as the cep halo -thorax,
which consists of at least six segments, for it bears six pairs of
appendages. The remainder of the body is made up of twelve
rings or segments, the last five forming a narrow tail which is
carried bent up over the body, and bears at its tip a pear-shaped
spine on the sharp end of which open two poison-glands. Here
we have an example of a true " sting", like that of a bee or
wasp, as contrasted with the biting arrangements found in
serpents, bugs, and gnats.

The appendages of a Scorpion differ strikingly from those of
an insect. Instead of antennae, three pairs of jaws, and three
pairs of walking-legs, we find two pairs of grasping organs and
four pairs of walking - legs, while feelers or antennae are not
present as such. The first pair of these appendages are short,
strong, forwardly- directed nippers (chelicerse), and they are
followed by a very large second pair (pedipalps), which end in
strong pincers much like those of a lobster and used for seizing
prey. The bases of these appendages are adapted for biting.
The possession of eight walking -legs is as characteristic of an
Arachnid as that of six is of an insect, and this affords the
simplest means of distinguishing the members of the two classes.

The two segments which immediately succeed the cephalo-
thorax also bear structures which are interpreted as appendages,
the first being fused into a small plate (operculum) notched
behind, while the others are comb-shaped organs (pectines) which
probably have a tactile function.

An ordinary insect breathes by means of air-tubes which
open to the exterior by a series of pores on each side of the
body, but the respiratory organs of a Scorpion consist of four
pairs of " lung -books" which open by a corresponding number
of oblique slits placed on the under side of the segments following
the one upon which the comb-shaped organs are borne. Each
of these breathing organs consists of a cavity into which a large
number of thin plates project, these being packed together in
a way which suggests the leaves of a book, hence the name
lung- " book". An insect, again, typically possesses a pair of
large compound eyes, and simple eyes may be present in addition.
Here the eyes are all simple and are arranged on the upper side

SPIDER-LIKE ANIMALS 387

of the cephalo-thorax, two of them being close together near the
middle line, while the remainder form a couple of groups, one
on each side, near the front end of the body. Each of these
groups contains from two to five separate eyes. The develop-
ment takes place without metamorphosis.

The class is divided into the following orders: —

1. Scorpions (SCORPIONID^E).

2. False Spiders (SOLPUGIM).

3. False Scorpions (PSEUDOSCORPIONID/E).

4. Whip-Scorpions (PEDIPALPI).

5. Harvestmen (PHALANGHXE).

6. Spiders (ARANEID^:).

7. Mites (ACARINA).

To these are usually added two small groups of doubtful
affinities, i.e.—

8. Tongue- Worms (LiNGUATULlD^E).

9. Bear-Animalcules (TARDIGRADA).

Order I. — SCORPIONS (Scorpionidae)

Scorpions, all of which conform to the description already
given, are widely distributed throughout the warmer parts of
the globe. Two common South European species are the little
House-Scorpion (Euscorpius Europ&us], which ranges as far as
the Tyrol and Carpathians; and the much larger Field- Scorpion
(Buthus Europceus), common in the Mediterranean countries.
The largest and most poisonous forms are the black Rock-
Scorpions of Africa and India, which may be as much as 9 inches
in length. These belong to the type-genus Scorpio.

Order 2. — FALSE SPIDERS (Solpugidae)

The order of False Spiders includes a small number of
species having a wide distribution, and mostly limited to warm
countries. A well-known type is the Common False -Spider
(Galeodes araneoides] (fig. 235), found in South Russia, Persia,
Arabia, and Egypt, and much feared on account of its poisonous
bite. The Kalmucks, Kirghiz, and other nomad tribes avoid
regions where it abounds, for its attacks are not limited to human

388 CHARACTERS OF INVERTEBRATE ANIMALS

beings, but extend also to domestic animals, such as sheep, and
camels. This creature resembles a large spider in appearance:
its body is about 2 inches long and its legs long and hairy.
There are great differences of structure and proportion as com-
pared with a Scorpion. The body is unique among Arachnids
in being distinctly divided into head, thorax, and abdomen, much
as in an insect, and the last region is composed of ten segments
and cylindrical in shape, there being no sting-provided tail. A

further resemblance to insects is
found in the breathing organs,
which consist of air-tubes, opening
to the exterior by three pairs of
stigmata situated on the under
surface, the first at the bases of
the second legs, and the others on
the abdomen. As in a Scorpion,
the first appendages are nippers,

here of very large size, and constituting the chief offensive
weapons, since poison glands open upon them. The pedipalps,
however, are not, as in a Scorpion, stout and pincer-bearing,
but slender leg-shaped structures having a forward direction and
acting as tactile organs. The first pair of legs closely resemble
the pedipalps in appearance and function, and it may be noted
that these four similar appendages are provided at their bases
with cutting projections, situated at the sides of the mouth and
serving as jaws. The remaining legs are attached to the thorax,
as in an insect, the resemblance being emphasized by the fact
that each ends in a claw-bearing tarsus. There is nothing to
correspond to either the operculum or the combs of a Scorpion,
and only two simple eyes are present, placed on the front of the
head close to the middle line.

Order 3.— FALSE SCORPIONS (Pseudoscorpionidae)

False Scorpions are minute widely -distributed animals not
unlike Scorpions in appearance, the resemblance being due to
similarity in the appearance and structure of the chelicerae, pedi-
palps, and walking-legs. There are, however, no poison-glands,
and the broad flat abdomen does not narrow into a tail. Breathing
is effected by means of air-tubes, which open on the under side

SPIDER-LIKE ANIMALS

389

Fig. 236. — Book-Scorpion (Cheli-
snlarged. Natural

of the abdomen by two pairs of stigmata. Spinning glands open
on the chelicerae, and the simple eyes, two or four in number, are
situated on either side of the front of the head.

A common European species is the little Book - Scorpion
(Chelifer cancroides) (fig. 236), often found in old books and
similar dark places.

Order 4. — WHIP-SCORPIONS (Pedipalpi)

The Whip -Scorpions make up a small
but widely-distributed order, the members of
which are fairly large in size. They are
found in the warmer parts of both hemi-
spheres, and considerable interest attaches to
them, owing to the fact that they are in some
respects intermediate between Scorpions and Spiders, on which
account they are sometimes called Scorpion- Spiders. A typical
Whip- Scorpion (Thelyphonus) looks not unlike a real Scorpion,
the pedipalps being large and provided with pincers, and the
abdomen narrowed into a sort of tail, reduced, however, to a
mere filament. The breath-
ing organs are lung-books
(two pairs), and the eyes are
arranged in a central and
two lateral groups. Among
the important differences
from Scorpions may be
noted — chelicerae provided
with claws, not nippers,
modification of the first
pair of legs into tactile
organs, and distinct mark-
ing off of the abdomen.

Some of the other genera
(as Phrynus] (fig. 237) ap-
proximate more closely to the Spiders, for the pedipalps possess
claws instead of pincers, and the abdomen is joined to the rest
of the body by a narrow waist. The tail filament is only repre-
sented by a button-like knob.

Fig. 237. — A Whip-Scorpion (Phrynus). a, Front of
cephalo-thorax, enlarged to show eyes.

390 CHARACTERS OF INVERTEBRATE ANIMALS

Order 5. — HARVESTMEN (Phalangidae)

Harvestmen constitute a large and almost universally-distri-
buted order, represented in this country by some two dozen
species. They are common in our fields and are generally mis-
taken for spiders, from which, however, the native species can
at once be distinguished by the great length and slenderness of

the four pairs of legs,
and the fact that the
small oval body is not
marked off distinctly
into regions. The
chelicerae are provided
with relatively large
pincers, but the pedi-
palps are usually short

Fig. 238.— A Harvestman (Phalangiuin opilio] J

and leg - like. 1 he

breathing organs are air-tubes which open by a pair of stigmata
on a forward prolongation of the abdomen, just behind the bases
of the first legs. A pair of simple eyes is borne on the upper
surface of the cephalo- thorax. Phalangium opilio is one of the
commonest native species (fig. 238).

Order 6.— SPIDERS (Araneidse)

Spiders make up a very large and widely-distributed order,
of which there are several thousand known . species. A well-
known British form is the large Garden- Spider (Epeira diadema)
(fig. 239), which constructs large regular webs resembling a
wheel in shape. The ground-colour varies from yellowish to
dark brown, diversified by darker markings, and usually pre-
senting a conspicuous white mark on the upper side of the
abdomen, whence the German name of " cross-spider " (Kreuz-
spinner], and the French name " cross-carrier " (porte croix).
As in a scorpion, the head and thorax are closely fused together,
but the large egg-shaped abdomen is connected by a narrow
wasp-waist to the rest of the body, and the segments of which
it is made up are so intimately united together that the boundaries
between them cannot be made out.

The first pair of appendages (chelicerte) are two-jointed, and

SPIDER-LIKE ANIMALS 391

of very characteristic construction, for the sharp curved end-
joint, at the tip of which a pair of poison-glands open, can be
folded down on the basal joint, much as the blade of a pocket-
knife folds down on the handle. In this way an efficient grasping
and holding organ is formed. The pedipalps are slender forwardly-
directed structures, looking something like antennae. As in

Fig. 239. —Garden-Spider [Epeira diadema] and Web. a, Female spider; b, arrangement of eyes.

Scorpion their basal joints are provided with cutting edges, which
work against one another and act as jaws. The tips of the
pedipalps are curiously modified in the male. The four pairs
of legs are strong and of considerable length. Each of them is
provided at its tip with several toothed claws, serving as efficient
grasping organs.

As regards breathing organs, the Garden- Spider combines
the arrangements characteristic of Scorpions on the one hand
and Insects on the other. There are two lung -books, which
open by a pair of slits on the under-side of the abdomen, near
its base, while farther back there is a single aperture, situated
in the middle, and opening into a set of air-tubes.

One of the most striking peculiarities of an ordinary spider
is its power of spinning webs: indeed the word spider probably
means " spinner ". The silk is made by a large number of
spinning-glands situated in the hinder part of the body, and

392 CHARACTERS OF INVERTEBRATE ANIMALS

opening upon conical " spinnerets ", of which there are six in
the Garden- Spider, four large and two small. They will be found
as conical projections on the under side of the abdomen near its
tip.

The Garden- Spider possesses eight simple eyes, situated on
the upper side of the cephalo-thorax, at its front end. Four of
them are comparatively large and are placed at the corners of
a square, on each side of which are a pair of rather smaller ones.
As in Arachnids generally the young resemble the adult when
hatched, except in size.

There are two sub-orders: i. Segmeated Spiders, and 2.
Unsegmented Spiders.

1. Segmented Spiders are represented by certain large East
Indian species, characterized by well-marked segmentation of
the abdomen and the possession of two pairs of lung-books,
agreeing in these respects with the whip-scorpions. A further
peculiarity is the presence of eight spinnerets, grouped together
on the under-side of the abdomen, and placed much farther for-
wards than in an ordinary spider.

2. Unsegmented Spiders include all the common forms, of
which Epeira has been taken as a type. The sub-order is again
divided into two groups according to the number of lung-books
present, i.e. into Four-lunged Spiders and Two-lunged Spiders.

Four - lunged Spiders include the largest members of the
order, which excavate burrows in the ground and line them with
silk, but do not construct snaring-webs. Some of these prey
upon small birds, e.g. the Bird-eating Spider (Mygale avicularia),
a gigantic South American form often seen in collections. Here
also are included the Trap-door Spiders (Cteniza and Nemesia)
of South Europe and elsewhere, which make hinged lids to their
burrows. There is only one British species (Atypus Sulzeri],
which burrows in damp earth.

Two-lunged Spiders, of which Epeira diadema is a typical
example, embrace the large majority of species, and all the British
forms except Atypus. Only some of the kinds construct snaring-
webs, and among these may be mentioned, in addition to the
Garden-Spider, the common House-Spiders (Tegenaria domestica
and T. civilis], and the Hedge- Spider (Agalena labyrinthica),
which constructs strong horizontal webs on bushes, hedges, &c.
A very interesting aquatic form is the Water- Spider (Argyroneta

SPIDER-LIKE ANIMALS

393

aquatica), which constructs a thimble - shaped nest under water,
attaching it to surrounding objects by mooring-strands which also
serve to snare prey. The nest is filled with air, which the spider
brings down in the form of bubbles adhering to its hairy abdomen.
Examples of common British spiders which do not construct
webs are the Wolf- Spiders (Lycosida)> and the handsomely-
striped Harlequin- Spider (Salticus scenicus), distinguished by its
leaping powers.

Order 7. — MlTES (Acarina)

Mites, as their name indicates, are minute forms, and they
embrace an exceedingly large number of species, some of which
are found almost everywhere. A typical example is the Cheese-
Mite ( Tyroglyph^ls siro) (fig. 240). The abdomen
is unsegmented and closely fused with the
cephalo-thorax, the entire body being oval in
shape. The small chelicerae are provided with
pincers, and the pedipalps are short and slender.
The usual four pairs of walking-
legs are present, two directed for-
wards and two backwards. Re-
spiratory organs are absent. An-
other well-known species is the
" Red-Spider " or " Money-Spin-
ner", which lives on the juices of leaves and spins a
protective web. One of the chief features in which
it differs from the Cheese- Mite is in the possession
of breathing organs, which are in the form of air-
tubes. Other examples of the group are Ticks,
Mange- Mites, and similar pests, which will be dealt
with in another section.

UTfl

Fig. 240. — Cheese - Mite
(Tyroglyphussiro},szz.n from
below. Enlarged forty times.

Order 8. — TONGUE- WORMS (Linguatulidae)

Fig. 241. — Tongue-
Worm (Pentastotnum
tcenioides}

Tongue -Worms are worm -like forms which in
the adult condition are found as parasites in the nasal cavities
of Dog and Wolf. The only appendages are two pairs of hook-
like structures in the neighbourhood of the mouth. Pentastomum
tcenioides is the type (fig. 241).

394

CHARACTERS OF INVERTEBRATE ANIMALS

Order 9.— BEAR-ANIMALCULES (Tardigrada)

Bear-Animalcules are minute creatures found in damp moss,
or sometimes in salt or fresh water. The shape of the body-
ludicrously suggests an unlicked bear-cub (fig. 242). There are

four pairs of stump -like legs, each pro-
vided with a pair of claws at its tip. The
only representatives of jaws are to be
found in a pair of sharp stylets which can
be protruded from the mouth.

PHARYNX

SALIVARY
GLAND

IV

Fig. 242. — A Bear- Animalcule (Ma-
crobiotus), much enlarged and seen
from above; i-iv, walking legs.

CLASS 3.— CENTIPEDES AND MILLI-
PEDES (MYRIAPODA)

Centipedes and Millipedes, which make
up the third class of Jointed -limbed Inver-
tebrates, are of simpler structure than the
average members of the preceding classes,
Insecta and Arachnida, contrasting strongly
with them in regard to the legs, of which
numerous similar pairs are present, though by no means so many
as the names " centipede" and "millipede" would seem to imply.
A common British centipede, the Thirty-Foot (Lithobius forficatus)
(fig. 243), may be taken as a type. This is a small chestnut-

/ coloured creature which lurks
under stones or among loose
earth, and glides rapidly away
when disturbed. The body
is flattened from above down-
wards, and is made up of a
head and trunk, the latter con-
sisting of sixteen segments,
of which the first is extremely
narrow, while each of the
remainder bears a pair of
jointed legs ending in pointed claws. The last two legs are much
larger than the others, and turn sharply backwards. The narrow
first segment of the trunk is provided with a pair of modified
limbs, the bases of which are fused together, while each of them
ends in a strong curved claw, near the end of which a poison
gland opens.

Fig. 243. — British Centipedes: i, (Geophilus lo>igicornis)\
2, 3, the Thirty- Foot (Lithobius forficatus]; 3 is under side of
head enlarged.

CENTIPEDES AND MILLIPEDES

395

The most obvious appendages of the head (fig. 244) are two
long-jointed antennae, while the remainder consist of three pairs
of jaws guarding the opening of the mouth, and overlapped by the
poison-claws. There is also a plate-like upper lip. The jaws,
which are comparable in some respects to those of a cockroach
(see p. 345), consist of a pair of hard-biting mandibles, followed
by delicate flattened
\st maxilla, and these
again by leg -like 2nd
maxilla.

As in an insect, the
breathing organs are
air - tubes ramifying
throughout the body,
opening to the exterior
on the sides by small
holes (stigmata), of
which the 3rd, 5th, 8th,
loth, 1 2th, and i4th
leg - bearing segments
each bears a pair. A
group of simple eyes is
to be seen on each side
of the top of the head.

It will be seen from
the above description
that a Myriapod is de-
cidedly simpler in struc-
ture than an Insect or

average Arachnid, only the front part being clearly marked off as
a head, while there is no distinction between thorax and abdomen,
though the first segment of the trunk is specialized. The presence
of numerous pairs of legs, extending right to the posterior end of
the body, is also characteristic, there being no restriction of walk-
ing-legs to the front part of the trunk, as in a scorpion or cock-
roach, where there are no legs on the abdominal region. As in
many other cases of animals with unspecialized trunk, there is
considerable variation within the limits of the class as to the
number of segments, and another noteworthy point is the similarity
these segments exhibit among one another.

Fig. 244. — Structure of Centipede

Mandibles; 2, first maxillae; 3, second maxillae; 4, first limbs of
trunk, with poison claws

396

CHARACTERS OF INVERTEBRATE ANIMALS

Myriapods are divided into the following five orders : —

1. Millipedes (CHILOGNATHA or DlPLOPODA).

2. Centipedes (SYNGNATHA or CHILOPODA).

3. Spider-legged Myriapods (SCHIZOTARSIA).

4. Insect-like Myriapods (SYMPHYLA).

5. Larva-like Myriapods (PAUROPODA).

Order i. — MILLIPEDES (Chilognatha or Diplopoda)

Millipedes (fig. 245) are vegetarian Myriapods, devoid of
poison-claws, and with cylindrical bodies. The legs are com-
paratively weak, and throughout the greater part of the trunk two

pairs of them are borne by
each segment, their bases
being close together instead
of wide apart, as in a Centi-
pede. The antennae are
short and club-shaped, while
each of them is made up of
seven somewhat bell-shaped
joints. The mouth is pro-

J

vided with a plate-shaped

imnpri:n ofrnncr msnrh'hlpc:
UppC lip, Strong man( CS,

^nd tWO OairS of maxillcE

fused together into a broad
plate. There are two pairs of stigmata on each trunk-segment,
and also two small pores (foramina repugnatoria], which are the
openings of defensive stink-glands. The eyes resemble those of
a Centipede in structure and position.

Millipedes differ very much in length. A common British
species of average length is the Earth Snake- Millipede (lulus
terrestris\ a sluggish creature about an inch in length, commonly
found under loose bark, &c., and with the habit of curling itself up
when alarmed. The Pill- Millipedes are short forms which roll
themselves into compact balls under similar circumstances. The
genus Glomeris is represented by British species.

Fig. 245.— British Millipedes
i, London Snake-Millipede (lulus Londinensis] ; 2, 3, Spotted

Snake-Millipede (7w/»^^«*), natural size and enlarged; 4,5,
Earth Snake-Millipede (/«/*« *«r«/w) and antenna of same, both
enlarged; 6, 7, Flattened Millipede (Polydesmus complanatus),
natural size and enlarged.

Order 2. — CENTIPEDES (Syngnatha or Chilopoda)

Centipedes conform in the main to the description already
given of the common British form (Lithobius\ The large Centi-

CENTIPEDES AND MILLIPEDES 397

pedes of tropical countries, feared on account of their painful bite,
constitute a widely-distributed family, including larger and longer
forms, provided with twenty-one pairs of legs. Some of them
may be as much as a foot in length. A well-known species is
Scolopendra morsitans. Another family of Centipedes, found all
over the world except in the coldest regions, includes slender
elongated forms, which are devoid of eyes, and burrow under-
ground in pursuit of earth-worms. Some of them are phosphor-
escent (fig. 243), as, e.g., the common British species (Geophilus
longicornis).

Order 3.— SPIDER -LEGGED MVRIAPODS (Schizotarsia)

Spider - legged Myriapods include the species of a genus
(Scutigera) which is widely distributed through the warmer parts
of the globe. The body is comparatively short, but antennae
and legs are very much elongated. The eyes are compound,
a unique peculiarity in the class. The breathing organs differ
considerably from those of ordinary Myriapods, and open to the
exterior by a single row of stigmata placed in the middle line
on the upper side of the trunk. They are very active creatures,
and most of them pursue their prey in broad daylight, even
when the sun is strong.

Order 4. — INSECT- LIKE MYRIAPODS (Symphyla)

Insect-like Myriapods include but one genus (Scolopendrella]y
of very small size, and represented by a number of widely-distri-
buted species, of which two are British. The adjective " insect-
like " is used on account of the very strong resemblance which
exists to the primitive insects of the order Thysanura (p. 384),
and some zoologists go so far as to state that we should look
upon these forms as coming very near to the ancestral stock
from which insects have been derived. There is at any rate
a close relationship.

Order 5. — LARVA- LIKE MYRIAPODS (Pauropoda)

The order of Larva-like Myriapods includes certain exceed-
ingly small creatures first discovered in Britain, and thought
at first by their discoverer (Sir John Lubbock) to be larvae.

398 CHARACTERS OF INVERTEBRATE ANIMALS

The most remarkable character is found in the antennae,
which are branched. No breathing organs have, so far, been
discovered. The type-genus is Pauropus.

CLASS 4. —PRIMITIVE TRACHEATES (PROTOTRACHEATA)

There are certain small groups of animals to which special
interest attaches on account of the speculations regarding gene-
alogy and origin of organs to which they have given rise. One
such group is the Hemichorda, already briefly described (p. 300);
another is the class now to be dealt with. It includes but a single
genus, Peripatus, a primitive type which appears to be of great
antiquity, and as the name of the class indicates, is supposed to
represent the ancestral stock from which the air-breathing arthro-
pods already described have been derived. The word ''represent"
must, however, as in such cases generally, be taken with some
qualification, for Peripatus can only be regarded as representing
that stock in a general sort of way, as no doubt it has to some
extent specialized on lines of its own, acquiring peculiarities which
adapt it to a special mode of life. To those who are not professed
zoologists it may appear that specialists make an unnecessary
fuss about an obscure creature that may briefly be described as
a "permanent caterpillar"; but the marvel is explained when we
remember that this lowly animal enables us to throw light upon
the origin and relationships of Myriapods, Arachnids, and Insects,
the last of which, taken by themselves, include the majority of
terrestrial species.

Like many archaic forms, Peripatus has an extremely wide
geographical distribution, and its included species are found in
South Africa, the Malay Peninsula, East Australia, New Zealand,
South America, Central America, and the West Indies. Yet
all these widely - separated species resemble one another with
sufficient closeness to be placed in the same genus.

Peripatus was first described (in 1826) as a Mollusc, later as
a Myriapod and as an Annelid (segmented worm). The late
Professor Moseley, in 1874, proved it to be an Arthropod, and our
detailed knowledge of its structure and development is mainly due
to the investigations of the late Professor F. Maitland Balfour and
Mr. Adam Sedgwick. It is now a widely-accepted view that
Arthropods have been derived from Annelids, and the special

PRIMITIVE TRACHEATES

399

interest attached to Peripatus centres in the fact that it, to use
the words of Sedgwick, " stands absolutely alone as a kind of
half-way animal between the Arthropoda and Annelida ".

Peripatus (fig. 246) is a small cylindrical animal, found among
rotting wood or the like, and comparable in appearance either
to a well-nourished caterpillar or a segmented worm. The
velvety skin is beautifully coloured, the tint varying according
to the species. The front part of the body, called
by courtesy the head, is not sharply marked off from
the trunk, and this again is not clearly segmented,
though its segmentation is indicated by the fact that
it bears numerous pairs of stumpy legs, the exact
number depending upon the species. These legs are
hollow, conical projections (much like the appen-
dages of some Annelids), and only exhibit in an
imperfect way the jointing that characterizes Arthro-
pods generally. Each of them is provided with a
couple of sharp claws at its tip. Upon the under side
of the head there is a swollen circular lip surrounding
a depression within which the mouth opens. On each
side of the head, outside this lip, is a short cylin-
drical projection known as oral papilla, and equiva-
lent to a pair of modified limbs. Upon the tip of
•each of these is the opening of a large slime-gland,
which secretes a sticky substance that can be forcibly
ejected either as a defensive measure or else (in the
New Zealand species at any rate) as a means of
capturing prey. Within the circular lip are a pair E^^££
of muscular jaws, each armed with two claw-like pro-
jections used for chewing the food. These jaws, again, are to
be looked on as modified limbs, and the presence of such limb-
jaws is an Arthropod character, though the limitation to a single
pair is a peculiarity of Peripatus. A pair of cylindrical imper-
fectly-ringed antenna project from the front of the head, and near
the base of each of them there is a simple eye.

Only a few of the more important details regarding the in-
ternal structure (fig. 247) can be mentioned here, and these
may be conveniently grouped into: i. Arthropod characters, and
2. Annelid characters.

i. Arthropod characters. — The organs of circulation con-

246. — Cape

400

CHARACTERS OF INVERTEBRATE ANIMALS

form in the main to the type which
Cockroach (p. 348). The heart is
to the upper surface of the body
containing pericardial space, from
through numerous pairs of valvular

A.

INTESTINE

Fig. 247.— Structure of Peripatus
A, Digestive organs (from below). B, Nervous system and

has been described for the

a slender tube placed close

and suspended in a blood-

which blood passes into it

apertures. The rest of the

blood -system consists of

larger and smaller spaces

which, together with the

heart and pericardial cavity,

make up a circulatory ar-

rangement of which the

parts communicate with one

another.

This appears to be a
suitable place in which to
speak more fully of the
nature of the Arthropod
heart, which is essentially a
blood-tube within a blood-
space with which it com-
municates by paired aper-
tures. In a Vertebrate
(p. 40) or a Mollusc (p. 308}
the heart possesses one or

m°re Wricks, IlltO which
blood IS pOUred by

pericardial

surrounding it does not

contain blood at all. Professor Ray Lankester explains the
arthropod condition by supposing that the heart was originally
a tube receiving blood by several pairs of lateral vessels which
later on dilated into auricles where they joined the central tube.
The fusion of these auricles into a large space round the heart
would give the state of things now existing. The pericardial
space of, say, Peripatus is therefore to be regarded, if the
theory be well founded, as equivalent to a big auricle surround-
ing the heart; the physiological problem solved in this case
being the evolution of an arrangement for storing blood about to
enter the heart.

Peripatus further agrees with typical air-breathing Arthropods

PRIMITIVE TRACHEATES 401

in the character of its breathing organs, which are air -tubes.
These are, however of very simple character, and open by very
numerous stigmata scattered over the surface of the body and
even present on the legs. The arrangement is not entirely
irregular, for some of these apertures are placed in a double
longitudinal row on the upper surface, while others are similarly
disposed on the under surface. We have, on the whole, what
may be considered a primitive or undifferentiated condition of
these organs, from which it is easy to imagine the derivation of
the more complex arrangements found in Myriapods, Arachnids,
and Insects.

2. Annelid characters. — These will naturally be better appre-
ciated after the Annelids have been considered, and will only
be briefly enumerated.

The body-wall, like that of an ordinary segmented worm,
consists of a thin skin covered by a delicate cuticle, and under-
lain by a muscular coat, consisting of an external layer of trans-
versely-running fibres and an internal layer of fibres having
a longitudinal direction. In regard to minute structure, the
muscle-fibres differ from those of Arthropods in being devoid of
transverse striations.

The mouth of Peripatus leads into a muscular pharynx, like
that of many Annelids, but quite unlike what is to be found in
Arthropods. A much more striking feature is to be found in the
excretory organs. These consist of a series of tubes known as
nephridia, one of which opens at the base of almost every leg,
on its under surface. Such segmentally-arranged renal organs
are extremely characteristic of segmented worms.

The central nervous system consists of a double brain- or
cerebral-ganglion above the pharynx, and two ventral cords con-
nected with these. A remarkable character is seen in the fact
that these cords are widely separated except at the extreme
posterior end of the body, where they unite together above the
gut. Very numerous slender transverse nerves connect these
two cords, much as the rungs of a ladder connect its sides,
though the " rungs " of the nervous system differ in being much
more narrow and numerous. The nerve cords are dilated at
regular intervals into ill-marked ganglia. This nervous system
agrees with that of various Annelids, and is also much like that
characteristic of the Pro to- Molluscs.

VOL. I. 26

402 CHARACTERS OF INVERTEBRATE ANIMALS

Lastly, it may be noted that the eyes of Peripatus agree more
closely with those of Annelids than with those of Arthropods.

B.— AQUATIC ARTHROPODS (BRANCHIATA)

The four Arthropod classes which have so far been reviewed,
i.e. Insects, Arachnids, Myriapods, and Prototracheates, together
make up the air-breathing or Tracheate division of Arthropoda ;
and we now come to the aquatic division, including the two classes
of Crustaceans (Crustacea) and King-Crabs (Xiphosura), to which
the Sea- Spiders (Pycnogonida) are doubtfally appended.

CLASS 5.— CRUSTACEANS (CRUSTACEA)

This very large class, of which a typical member, the Lobster,
has already been partly described in contrasting Vertebrates with
higher Invertebrates (p. 302), includes animals which are for the
most part marine, though many, of the minute forms especially,
inhabit fresh water, and some few are terrestrial.

So great is the diversity of structure within the limits of the
class that no single type fully illustrates it; but it may be as
well to enlarge somewhat upon the description already given
of the Lobster (Homarus vulgaris), taking it as a good average
example of the higher Crustacea. To those desirous of seeing
how a single type may be made to illustrate the whole of the
class in question, and at the same time give a sound knowledge
of the principles of zoology generally, a perusal of Huxley's classic
work, The Crayfish, is recommended.

External Characters of the Lobster (fig. 248). — The body is
bilaterally symmetrical, and the hinder part of it, or tail, is clearly
divided into segments. The front part of the body is a cephalo-
thorax, consisting of head and thorax closely fused together,
though segments are present here too, as shown by the
numerous paired appendages. The head is marked off from
the thorax by means of a distinct groove (cervical groove).
The number of segments appear to be as follows : — head, 5 ;
thorax, 8 ; tail or abdomen, 7. All the segments, and the ap-
pendages they bear, are constructed on the same common plan
(see p. 195), but there are many differences in detail, to serve
various physiological ends.

CRUSTACEANS 403

Appendages. — The letter Y may be taken as a diagram of
the type on which the appendages are constructed, the stem
of the letter corresponding to a basal stalk by which the appen-
dage is attached to the body, while its two forks represent
outer and inner branches. The same thing may be expressed
in another way, by saying that the typical limb in a higher
Crustacean is cleft or forked. We will now see how far the
appendages of the Lobster conform to this Y diagram, and in
doing so it will be convenient to start with the abdomen, as
this is the least modified region. Of the seven segments here
present only the first six bear appendages, while the last, usually
known as the telson, forms the middle portion of the tail-fin.
An average abdominal segment, say the fourth, bears two small
forked appendages somewhat inappropriately termed swimmerets,
and it will be seen from the diagram that these conform to the
Y type. The appendages of the sixth abdominal segment are
relatively large, and form the side-parts of the great tail-fin.
Each of them, however, may be compared to a Y in which the
main stem is shortened and broadened, while the two branches
are flattened out into oval plates. It may further be noted
that the first two pairs of abdominal appendages in the male
are curiously modified, while in the female the appendages
of the first abdominal segment are either absent or very much
reduced.

Thoracic appendages. — The four hinder most segments of the
thorax are stout walking- legs, of which the two first pairs end
in small pincers. At first sight they deviate entirely from the
Y type, for they are obviously not forked. This is a case
where the facts of development are of use in throwing light on
a problem, for we find that in a very young Lobster these
walking-legs are forked, but their outer branches are compara-
tively feeble and ultimately disappear altogether. There is a
further peculiarity about the three first pairs of these legs to
which attention must be called, but before doing so it is
necessary to speak of the nature and position of the breathing
organs or gills. These are delicate plume -like outgrowths of
the body, limited to the thoracic region, and sheltered in a gill-
cavity on either side, the outer wall of which is formed by a
firm gill-cover that constitutes the side of the carapace or hard
shield covering the greater part of head and thorax. When

404 CHARACTERS OF INVERTEBRATE ANIMALS

this gill-cover is cut away the gills will be seen, and it can
readily be shown that they are of three kinds, attached to
different regions of the body. It need only be mentioned here
that some of them are limb-gills, so called because they are borne
by the bases of certain limbs, among which are the three first
pairs of legs. When one of these is carefully detached it will
be seen that a thin plate (gill-plate) projects from its base, and
that to that plate a feathery gill is attached. The fourth thoracic
segment bears the most conspicuous appendages of all, i.e. the
great pincers, which are constructed on the same lines as the
nipper-bearing walking-legs, and like trfem are provided with
gill-plates and limb-gills.

The three first segments of the thorax bear appendages
modified for chewing, and called foot-jaws, the name suggesting
the idea that they were once locomotor in function but have
acquired new duties. The last or third pair of foot-jaws are
decidedly leg-like, but are provided with stiff bristles where they
bite against one another. An outer branch is present, though
small, and gill-plate with limb-gill will be readily recognized.
The middle or second foot-jaws are like the preceding, but a
good deal smaller, while the first foot-jaws are very delicate,
with broad biting basal stalk, small outer and inner branches,
and gill-plate devoid of gill.

Head Appendages. — These consist of three pairs of jaws
behind and two pairs of feelers in front. The jaws, beginning
with the last pair, are named, as in an insect, second maxillae,
first maxillae, and mandibles. The second maxillcz somewhat
resemble the first foot-jaws, but gill -plate and gill are absent,
while the outer branch is broadened into an oval " baler", which
lies in the front of the gill-chamber, and by its constant scooping
movement brings about a forward movement of water over the
gills. Theyfotf maxillce are still more delicate, and are reduced
to a two-jointed basal stem and an insignificant inner branch.

The same parts are present in the first jawrs or mandibles •,
but their proportions and texture are very different. The basal
joint of the stalk is broadened into a hard biting-piece, strongly
toothed on its inner margin, while the second joint of the stalk
together with the inner branch make up a small three-jointed
" palp " which probably has sensory functions.

Owing to an upward bend of the region in front of the

CRUSTACEANS

405

mouth, the two pairs of feelers are directed forwards, an ad-
vantageous position for sense organs of the kind. The second
or smaller pair, the antennules, conform markedly to the Y-
diagram, and consist of a basal stalk and slender outer and
inner branches. But it is by no means certain that we are
justified in comparing the parts of this appendage with those

FEELER 2

FOOT JAW 3 -fS

FIN

Fig. 248. — Appendages of Lobster (Homarus vulgaris}

PR. Base of appendage (protopodite); EN. inner branch (endopodite) ; EX. outer branch (exopodite), (the right-
hand EX. in feeler 2 indicates excretory aperture; EP. gill-plate (epipodite); G, gill.

of the other ones, for it must be noted that the basal stem is
here three-jointed instead of two-jointed as in all the other cases.
The large feelers or antennce conform to the type, for they con-
sist of a two-jointed stalk, to which are attached a scale-shaped
outer branch, and an excedingly long inner branch, which can be
swept round so as to explore a considerable area in the neighbour-
hood of the body.

We have in the Lobster appendages an excellent example
of the principles enumerated on p. 195, whereby structures of
generalized type are modified in various ways to bring about
special ends. These principles are: i. variations in shape'.

406

CHARACTERS OF INVERTEBRATE ANIMALS

compare, e.g., the antennae, great pincers, and swimmerets ; 2,
variation in relative size: compare outer and inner branches in
an average swimmeret and the side-pieces of the tail-fin; 3.
variation in number: taking the Y- shape as the generalized
type, the gill-plate is an addition in some cases, while in others
the outer branch is much reduced or even absent; 4. f^ls^on
of parts: seen in the first abdominal appendages of the male.

This is a convenient point at which to compare some of
the anterior appendages of Lobster with those of air-breathing
Arthropods. One view is represented by the following table :—

Cockroach.

Scorpion.

Centipede.

Peripatus.

Lobster.

Antennae

Absent.

Antennae

Antennae

Antennules

Absent

Chelicerae

Absent

Absent

Antennae

Mandibles

Pedipalpi

Mandibles

Jaws

Mandibles

ist Maxillae

ist Legs

ist Maxillae

Oral Papilla

ist Maxillae

2nd Maxillae

2nd Legs

2nd Maxillae

ist Legs

2nd Maxillae

ist Legs

3rd Legs

Poison Claw

2nd Legs

ist Foot-jaws

2nd Legs

4th Legs

ist Claw

3rd Legs

2nd Foot-jaws

3rd Legs

Operculum

2nd Legs

4th Legs

3rd Foot-jaws

Absent

Combs

3rd Legs

5th Legs

Great Pincers

It should be mentioned that many difficulties attend the
comparison of segments and appendages in different groups, for
one can seldom be quite certain that segments really correspond
in different cases, and, even if they did, some appendages are
apt to shift their position in the course of development.

The Lobster is protected by a firm shelly exoskeleton in the
form of a horny layer which is thin where mobility is required,
while elsewhere it is thick and hardened by the deposit of salts
of lime. This strong suit of armour cannot increase in size as
the animal grows, and the exigencies of the case are met by a
process of moulting, which takes place frequently in young
animals and at longer intervals later on. At the commence-
ment of the operation a transverse split appears along the back,
where the carapace joins the tail, and through this opening the
animal painfully makes its way out. A sheltered corner is
chosen for moulting purposes, and here the soft and defenceless
creature remains till its new armour is properly developed and
hardened.

The chief external characters have now been reviewed, and

CRUSTACEANS

407

the remainder may be taken in connection with the various
systems of organs which are concerned, and which will be con-
sidered seriatim.

Digestive Organs (fig. 249). — These consist of (i)a symmetrical
food-tube or gut, which runs back from the oval mouth situated
on the under side of the head to the vent placed below the telson,
and (2) of digestive glands. It will be noted that, as in Arthropods
generally, the jaws are modified limbs, which work against one
another from side to side and are entirely outside the mouth,

OPHTHALMIC
ARTERY

ANTENNARY
ARTERY

DORSALABDOMTNAL
ARTERY.

PINC

AB 7. (TELSON)

Fig. 249.— Side-dissection of Lobster (Homarus -vulgaris], reduced.. ABI-AB/, Abdominal segments.

which is bounded by upper and lower lips. It is found convenient
to speak of the gut as being divided into three sections — a large
fore-gut, a very small mid-gut into which the digestive glands
open, and a long hind -gut. The distinction between these sec-
tions is based on the mode of development, for the first and last
of them begin as pits which extend farther and farther inwards
till they join the developing mid-gut, and form with it a continuous
tube. It is not therefore surprising to find that the fore- and
hind-guts, since they are formed by inpushing of the body-wall
(see p. 261), are lined by a firm horny layer continuous with
the exoskeleton, and that this layer is shed and replaced every
time the animal moults. The fore-gut consists of a short gullet
dilating into a very large stomach, divided into a cardiac section
in front and a pyloric part behind. The stomach is said to be
a "masticatory" one, for it contains a chewing apparatus or
gastric mill, consisting of a number of hard pieces formed by
thickening and calcification of the horny lining. These pieces

4o8 CHARACTERS OF INVERTEBRATE ANIMALS

make up an elastic framework on which are borne two large
lateral and one median tooth, and by the action of appropriate
muscles these can be brought together so as to effectually chew
anything that happens to be between them. The cavity of the
pyloric part of the stomach is narrowed, and numerous inter-
lacing bristles project from its walls, constituting a very effective
" strainer ", which prevents any but finely-divided particles from
passing back into the short mid-gut. The lining of this part
of the food- tube is soft, and a pair of large digestive glands,
commonly called the liver, open into it. These organs are
physiologically equivalent to liver and paticreas of a Vertebrate
(see pp. 37 and 38). The hind-gut or intestine is a narrow tube
continuous with the mid-gut, and possessing a firm lining raised
up into longitudinal ridges.

The Circulatory Organs (fig. 350) conform to the Arthropod
type already described in dealing with the Cockroach (p. 348),
but the heart, instead of being a long slender tube, is a short
broad sac, possessing only three pairs of valvular apertures, and
suspended in the blood-containing pericardial cavity by means of
fibrous cords. It is systemic (see pp. 308 and 348), i.e. contains
purified blood, which it pumps through delicate branching arteries
to the body at large. Sooner or later these arteries communicate
with irregular spaces which ultimately open into a large sternal
sinus running along the lower part of the body just within the
body -wall. Meanwhile the blood has become impure by loss
of much of its oxygen and receipt of carbon dioxide as a waste
product. It therefore passes to the gills for purification, and, when
this has been effected, is carried to the pericardial cavity, whence
it passes into the heart through the valvular openings with which
that organ is provided.

The Excretory Organs, by which nitrogenous waste is removed
from the blood, consist of a pair of antennary- or green-glands,
situated in the front part of the head and opening on the bases
of the antennae. Each is essentially a coiled tube, possibly equi-
valent to a nephridium (see p. 401).

The Muscular System is complex. The largest muscles are
to be found in the tail, which is the organ by means of which
the lobster is able to swim backwards through the water with
great rapidity. Powerful flexor muscles, lying below the gut,
bend the tail down and enable it to give its effective stroke, while

CRUSTACEANS 409

less powerful extensor muscles, lying above the gut, straighten it
for another downward movement.

The Central Nevvous System (fig. 249) consists of a nerve-
ring surrounding the gullet, and a double ventral cord upon which
numerous pairs of ganglia are developed. The upper part of the
nerve-loop is thickened into a pair of brain or cerebral ganglia,
by which the important sense organs of the head are supplied.
The general arrangement conforms to that described in the Cock-
roach (see p. 349).

Organs of Sense are well developed, and many of them are
in the form of specialized bristles or seta, the soft axes of
which are in communication with the skin by means of vertical
canals which perforate the hard exoskeleton. Many of these
setae minister to the sense of touch, and this is especially true of
those found on the antennules and antennae. Organs of taste
are not definitely recognized, though they probably exist on some
of the appendages of the mouth. Certain peculiar spatula-shaped
setae present on the external branches of the antennules have to
do with smell.

The extremely interesting auditory organs consist of two pear-
shaped sacs, one of which is lodged in the basal joint of each
antennule, and opens by a slit to the exterior. These sacs are
really in-pushings of the skin, and they contain numerous special-
ized auditory setae, and also grains of sand which have been
introduced from the exterior. The chief interest attaching to
these organs lies in the fact that they correspond to a stage in
the development of more complex structures, such, e.g., as the
membranous labyrinths of Vertebrates, which start as pits in the
.skin (see p. 56).

The visual organs are in the form of two compound eyes
situated on the front of the head near the antennules, and borne
on stalks.

Development. — The Lobster passes through a metamorphosis
in the course of its life-history, for it hatches out as a larva, which
differs in many ways from the adult.

Crustacea, of which the Lobster has been described as a type,
may be defined as aquatic Arthropods possessing two pairs of
feelers (antennules and antennae), and breathing organs (when such
are present) in the form of gills. The appendages are typically
forked. The class is subdivided in the following way: —

4io CHARACTERS OF INVERTEBRATE ANIMALS

Sub-class i. — Higher Crustacea (MALACOSTRACA).

Order I. Stalk-eyed Crustacea (THORACOSTRACA). — Lobster, Cray-
fish, Crab, Locust-Shrimp, Opossum-Shrimp, &c.

Order 2. Sessile-eyed Crustacea (ARTHROSTRACA). — Sand-Hop-
pers, Wood-Lice, &c.

Order 3. Intermediate Crustacea (LEPTOSTRACA). — Mud-Shrimps
(Nebalia and its allies).

Sub-class 2.— Lower Crustacea (ENTOMOSTRACA).

Order i. Barnacles (ClRRlPEDlA). — Ship-Barnacle, &c.
Order 2. Bivalve Crustacea (OSTRACODAJ. — Cypris, &c.
Order 3. Fork-footed Crustacea (COPEPODA). — Cyclops, Fish-Lice.
Order 4. Leaf-footed Crustacea (PHYLLOPODA).— Apus, Water-
Fleas, &c.

Sub-class i. — HIGHER CRUSTACEA (Malacostraca)

The following features, illustrated by the Lobster, are char-
acteristic of the sub-class: — The body is made up of a constant
and limited number of segments, each of which, with the exception
of the last (or telson), bears a pair of appendages. In nearly all
cases the segments are twenty in number, distributed as follows:—
head, 5 ; thorax, 8 ; Abdomen, 7. The excretory organs typically
present are antennary glands. The development is complex,
and in most cases there is a larval form, differing markedly in
appearance from the adult.

Order i. — STALK-EYED CRUSTACEA (Thoracostraca)

In this order the head and thorax are fused together, and the
eyes are usually situated on stalks. There are four sub-orders :—
i. Ten-legged Crustacea (Decapoda\ 2. Opossum -Shrimps.
(Schizopoda), 3. Locust-Shrimps (Stomatopoda), and 4. the
Cumacea.

i. The Decapoda are so named because the last five pairs of
thoracic limbs are seven-jointed locomotor organs, in which the
outer branches are absent in the adult.

One large section of the sub -order is distinguished by the
presence of a long and powerful tail, provided with a well-
developed tail-fin. Among the British species here included are
the following: — Lobster (Homarus vulgaris) (fig. 250), Rock-

Fig. 250. — Mediterranean Crustacea

i, Lobster (Hontarus vulgaris); 2, Bear-Crab (Scyllarus arctus); 3, Dromia Crab (Drotnia};

4, Spider-Crab (Maia squinandd); 5, Locust-Shrimp (Squilla mantis}.

411

412

CHARACTERS OF INVERTEBRATE ANIMALS

Lobster (Palinurus vulgaris) (fig. 251), Norway Lobster (Nephrcps
Norvegicus), Prawn (Pal&mon serratus\ Shrimp (Crangon vul-
garis]. The Crayfish (Astacus fluviatilis] is a fresh-water form.

Fig. 251. — K ode-Lobster (Falinurzts vulgaris)

Another section, including forms intermediate between the
above and Crab, is formed by the Hermit-Crabs (fig. 252), which

take up their abode in the empty shells of
various sea-snails, and possess a soft un-
symmetrical tail that has lost most of its
appendages except the last pair. These
have lost their original function of acting
as swimming organs, and are modified
into hook-like structures, by which the
hermit-crab holds on to its house.

The last section of the Decapods
includes the true Crabs (fig. 250), in which
the cephalo- thorax is very broad, and the
tail so much reduced as to be useless
as a swimming organ. Among British
species may be noted the Edible Crab
( Cancer pagurus) and the Shore Crab (Carcinus mcenas).

2. The Opossum- Shrimps (Schizopoda) are small marine forms

Fig. 252. — A Hermit-Crab (Pagurus
Bernkardus), removed from its dwelling

CRUSTACEANS

often found swimming in large shoals, and superficially resembling
ordinary shrimps, though of much smaller size. They differ, how-
ever, from these in many important particulars, among which
may be mentioned the delicate nature of the shield covering the
cephalo-thorax, to which at least one thoracic segment is not united.
There are, further, eight pairs of closely similar thoracic legs, pro-
vided both with outer and inner branches. The commonest
British genus is My sis (fig. 253), which so closely resembles a

Fig. 253. — Opossum-Shrimp (Mysis), enlarged

stage in the development of the Lobster that this is spoken of as
the " Mysis stage ". Closed auditory sacs are present in its tail.

3. Locust- Shrimps (Stomatopoda) are much larger (fig. 250) than
the members of the last sub-order, and are commonest in tropical
seas, where they may attain a length of as much as 8 inches or
more. The cephalo-thorax is small, for not only does it not include
the last three thoracic segments, but it does not have to shelter the
gills, as these are attached to the appendages of the broad well-
developed abdomen. The thoracic appendages are remarkable, for
the first five pairs of them are modified into foot-jaws, of which the
second are extremely large and modified as seizing-limbs (fig. 250).
So strikingly do they resemble the first pair of legs of the Praying
Mantis (see p. 381), which have a similar function, that these
Crustacea are often known as " Mantis- Shrimps ". The last three
pairs of thoracic appendages are leg-like, and possessed of both
outer and inner branches. Two species are not uncommon in the
Channel Islands (Squilla Desmaresti and Squilla mantis). Like

CHARACTERS OF INVERTEBRATE ANIMALS

the other members of the sub-order, they are burrowing forms
found in shallow water.

4. The sub-order Cumacea includes small shrimp -like forms
(fig. 255) which live in shoals, and inhabit fairly deep parts of the
sea where the bottom is of sand. The cephalo-thorax is even more

ANT. 1

CEPHALOTHORAX

Fig. 254. — Cuma (enlarged). ANT. i, Antennule iv-vni, posterior thoracic segments; 1-7, abdominal segments.

limited than in the Locust- Shrimps, for the last five thoracic seg-
ments are not united with it. Some at least of the thoracic limbs
possess outer as well as inner branches, and the two compound
eyes are stalkless and sometimes fused together.

Order 2.— SESSILE-EYED CRUSTACEA (Arthrostraca)

The order includes a large number of comparatively small
Crustacea, in which, as a rule, only one thoracic segment is fused
to the head, and there is consequently no great armour-covered
cephalo-thorax, as in a Crab or Lobster. The head-appendages
correspond to those of a Lobster, but the thorax possesses only
one pair of foot-jaws, its other appendages being seven pairs of
legs, none of which are provided with pincers. There are usually
six pairs of limbs on the abdomen. The eyes are devoid of
stalks, i.e. are sessile

Two sub-orders are recognized, one containing laterally-flattened
animals (Amphipoda), and the other forms which are flattened from
above downwards (Isopoda).

i. The most typical members of the Amphipoda (fig. 255) are
springing forms, of which the Sand- Hopper (Talitrus locusta) is
a good example. During the summer months this may often be
seen in leaping myriads between tide-marks on sandy shores. Its
appearance suggests a strongly-curved flattened shrimp. Very
similar in character are the species of the genus Gammarus,
common in shallow water, both salt and fresh. The curious

CRUSTACEANS

Whale- Louse (Cyamus ceti) is exceptional among the members of
the sub-order in having its body flattened from above downwards,
and it is found parasitic on the skin of whales, to which it is
enabled to hold fast by
means of strongly -hooked
legs. The abdomen is re-
duced to a mere stump en-
tirely devoid of appendages.
The weird-looking Skeleton
Shrimps (Caprella] are also
distinguished by the presence
of a much-reduced abdomen.
They are found climbing like
monkeys among the branch-
ing colonies of various zoo-
phytes.

2. Isopods (fig. 256) differ
from Amphipods in being
flattened from above down-

ABDOMEN

Fig. 255. — Amphipods (enlarged)
A, Sand-Hopper ( Talitnis locusta] ; 2-8, last seven segments of

wards while the abdomen is tnorax> tne first is fused with the head- B> whaie-Louse (Cyamus

ceti}. c, Skeleton Shrimp (Caprella}.

shortened and bears plate-
like appendages. Most of the species are marine, and of these
Cirolana, Idotea, and Sphceroma may be taken as typical British
genera. Some, however, are
fresh - water, as, e.g., the
•common native Fresh-water
Shrimp (Asellus aquaticus),
distinguished by its long
limbs; and still others are
terrestrial, of which the most
familiar is the Wood- Louse
{Oniscus murarius\ com-
monly found Under damp Fte- 256.— Isopods (enlarged)

crrm^c anrl in cimilar r»1ar^c n-viii, Free segments of thorax, the first is fused with the

piaCeS. nead A> Cirolana boreaiis. B> Wood-Louse (Oniscus murarius).

Some of the Isopods are

curiously modified to fit them for a parasitic life, and these will

be mentioned elsewhere.

416

CHARACTERS OF INVERTEBRATE ANIMALS

EYE

Order 3. — INTERMEDIATE CRUSTACEA (Leptostraca)

The Mud- Shrimps, including Nebalia (fig. 257) and allied
genera, form a restricted but interesting group of small marine
shrimp -like creatures found in all parts of the world. These
forms have attracted a good deal of attention, because they are a
connecting-link between the higher and lower Crustacea, and

some zoologists place them in a
separate sub -class. Like the
higher Crustacea they are com-
posed of» a definite and limited
number of segments, in this case,
however, twenty -one instead of
twenty, for the abdomen has an
extra segment, and the excretory
organs include antennary glands.
They also agree with stalk -eyed
forms as regards their visual organs. But on the other hand all
the segments of the thorax are free, though it and part of the
abdomen are covered by a bivalve shield which grows back from
the head, and the eight pairs of thoracic appendages are leaf-like,
as in some of the lower Crustacea. The first four segments of the
abdomen bear forked appendages as in certain other of the lower
Crustacea (Copepoda), a further agreement with which is found
in the fact that the tail also is forked. In addition to antennary
glands they possess other excretory structures (shell -glands) re-
sembling those of lower forms. On the whole it may be taken as
fairly certain that the Leptostraca resemble in many points the
ancestral stock from which the various groups of higher Crustacea
have diverged.

Fig. 257. — Mud-Shrimp (Nebalia), enlarged
Left half of shield cut away.

Sub-class 2. — LOWER CRUSTACEA (Entomostraca)

This is an exceedingly large and greatly diversified group,
including both marine and fresh-water forms, of which the large
majority are very small, and which play a very important part
in nature as the food of higher animals. There is a very large
amount of variation as regards the number of segments, and a
similar wide range in the nature of the appendages, which are often
flattened and leaf-like, though in other cases they may exhibit

CRUSTACEANS

the bifurcated type which we have found to be characteristic of
higher forms. There is no gastric mill, and it is common to find
three functional eyes present in the adult, two compound and one
simple. As a general rule the
embryo hatches out as a Nau-
plius larva (fig. 258), which typi-
cally possesses an unsegmented
ovoid body and three pairs of
appendages by means of which
it swims, these corresponding
to the antennules, antennae, and
mandibles of the adult. A larva
of this kind is rarely found
among higher Crustacea.

The four included orders
have already been enumerated ;

T> 1 / /"* ' ' / J ' \ Fig. 258. — A Nauplius larva (seen from below and greatly

i.e. i. Barnacles (Lirripedia}\ enla*ge^ Note unppaired eye ^and three pairs of appgendagyes
2. Bivalve Crustacea (Ostra-

coda); 3. Fork-footed Crustacea (Copepodd]\ and 4. Leaf- footed
Crustacea (Phyllopodd).

Order i. — BARNACLES (Cirripedia)

This is a remarkable group of marine Crustacea, all of which
are either fixed or parasitic, and have undergone considerable,
or, it may be, profound modifications resulting from their mode
of life. Leaving out of consideration the degenerate parasitic
forms, which will be dealt with elsewhere, there remain the widely-
distributed group of Barnacles, of which the best known are the
Ship- Barnacle (Lepas anatifera), and the Acorn- Barnacles (species
of Balanus), which encrust the rocks between tide-marks on the
coasts of Britain.

The characters of the group will best be understood by briefly
describing the Ship- Barnacle {Lepas anatifera], large numbers
of which are often found attached to floating objects which have
been cast up on our shores, and which in old days were a serious
nuisance to sailors, as they attached themselves to the wooden
bottoms of ships in such vast numbers as to impede movement.

The Ship-Barnacle (fig. 259) is attached by means of a long
soft stalk covered with corrugated skin, and upon this is borne a

VOL. I.

27

4i8 CHARACTERS OF INVERTEBRATE ANIMALS

flattened oval swelling, covered by a number of shelly pieces,
suggesting at first sight that the animal is a kind of Mollusc. A
closer examination will show that these pieces are attached to
a couple of soft flaps united together on one side, but leaving
between them on the other side a slit through which, in a living
specimen, a bundle of hairy -jointed filaments will from time to
time protrude and spread out, being drawn back again immediately

afterwards. These tendril -like struc-
tures, or cirri, which act as a kind of
casting-net, whereby food is swept into
the mouth, aro six pairs of bifurcated
appendages, and their jointed nature
shows that we are dealing, not with a
Mollusc, but with an Arthropod. To
a strong imagination they might sug-
gest feathers in a vague way, and they
are probably responsible for the well-
known natural-history legend, according
to which the Solan Goose ("Barnacle" Goose) develops from
a barnacle, the chicks falling into the water when sufficiently
grown to look after themselves. The specific name of the
barnacle, "anatifera" (L. anser, a goose; fero, I bear), has
reference to this old belief. On removing the shell, and the
flaps to which it is attached, the soft body of the animal will
be found, consisting mainly of thorax, to which the tendril-like
appendages are attached. The abdomen is reduced to a limbless
process ending in a long filament. The under (ventral) side of
the thorax is turned upwards, and at its front end will be found
the mouth, provided with upper lip, mandibles, and two pairs of
maxillae. No eyes or feelers are visible, but study of the develop-
ment shows that the animal is fixed by its head, which has grown
into a long stalk, at the end of which were situated the short
antennules, that served as organs of attachment from which the
sticky secretion of special " cement glands " was poured out. The
antennae, present in the larva, entirely disappear in the adult.
Huxley has graphically compared a barnacle to a man lying upon
his back and kicking his food into his mouth.

A corn- Barnacles agree essentially in structure with the ship-
barnacle, but do not possess a stalk, and there is an extra pro-
tection to the body in the form of a sort of shelly cup made up

CRUSTACEANS

419

of a number of pieces fused together. The appearance of the
body within the cup has no doubt suggested the name ' 'acorn "-
barnacle, but the resemblance is remote.

Order 2.— BIVALVE CRUSTACEA (Ostracoda)

These are small Crustacea common both in salt and fresh
water, especially where the bottom is muddy. Examination of
the mud from almost any pond will often reveal the presence of

A.

NT.1

Fig. 260. — Small Fresh- water Crustacea (much enlarged to various scales)

A, Water-Flea (Daphnia) : i, antennule; 2, antenna; 3, mandible; 5-9, flattened thoracic appendages.
B, Cyclops (seen from above): ANT.I, antennule; ANT. 2, antenna. B', Swimming-foot of Cyclops,
showing the typical forked shape, c, Mussel-Shrimp (Cypris)'. ANT.I, antennule; ANT.2, antenna;
MND., mandible; MX.I, first maxilla; MX.2, second maxilla.

one or more Mussel-Shrimps, species of the typical genus Cypris
(fig. 260), which therefore furnishes a convenient type. The most
striking feature is the presence of a bivalve shell, comparable to
the carapace of other forms and entirely enclosing the body,
reminding one of the arrangement characteristic of bivalve
Molluscs (p. 311). The resemblance is enhanced by the fact
that the two valves of the shell can be closed by the contraction

420 CHARACTERS OF INVERTEBRATE ANIMALS

of an adductor muscle and opened by the action of an elastic
ligament. The body of the animal is extremely short, and the
abdomen is a mere vestige. Segmentation is only indicated by
the appendages, of which there are but seven pairs, five belonging
to the head (two pairs of feelers and three pairs of jaws), and the
remainder, in the form of narrow pointed legs, to the thorax.
These legs can be protruded from the shell, and so can the well-
developed feelers which are used as organs of locomotion, as in
a nauplius larva. There is an unpaired eye on the front of the
head.

Order 3. — FORK-FOOTED CRUSTACEA (Copepoda)

This order is a vast assemblage of species which are for
the most part minute, and occur in all parts of the world both
in salt and fresh water. Many of them are found in huge shoals
at the surface of the open sea, forming a variety of plankton, as
such assemblages are termed, which furnishes an important item
in the food of whales and of many fishes, such as the herring.
The fishes, however, do not have it all their own way with the
Copepods, for attached to their eyes and gills may often be found
parasitic members of the order, which are often strangely modified.
Leaving such forms out of consideration for the present, and
turning our attention to free-living Copepods, we may take a
common fresh- water genus, Cyclops (fig. 260), as a type of the
order. It can be distinguished with the naked eye as an active
whitish creature with a jerky mode of progression.

The body of Cyclops has not unaptly been compared in shape
to half of a split pear, with the convex side dorsal and the stalk
corresponding to a tail. The body is distinctly segmented, and
the five head-segments are fused with one another and with the
first thoracic segment. Then follow the five free segments of
the thorax and the four narrow segments of the tail, the last of
which bears a tail-fork provided with two bunches of bristles.
Upon the front of the head is a single reddish eye, which has
suggested the generic name of Cyclops; and there are the usual
five pairs of head appendages, of which the first, i.e. the antenna,
are very large and employed as oars. The thorax bears four
large pairs of forked swimming- feet, and the abdomen is limbless.
The female, which is the sex commonly seen, usually has a pair
of large egg-sacs attached at the base of the abdomen.

CRUSTACEANS

421

UNPAIRED EYE

Order 4.— LEAF-FOOTED CRUSTACEA (Phyllopoda)

These are Crustacea of varying size, though none are very
large, which are for the most part inhabitants of fresh water.
The segments and appendages differ greatly in number in different
cases, but those of the latter situated farther back than the head
are characterized by their flattened leaf-like form. There are two
sub -orders — i. Gill -footed Phyllopods (Branchiopoda), and 2.
Water- Fleas (Cladocera).

i. Gill-footed Phyllopods possess a special interest, since they
not improbably present many of the characters distinguishing the
ancestral stock from which all the different groups of Crustacea
are descended.

A typical genus is Apus (fig. 261), species of which are
sometimes found in great abundance at various localities on the
Continent, especially in flooded
meadows. For the sub-order
this form reaches a consider-
able size, being as much as
2 inches in length. It is of
a greenish -brown colour, and t3.
though segmentation is not
apparent in the head, the rest
of the body is composed of a
large number of obvious seg-
ments. The most striking fea-
ture is the presence of a broad
thin carapace, which, arising
from the head, extends back- Fig. 26i.-APus

wards so as to cover the back

and sides of the greater part of the body. The narrow posterior
end of the abdomen remains uncovered by the carapace, and
terminates in a pair of long jointed filaments. The head bears
a minute pair of antennules, vestiges of antennae, strong mandibles,
and two pairs of maxillae. A very large number of flattened
lobed swimming-feet are attached to the thorax and front part
of the abdomen. They are not limited in number to one pair
per body-segment, as is the case with the majority of Crustacea.
A typical swimming-foot consists of a number of small pointed
inner lobes, and two larger outer lobes, of which one is a soft

422 CHARACTERS OF INVERTEBRATE ANIMALS

pear-shaped gill. Such a limb is regarded by many as the
generalized type from which the varied kinds of appendages
found within the class have been derived by modifications of
different kinds (see p. 403). This may perhaps be so, but it is
not always easy to recognize with certainty the equivalent parts
in different Crustacean groups. All these swimming-feet in Apus
are by no means exactly alike; e.g. some of the inner lobes of the
first thoracic pair are drawn out into long filaments, liable at first
glance to be mistaken for antennules and antennae, while in the
female the eleventh thoracic pair are partly modified into brood-
capsules within which the eggs develop."

The excretory organs of Apus are not, as in the higher
Crustacea, antennary glands, but coiled tubular shell-glands which
open to the exterior upon the second maxillae. Three eyes are
placed close together on the top of the head — a small unpaired
" nauplius eye " in the middle line, and a pair of compound eyes.

2. The Water-Fleas (Cladocera) are small active Crustacea,
mostly inhabiting fresh water. The characters of the sub-order
are well seen in the very common form Daphnia (fig. 260). The
thorax and shortened abdomen are enclosed in a bivalve carapace
which grows back from the head. The usual five pairs of
appendages are borne by the head (except second maxillae, which
are absent), and of these the most remarkable are the large forked
plume-like antenna which are used as swimming organs. There
are five pairs of flattened thoracic swimming-feet, something like
those of Apus, but the short abdomen is limbless. It bends
sharply round to the under surface and ends in a pair of curved
bristles. When Daphnia is examined alive under the microscope,
an oval pulsating sac will be noticed near the dorsal surface.
This is the heart. The coiled shell-gland is also easily seen, and
the large compound eye is a conspicuous object. It lies in the
front of the head, and has been formed by the fusion of two lateral
eyes. It is constantly in a state of trembling movement. A small
nauplius eye is present a little farther back.

CLASS 6.— KING-CRABS (XIPHOSURA)

This small but interesting class includes only the King-Crab
(Limulus\ a large marine Arthropod living in shallow water in
the East Indies and the warmer parts of the West Atlantic and

KING-CRABS

423

B.

West Pacific. There has been much discussion as to its classi-
ficatory position, some authorities including it in the Crustacea
and others in the Arachnida. In support of the latter view it
has been shown that, allowing for the fact that one is a marine
and the other a terrestrial ani-
mal, there is a very remarkable
agreement between K ing-Crab
and Scorpion. As, however, the
matter is by no means settled,
it appears better for the present
to assign Limulus a class of its
own.

The body of a King -Crab A.
(fig. 262) is divided into two re-
gions, a cephalo-thorax covered
above by a large horse-shoe-
shaped shield, and a similarly
protected abdomen to which a
long movable spine is attached

° • r ' / Fig. 262. — King-Crab (Limuhts), reduced

behind. A pair of simple eyes A, From above; B) frombelow. ,, cheiice« : 2-6, legs,
are situated on the top of the The mouth is seen in B as a darkly-shaded slit between the

bases of the legs.

front shield towards the anterior

end, and farther back there are a pair of large compound eyes,

placed at a greater distance from each other.

Upon the under side the appendages can be seen, the first
of which are the small chelicerte, provided with pincers and
situated in front of the mouth. Then follow five pairs of legs
apparently equivalent to the pedipalps and legs of the Scorpion.
The first of them are provided with pincers in the female, and
the next three pairs are similarly provided in both sexes. The
ends of the last pair are curiously modified so as to fit them for
digging. The bases of these leg-like appendages are provided
with biting projections, which between them almost surround the
elongated mouth. The remaining appendages are six plates,
obviously formed by the fusion of pairs of appendages and borne
by the abdomen. The first of them is equivalent apparently to
the operculum of Scorpion (p. 386), and the second to the combs
of that animal. All but the first bear numerous gill-folds.

424

CHARACTERS OF INVERTEBRATE ANIMALS

CLASS 7.— SEA-SPIDERS (PYCNOGONIDA)

These are widely distributed marine Arthropods, mostly of
small size. Some zoologists associate them with Arachnida, but

their real affinities are as yet un-
determined. Taking such a typical
form as the Shore Pycnogon (Pycno-
gonum littorale), we see (fig. 263)
that the spider -like appearance is
due to the presence of four slender
pairs of thoracic limbs, in front of
which are three pairs of smaller
appendages, of which the first bear pincers and the last are used
by the males for carrying the eggs about. The mouth is situated
on the end of an elongated snout, and there are four simple eyes.

Fig. 263. — Shore Pycnogon (Pycnogonum littorale)

A, From above; B, from below. AB. Abdomen
ov. egg-bearing appendages; PR. snout.

CHAPTER IX

STRUCTURE AND CLASSIFICATION OF SEGMENTED
WORMS, SIPHON-WORMS, WHEEL-ANIMALCULES,
MOSS-POLYPES, AND LAMP-SHELLS

SEGMENTED WORMS (ANNELIDA)

This phylum is a vast assemblage of marine, fresh-water, and
terrestrial worms and leeches, which agree with one another in
the possession of a bilaterally symmetrical body divided into
numerous similar segments, and in the absence of jointed appen-
dages like those of the Arthropoda. The body-wall consists
of skin with underlying layers of muscle; there is a muscular
pharynx; and the excretory organs are in the form of numerous
pairs of convoluted tubes (nephridia), each of which, in typical
cases, opens on the one hand to the exterior, and on the other
into a body-cavity of the same nature as that found in Verte-
brates (see p. 42). The phylum is divided into two classes:
i. Bristle- Worms (Chaetopoda), and 2. Leeches (Discophora).

CLASS I.— BRISTLE-WORMS (CH^TOPODA)

A good typical example of this group is the so-called Sea-
Centipede (fig. 264), a name popularly applied to several species
of the genus Nereis which are found upon our coasts. The
body is bilaterally symmetrical and obviously divided into a large
number of segments, all very similar to one another except those
at the extreme ends. There are clearly none of the jointed lateral
appendages which are so characteristic of Arthropods, their place
being taken by hollow " foot-stumps " (parapodia), of which each
segment bears a pair. The nearest approach to such foot-stumps
among the Arthropoda are the legs of Peripatus (p. 399). Ex-
amination by means of a lens will show that each foot-stump is
divided into an upper lobe and a lower lobe near each of which

425

matic

426 CHARACTERS OF INVERTEBRATE ANIMALS

is a short feeler or cirrus. Imbedded in each lobe is a bundle of
bristles or setce, of which one is much larger and stronger than
the rest, though it only just projects from the surface of the body.
These setae are of great use in locomotion, acting
as holdfasts by means of which the body gets a
purchase on sand or the like. They exhibit all
sorts of variations in shape. The head, upon the
under side of which is the mouth, consists of a
mouth-segment, and a projection in front of the
mouth which may conveniently be called the head-
lobe. From the upper side of this lobe, near the
front, spring two short tentacles, and there are
two much larger " palps " which arise from the
under side of the same region rather farther back.
Fig 264 -A sea-centi- ^ot onty ^° tnese last-named structures act as
(Nereis], diagram- sensory organs, but they also serve as lips to some
extent. Upon the upper side of the head-lobe are
four simple eyes, looking like black specks. The mouth-segment
is provided with four pairs of slender feelers, which are apparently
of the same nature as the cirri of the trunk-segments. None
of the appendages are converted into jaws, a feature which is
so characteristic of Arthropods, though it may be remembered
that in the member of that group which comes nearest to the
Annelida, i.e. Peripatus, there is only one pair of these structures
(see p. 399). Turning now to the last tail segment, which is
perforated by the opening of the intestine, we shall find that it
is comparatively small and devoid of foot -stumps, though it
possesses one pair of long backwardly-directed cirri.

Internal Structure of Nereis (fig. 265). The body -wall pre-
sents very primitive features, reminding one of Peripatus (see
p. 401). It consists of the skin, which is practically little more
than a thin epidermis covered by a tough cuticle, and two under-
lying muscle- layers, of which the outer is composed of fibres which
run transversely, while the other consists of four prominent bands
in which the fibres have a longitudinal direction. Closely con-
nected with the body-wall, though scarcely perhaps forming part
of it, is an oblique sheet of muscle on each side, made up of
numerous fan-shaped sections, which take origin near the sides
of the ventral nerve-cord and spread out in the foot-stumps, to
which they are attached and serve to move.

BRISTLE-WORMS

427

Digestive organs. — On cutting open the body of Nereis we
find that it is traversed by a straight food-tube, between which
and the body-wall is a large body-cavity which corresponds with
that of Vertebrates in being a coelom, i.e. a body-cavity which
contains a lymph-like fluid and communicates with the exterior
by excretory tubes (see p. 428). The segmentation prominent
externally is emphasized internally by the division of the body
into a series of compartments by transverse partitions attached

A.

RAIN

Fig. 265.— Structure of Sea-Centipede (Nereis], enlarged to various scales

A, Head, with mouth-cavity everted and jaws protruding. B, A foot-stump (parapodium). c, Front part of

nervous system.

to the body-wall at the places where grooves mark off the seg-
ments from one another. These partitions are also attached
to the gut, and keep it in place, thus acting like the folds known
as mesentery in Vertebrates. The food-tube or gut consists suc-
cessively of mouth -cavity, pharynx, gullet, and intestine. An
interesting peculiarity of the short mouth-cavity is found in the
fact that it can be everted, or turned inside out, by means of
appropriate muscles, and under those circumstances the absence
of limb-jaws is made up for by the protrusion of a pair of hard
horny jaws carried on the inner side of the pharynx and acting
as a very efficient pair of pincers for seizing food. The everted
mouth-cavity is restored to its normal position by certain muscles
attached to its wall which draw it back. The pharynx has ex-
tremely thick walls, another point in which one is reminded of
Peripatus (see p. 401). The gullet is short and narrow, and
a pair of glands open into it. The intestine, which makes up
the greater part of the gut, is thin-walled.

Circulatory Organs. — We can here distinguish, as in a Verte-

428 CHARACTERS OF INVERTEBRATE ANIMALS

brate (see p. 38), between a blood-system and a lymph-system.
The blood- system contains red blood, the colour being due to
the presence of the same pigment (haemoglobin) as that found
in Vertebrates (see p. 265), though here it is dissolved in the
plasma, or liquid part of the blood, and is not contained in cor-
puscles. The vessels which carry the blood form a closed system,
the smaller branches of which break up into capillary net-works.
No heart is present, and the pumping is effected by the larger
vessels, along which waves of contraction pass, forcing the blood
onwards, much as digesting food is carried onwards in an intes-
tine by peristaltic contraction (see p. 37). "The two most impor-
tant blood-vessels are longitudinal in direction, one being a dorsal
vessel above the gut and the other a ventral vessel below it. The
blood flows forwards in the dorsal vessel and backwards in the
ventral one. From these two chief trunks transverse vessels are
given off regularly in segmental order, and the branches of these
break up into net- works in the substance of the various organs.

The lymph system consists of the body-cavity, and the colour-
less lymph which it contains consists of plasma in which float
numerous irregular lymph corpuscles (see p. 42).

Respiration is effected in the body-wall, which is richly pro-
vided with blood-vessels branching just below the epidermis.
Most likely the foot-stumps play an important part in regard
to this function.

Excretion of nitrogenous waste is performed by the excretory
tubes or nephridia, of which a pair are present in almost every
segment. Each of these organs is essentially a ciliated tube
which begins by a small funnel placed just in front of one of
the partitions which cross the body-cavity, runs back to pierce
this, and ultimately opens to the exterior on the under surface
of the body close to the base of a foot-stump.

The Central Nervous System consists of a nerve -ring en-
circling the commencement of the gut and a ventral nerve-cord,
resembling, therefore, the corresponding organs of a lobster. The
nerve-ring is thickened dorsally into a closely-fused pair of brain-
or cerebral-ganglia, and the ventral cord, which is of double nature,
swells into a pair of closely approximated ganglia in each segment.

Sense Organs. — The cirri and head-tentacles are presumably
organs of touch. It is likely that the palps borne on the head
have to do either with taste or smell, or it may be with both.

BRISTLE-WORMS 429

There are no auditory organs, and the four simple eyes have
already been mentioned.

The Bristle-Worms are subdivided into three orders: — i.
Many -bristled Worms (Polychaeta); 2. Few-bristled Worms
(Oligochaeta); and 3. Simple Segmented Worms (Archiannelida).

Order i. — MANY-BRISTLED WORMS (Polychaeta)

This order embraces an enormous number of marine worms,
possessing a considerable number of setae and agreeing in many
other particulars with Nereis, which is a type of the order. Over
thirty families are recognized, grouped into seven sub-orders, but
it will be sufficient for our purpose to divide the order into two
groups: — i. Free-living Polychaetes (Errantia), and 2. Sedentary
Polychaetes (Sedentaria).

1. Free- living Poly chat es (Errantia), of which Nereis is a
good example, are carnivorous forms which move actively about
in pursuit of their prey, swimming, crawling, or burrowing. To
this habit their structure corresponds, for the locomotor organs
(foot-stumps) are well-developed, as also is the head-lobe, with
its eyes and feelers. The pharynx can be protruded in the way
described for Nereis and is usually armed with horny jaws.

Only one or two British forms can be mentioned here in
addition to Nereis. Species of Polynoe are common on our
shores, and these can readily be identified by their elongated
oval form and the presence of a double series of scales (elytra)
on the upper surface, which appear to be organs of respiration.
Built on somewhat the same lines is the conspicuous " Sea-
Mouse" (Aphrodite], the plump body of which attains a con-
siderable size. The setae are of several kinds, and very numerous :
some of those arising from the upper divisions of the foot-stumps
are much elongated and beautifully iridescent. As in Polynoe,
there is a double row of elytra, but they are here covered by
a tough membrane consisting of numerous small setae matted
together.

2. Sedentary Polychcztes (Sedentaria) either have limited
powers of movement, and inhabit permanent tubes, or else are
burrowing forms, which pour out a sticky fluid from the skin that
glues together the sand or mud surrounding them into a sort of
temporary case. They live upon vegetable matter. The head-

430

CHARACTERS OF INVERTEBRATE ANIMALS

region may either be much reduced and devoid of appendages,
or else, in the forms inhabiting firm permanent tubes, provided
with numerous tactile filaments and large branching gills. The
foot-stumps are not so well developed as in the free-living forms,
and the pharynx is not armed with horny jaws or teeth.

The Lugworm (Arenicola piscatorum) (fig. 266) is well known
as a burrowing form highly esteemed as bait. It makes U-shaped
passages in the mud or sand, near which may be seen
coiled " worm-castings " made up of sand and undi-
gested food, which have been voided from the body.
The most striking external feature consists in the
presence of branching gills projecting from the middle
region of the body. Cirratulus is a long cylindrical
worm often to be found buried in the sand under-
neath stones. Its locomotor organs are much reduced,
and the dorsal cirri are slender elongated filaments
which project above the surface of the sand and act
as gills, the active wriggling movements which they
constantly execute giving them a resemblance to small
red worms, for which they are often mistaken. Another
common shore form is Sabellaria, which glues sand-
grains together into a tube. Large numbers of these
animals live associated together, and their tubes often
form compact masses of considerable extent. Every-
one must have noticed at the sea-side small convoluted
limy tubes encrusting oyster-shells or stones. These
belong to Serpuia, a particularly beautiful form, in
which the head bears two brightly-coloured bunches of
gill-filamentSc One of these is converted into a conical
horny stopper, which closes the opening of the tube
when the worm has withdrawn itself. Spirorbis is a
related but much smaller form, in which the tube is
rig. 266. coiled into a flat spiral. Large numbers of these
may often be found adhering to brown sea-weeds.

Order 2. — FEW-BRISTLED WORMS (Oligochseta)

These are segmented worms which for the most part live in
fresh water or burrow in the earth. They lack feelers and foot-
stumps, while gills are but rarely present. Locomotion is effected

BRISTLE-WORMS 431

by the contractions of the muscular body-wall aided by the setae,
which are comparatively few in number. The pharynx is not
armed with horny jaws.

A common fresh-water form is the Red River- Worm (Tubifex
rivulorum), a small creature often to be seen in ponds, partially
imbedded in the mud. Large numbers often occur together,
making up conspicuous red patches, that disappear when the
mud is disturbed, which means that the worms have withdrawn
themselves into it.

Earth-Worms are found in almost all parts of the world, and
there are a number of British species, of which the Common
Earth- Worm (Lumbricus herculeus] is usually taken as a type.
The head consists of a mouth-segment and a head-lobe, and is
entirely devoid of eyes or feelers. The trunk is divided into
a large number of clearly - defined segments, and not very far
from its front end is a band-like thickening, the girdle (clitellum),
which has to do with the formation of capsules in which the eggs
are enclosed. The popular notion is that it is the result of injury
by the spade. If an earth-worm be passed, tail first, between the
finger and thumb, a distinct feeling of roughness will be experienced,
due to the presence of four double rows of setae, the tips of which
just project above the skin.

Order 3. — SIMPLE SEGMENTED WORMS (Archiannelida)

A small number of simple marine worms constitute this order,
and it is probable that to some extent at least they possess the
characters of the ancestral stock from which all segmented worms
are descended. It must not be forgotten, however, in this
connection, that simplicity of structure is often the result of de-
generation, and it is possible that some of these creatures may
be on the down-grade. There is a small head-lobe, and most
of them have a fairly long trunk. The segmentation of the body
is well marked by the presence of encircling grooves, and also by
rings of cilia, which serve as locomotor organs in place of setae
and foot-stumps, here entirely absent. Probably the most primitive
of the Archiannelids is the minute Dinophilus, of which more than
one species occur on our coasts. It presents certain points of
affinity to the unsegmented worms, which will be considered
later. Polygordius is a slender cylindrical red worm found,

432 CHARACTERS OF INVERTEBRATE ANIMALS

among other places, in the Mediterranean, and also recorded
from the British coasts. It is more complex in structure than
Dinophilus.

A word of explanation may be here necessary of the statement
that the simplicity of these creatures is possibly not primitive, but
the result of degeneration. Take, for example, the absence of
setae as a character to be criticised from the two points of view.
This negative character may mean that the ancestors of the group
did not possess setae, and, if so, the character is a primitive one.
To maintain, on the other hand, that the absence of setae is due
to degeneration is to suppose that the ancestral forms did possess
setae, which have been lost in the course of evolution. That such
a thing is possible becomes clear if we compare Nereis with some
of the sedentary Polychaetes and with earth-worms. In the first
case well-developed foot-stumps provided with setae are present;
in the second, these structures are reduced; while ordinary earth-
worms possess setae but no parapodia. A further reduction would
give the condition found in Polygordius (and some Oligochaetes).

CLASS II. — LEECHES (DISCOPHORA)

Leeches are typically fresh-water or terrestrial forms, though
some of them are marine. Most of them are blood-sucking para-
sites. The average characters of the group are conveniently
exemplified by the common Medicinal Leech (Hirudo medicinalis\
The elongated body, capable of considerable variation in form,
is flattened from above downwards, and is provided with a
sucker at each end, by alternate attachment of which looping
movements can be performed, much as in a looper caterpillar
(see p. 364). Both the suckers face downwards, and the mouth is
in the middle of the front one, while the intestine opens outside
and just above the hinder one. Encircling grooves divide the
body into a large number of narrow rings, several of which go to
a segment. Feelers, foot-stumps, and setae are entirely absent.
Ten simple eyes may be seen as black specks on the front end of
the body, placed close to the margin of the upper surface. The
well-known three-rayed bite of the leech is made by the finely-
toothed margins of three horny jaws with which the muscular
pharynx is provided.

Although the Medicinal Leech does occur in this country, a

SIPHON-WORMS

433

much commoner native form is the Horse- Leech (Aulastomum
gulo], which has smaller jaws and does not attack animals, preying
only upon small aquatic animals. Some of the Leeches do not
possess jaws, and these are grouped in a special subdivision of the
class. Many of them are parasitic upon fish.

A.

SIPHON-WORMS (GEPHYREA)

This group is retained as a matter of convenience to include
a number of worm-like animals, little known except to professed
naturalists, and some of which appear to be related to Annelida.
None of them exhibit other than feeble traces of segmentation.

The Bristle-Tail (Echiurus) (fig. 267) has a stout cylindrical
body covered by tough skin. The mouth is situated on the under
surface at the base of a narrow forward
projection known as the proboscis, which
must be regarded as a head-lobe. It is
used both as an organ of locomotion and
in procuring food, a deep groove running
along its under side to the mouth. Among
the characters which show affinity to the
Bristle -Worms may be mentioned the
possession of setcz, one or two rings of
which encircle the hinder end of the body,
while there are a pair of hook-like bristles
imbedded in the skin not far behind the
mouth. Behind them are the openings of
two pairs of excretory tubes (brown tubes)
comparable to the nephridia of segmented
worms.

The Siphon- Worm (Sipunculus] (fig. 267), numbers of which
are sometimes cast up on the shore by storms, burrows in the
sand, which it swallows for the sake of the contained organic
debris, in this respect reminding one of the earth-worm, to which
also our native species have a slight external resemblance, though
they differ markedly in the absence of setae and body-segments.
Examination of a living Sipunculus shows that the animal pos-
sesses the peculiar power of turning the narrow front part of its
body outside in (introverting it), this and the reverse process
often being rapidly repeated for a considerable number of

M« mouth; s?s' s?t£E; EX' excretojy

apertures ; i, intestinal aperture.

VOL. I.

28

434

CHARACTERS OF INVERTEBRATE ANIMALS

times. When the body is fully extended the mouth will be seen
at its tip surrounded by a series of short, grooved tentacles. This
creature departs further than Echiurus from the typical segmented
worms, for it is entirely destitute of setae, and the convoluted
intestine opens to the exterior not far from the front end of the
body on the upper surface.

WHEEL-ANIMALCULES (ROTIFERA or ROTATORIA)

The Wheel - Animalcules are minute transparent animals
mostly found in fresh water, though some are marine, and others
are to be found on damp earth and vegetation. They present
great diversity in form and structure, and as microscopic objects
are unsurpassed for beauty and interest. Unfortunately their
true affinities are yet to seek, but as likely as not they are the
degenerate descendants of higher worms, and are to be looked
upon as permanent larvae, the original adult form having been
dropped out of the life-history. That such a thing is possible is

shown by the case of the Axolotl among
Amphibia, to which reference has already
been made (p. 249).

A common and beautiful fresh-water form,
which will serve as a type, is the Rose-coloured
Rotifer (Philodina roseola) (fig. 268). The
elongated body is covered by a firm cuticle,
and this is marked by transverse furrows so
as to give a deceptive appearance of seg-
mentation. The posterior end of the body
tapers into a jointed tail ending in a pair of
forceps, and by means of this region the
animal is able to progress somewhat like
a leech. When the rotifer is fully extended,
a couple of prominences are seen at the
front end which together constitute the wheel -organ. Each of
them is fringed with a circlet of cilia, and as these move one
after another in a very regular way a deceptive appearance of
rotation is produced, suggesting the movement of a wheel. By
means of this organ the animal is able to swim and it also sets
up currents by which food particles are brought to the mouth,
lying in a depression at the front end of the body. The mouth

GIZZARD

STOMACH

1NTESTIN

Fig. 268.— Rose-coloured Rotifer
(Philodina roseola], greatly en-
larged

MOSS-POLYPES AND LAMP-SHELLS 435

opens into a mouth-cavity <md this again into a muscular pharynx,
which is continued by a short gullet into a wide stomach, this
narrowing into an intestine which opens to the exterior on the
upper surface of the body. Special interest attaches to the
pharynx, usually known as the mastax, for the lining of this is
thickened into a complicated set of hard pieces which work upon
one another to crush the food. The transparency of the animal
permits this process to be clearly seen, and it presents a curious
and interesting spectacle, reminding one of the gastric mill in a
lobster or the gizzard of a bird. The last section of the intestine
is a cloaca, into which opens a thin-walled bladder (contractile
vesicle) receiving nitrogenous waste matter from a couple of
branched excretory tubes, which in some respects resemble the
corresponding structures of the unsegmented worms. There are
no specialized organs of circulation, these being represented
simply by a fluid - containing body - cavity. Respiration takes
place by the general surface of the body, as is very commonly
the case with minute animals of all grades. The nervous system
consists of a comparatively large brain, placed in the front part
of the body above the mouth. A red eye-spot is imbedded in its
upper surface, and it is also connected with a stiff dorsal tentacle
(calcar) which ends in a bunch of stiff hairs and is probably an
organ of touch.

Rotifers differ from one another in many respects. The
general outline of the body varies largely, and the wheel-organ
in particular may be very variously shaped. Sometimes the
cuticle is thickened into a firm protecting carapace. Many
members of the group are fixed, and in this case a cement-gland
opens on the tip of the tail. Such forms often live in a cup or
" house " of various nature. Some of these will be described
later on, when the defences of animals are dealt with in detail.

MOSS-POLYPES AND LAMP-SHELLS (MOLLUSCOIDA)

Considerable doubt attaches to the affinities of the two aquatic
groups here associated together for convenience sake. They
were originally included in the Mollusca, and at a later date
Huxley established a sub-kingdom Molluscoida in which they
and the Tunicates were placed. When, however, it was proved
beyond doubt that the last group should without doubt be asso-

436 CHARACTERS OF INVERTEBRATE ANIMALS

ciated with the Vertebrata (pp. 297-300), the term Molluscoida
gradually fell into disuse, and the Moss -Polypes and Lamp-
Shells were cut adrift. There is good reason for thinking that
they are distantly related to the segmented worms, but their
relationships to that group and to one another are very obscure.
Perhaps the simplest plan will be to retain the old division
Molluscoida for their reception, provided the derivation of the
word ( = mollusc-like) be ignored. As Huxley himself has re-
marked, it is often necessary to retain terms, the derivations of
which have reference to obsolete views. The two included classes
—i. Moss-Polypes (Polyzoa) and 2. Lamp*Shells (Brachiopoda)—
are best considered separately.

CLASS I.— MOSS-POLYPES (POLYZOA)

These are small aquatic animals which are for the most part
inhabitants of the sea, though some of them are found in fresh
water. They present a peculiarity which so far we have only
met with in some Tunicates (p. 300), i.e. they are colonial. From
the egg a free-swimming embryo is hatched, which after a time
becomes fixed and founds a colony by means of budding, in a
way suggestive of what takes place in plants. The shape of
the colony naturally depends upon the manner in which the buds
arise, and it may be compact, flattened, or branched in a more
or less elaborate way. There is a firm external skeleton for the
support of the colony, sometimes of gelatinous nature, but more
frequently of horny or calcareous consistency. Each member
of the community (zooid) is lodged in a special depression or cup,
and within the protective investment the bodies of the different
zooids are connected together. The Polyzoa are among those
animals included in the old group of " zoophytes " (Gk. zoon, an
animal; phyto, a plant), so named from the exploded idea that
they partook of the nature both of plants and animals, a notion
largely due to the fixed nature and plant-like appearance of many
kinds. Even yet some of them are popularly confounded with
sea-weeds, and of such one of the most abundant is the Sea- Mat
(Flustra) (fig. 269), the flattened branching skeletons of which
are among the common objects thrown up on the shore by the
action of the waves. Examination of one of these with a lens
reveals the presence of innumerable closely-packed little cups,

MOSS-POLYPES

437

the homes of the constituent zooids. Another common form is
the Lace-Coralline (Membranipora), seen at low tide as a delicate
lace-like encrustation upon the large brown sea-weeds.

Fig. 269.— Polyzoa. i, Sea-Mat (Flustra}; 2, Sea-Net (Retepord)

The structure of the group is most conveniently studied in
one of the fresh-water Polyzoa, the Plume Coralline (Plumatella
repens] (fig. 270), the branched creeping colonies of which are

A.

CHETORY
OPENING
NGLION.

Fig. 270. — Polyzoa, enlarged

A, Small colony of Lophopus crystallinus, showing some individuals fully extended, and
others in different states of retraction. B, Diagram of a single individual of Plumatella, cut
through centre of body.

found creeping upon stones and other objects. The skeleton is
horny, and from the tip of each branch a zooid can be protruded,
the free end bearing the mouth surrounded by a double crown

438 CHARACTERS OF INVERTEBRATE ANIMALS

of ciliated tentacles in the form of a double horse-shoe, within
the concavity of which is the external opening of the intestine.
This protrusible part of the body is drawn back or introverted
by a retractor muscle at the approach of danger, reminding one of
the arrangement which obtains in Sipunculus (p. 433), a further
resemblance to which animal is found in the approximation of
the two openings of the gut, an obvious convenience in a tube-
inhabiting animal. Within the translucent body the U-shaped
food-tube can be clearly seen, consisting of gullet, stomach, and
intestine. There are two small excretory tubes opening near the
tentacular crown. The nervous system consists of a nerve -ring
closely surrounding the beginning of the gullet, and thickened
on the side next the intestine into a bilobed brain- or cerebral
ganglion. The stomach is connected with the body-wall by a
fibrous cord (funicle) in which are developed buds known as
statob lasts. In winter the colony dies down, to be replaced in
the following spring by fresh colonies to which these buds give
rise.

Another very beautiful fresh-water form, the transparency of
which makes it an attractive object under the microscope, is
Lophopus crystallinus (fig. 270), in which the skeleton is gela-
tinous, and the entire colony, instead of being fixed, is able to
creep slowly along the surface of water-plants.

CLASS II.— LAMP-SHELLS (BRACHIOPODA)

These are exclusively marine forms, including only about
1 20 recent species, though their area of distribution (which in-
cludes the British seas) is extremely wide. In some of the
geological periods, however, they were extremely abundant, but
have gradually declined in importance from very early times.

The most striking characteristic is found in the presence of a
bivalve shell, which for a long time caused them to be confounded
with bivalve Molluscs (pp. 328-338). Taking a typical Lamp-
Shell (Waldheimia or Terebratula) (fig. 271) as an illustration,
we shall find many well-marked differences from the members
of the last-named group. To begin with, the two valves are not
right and left, but upper and lower, and they are of unequal
size, the upper being the smaller. In side view the shell resembles

LAMP-SHELLS

439

in outline an ancient lamp, a fact which has given the popular
name to the class. The projecting beak of the lower valve is
perforated by a rounded hole from which projects a cylindrical
stalk by which the animal is attached, and which occupies the
same relative position as the wick of the imaginary lamp. The
fixed sedentary habit of this and most other forms is another
point of contrast with the bulk of bivalve molluscs, which possess
a marked power of
locomotion. We shall
further find that either
valve is bilaterally sym-
metrical, which is not
the case with a shell of
cockle, mussel, or the
like, while the two valves
of such a mollusc are
typically equal in size.
There are also great dif-
ferences in the minute
structure of the shell.

Upon opening the
shell of a Waldheimia,
the animal will be
found to occupy but a
very small proportion
of the space within,

but there are two large coiled "arms", one on either side ot the
mouth, fringed with a double set of tentacles and suggesting the
tentacular crown of Plumatella, if one supposes the halves of this
to be much elongated and coiled up. The so-called arms are
supported by a curved loop, shaped like a carriage-spring and
projecting from the upper valve. A mantle -lobe lines either
shell.

In a bivalve mollusc the two halves of the shell are drawn
together by adductor muscles and opened by an elastic ligament
(p. 330). In a Lamp - Shell both processes are effected by
muscles, and the hinge along which movements take place runs
transversely and is placed close to the beak. The gut is a bent
tube placed in the middle plane and consisting of gullet, stomach,
and intestine, the last ending blindly. A good-sized digestive

Fig. 271.— Lamp-Shell (Waldheimia)

A, Diagram of structure ; body cut through centre. B, Shell, seen from
left side, c, Interior of dorsal valve, to show "carriage-spring" support

440 CHARACTERS OF INVERTEBRATE ANIMALS

gland opens into the stomach. The excretory organs are in the
form of a pair of funnel-shaped tubes, which, like the nephridia
of a segmented worm (p. 425), place a spacious body-cavity in
communication with the exterior. The central nervous system
essentially consists of a ring surrounding the gullet, and thickened
above into brain- and below into other ganglia. (The upper part
of the ring is not shown in fig. 271, A.)

CHAPTER X

STRUCTURE AND CLASSIFICATION OF FLAT-WORMS
(PLATYHELMIA) AND THREAD-WORMS (NEMATHELMIA)

FLAT-WORMS (PLATYHELMIA)

The animals included in this phylum are unsegmented worm-
like creatures which in most cases are markedly flattened from
above downwards, hence the name of the group. They are
divided into three classes: — i. Tape- Worms (Cestoda); 2. Flukes
(Trematoda); 3. Planarian Worms (Turbellaria).

CLASS I.— TAPE-WORMS (CESTODA)

Tape- Worms are all internal parasites, and in the adult stage
are almost without exception inhabitants of the food -tube in
various Vertebrate animals, from Man downwards. The structure
has undergone profound modification as a result of the parasitic
habit, and the life-history is often complicated for the same reason.
Details must be deferred till the phenomenon of parasitism is fully
discussed, it being sufficient for the present purpose to briefly
consider an average example of the group, the Common Tape-
Worm (T<znia solium) (fig. 272), the adult stage of which is found
in the human intestine. The long flattened whitish body has
earned the name of " tape "-worm for the animal, which may be
as much as 3 yards in length. It consists of a very small head
and neck followed by the trunk, which gets broader and broader
as we pass back to the hinder end, and is divided into a very large
number of joints, which must not be mistaken for true segments
such as have been described for annelids. It is indeed doubtful
whether a tape-worm should be regarded as- a single individual.
An alternative view is that the head and neck together represent
what may be called the primary individual, from which a long
series of secondary individuals, the joints, has been originated.

Close examination of the minute head by means of a lens

441

442

CHARACTERS OF INVERTEBRATE ANIMALS

D.

B.

shows that at its front end there is a double circlet of horny hooks?
behind which four large slickers are arranged at equal distances.
By means of these hooks and suckers the head is attached to
the mucous membrane lining the intestine of the host. Such
organs of attachment are common among parasites, and another

example is given further
on in the case of the
Thorn -headed Worm (p.
449). No trace of a mouth
can be seen, and indeed
digestive organs are en-
tirely absent, for the para-
site lives at the expense of
its host, absorbing through
the general surface of its
body the digested food
with which, in the small
intestine, it is surrounded.
Nor are there any special
circulatory or respiratory
organs. The excretory
organs are well developed
and quite unlike the paired
nephridia of segmented

Fig. 272. — Common Tape-Worm ( Ttznia solium)

A, slightly reduced ; B-G, enlarged to various scales. A, Adult,
showing head and chain of joints (proglottides), the ripest of which
have been broken off. B, Head, c, Six-hooked embryo. D-F, Stages
in development of head. G, Bladder- Worm stage (cysticercus), with
head protruded.

worms.

narrow

They consist
of a

tube on each side,
not far from the edge of
the body, and connected

with elaborate branching canals which ramify in the soft substance
filling up the spaces between all the internal organs. The nervous
system is feebly developed, and its central parts consist of a longi-
tudinal cord on either side, these being connected in the head by
a transverse band, which may be regarded as equivalent to a pair
of brain-ganglia.

The joints which make up the trunk are largely filled with
egg-producing organs, and, at the hind-end, joints full of ripe eggs
are from time to time detached, passing out of the body of the
host to the exterior. If any of these joints are devoured by a
pig, the eggs hatch out in its stomach and the spherical embryos,

FLUKES

443

each of which is provided with three pairs of hooks, bore their
way into the blood-vessels and are carried to the muscles, where
they pass into a quiescent stage (pork " measles "). If a pig so
infested is slaughtered, and some of the " measly " pork eaten in
an imperfectly cooked condition by a human being, the life-history
of the parasite is continued a step further, for the " measles "
develop in the intestine of the new host into adult tape-worms.

CHIEF NERVE

NERVE RING

ENTRAL SUCKER

CLASS II. — FLUKES (TREMATODA)

Flukes, like Tape-Worms, are parasitic forms, but they present
a wider range in the nature of their parasitism, some being found
attached to the exterior of other animals, while some live within
various organs of the host. Here
again it will, for the present, suffice to
describe a single member of the group,
and the type which is most commonly
taken as a sample is the Liver -F hike
(Fasciola hepatica) (fig. 273), a much-
dreaded agricultural pest which gives
rise to the disease of sheep known as
" liver rot ".

The adult Liver -Fluke is found
often in large numbers in the liver of
the sheep, or, in some cases, of other
animals, among which Man must be
included. The body is leaf-shaped and
about an inch long. At the front end
there is a conical projection on the tip
of which is a sucker surrounding the
opening of the mouth. There is a
second sucker placed farther back on
the under surface of the body, and the
excretory organs open by a minute
pore at the tip of the hinder end. The

animal is covered by a thick cuticle beset with minute backwardly-
directed spines, which facilitate an onward wriggling through the
substance of the diseased liver, which gradually becomes more and
more degenerated and broken down, till at last, in bad cases, the
sheep dies.

EXCRETORY DUCT

^W EXCRETORY

PORE

Fig. 273. — Liver- Fluke (Fasciola. hepatica),
diagrammatic

444 CHARACTERS OF INVERTEBRATE ANIMALS

The internal structure of the Fluke is extremely complex,
and only a few of the more salient features can be mentioned.
The body-wall consists of the cuticle with underlying epidermis
and muscular layers, within which the spaces between the various
organs is filled up by a soft packing material, much as in a tape-
worm. Here, however, though special circulatory and respiratory
organs are absent, the digestive system is well developed, for the
surrounding food, consisting of blood and broken - down liver
substance, requires to be digested. The mouth leads into a
muscular pharynx, acting as a suction-pump, this into an extremely
short gullet, and this again into a forkecV intestine, the halves of
which are much branched and end blindly. The excretory organs
are much of the same kind as in the Tape- Worm, and the nervous
system is not much better developed. It consists of a ring round
the front part of the gut, thickened above into ill-defined cerebral
ganglia, and giving off a number of nerves, of which the most
important take a backward direction.

The life-history is more complex than that of the Tape-Worm,
and of a remarkable character, including a number of stages. The
ripe eggs produced by the adult Fluke may be taken as the point
of departure. These pass down into the intestine of the sheep
and thence to the exterior. If they happen to fall into water or
on to damp grass, and fluke-disease is commonest in wet fields, an
elongated ciliated embryo hatches out from each of them. This
swims actively about for some time, and if it happens to come
across a certain small water-snail (Limncea truncatula], further
stages in its life-history become possible. The larva makes its way
into the lung of the snail and becomes a shapeless bag (sporocyst}
within which are developed a number of cylindrical rediae, con-
stituting the next stage. The redia makes its way out of the
sporocyst and travels to the liver of the snail, upon which it preys.
The last stage in the life-history is the tadpole-like cercaria, which
is really an immature fluke, and this is produced within the redia
much as that took origin within the sporocyst. The cercaria
leaves the snail, and, swimming through the water by means of
its tail, reaches a piece of grass or some other plant, to which it
becomes attached. The tail is lost and the body becomes invested
in a firm limy coat or cyst. If now a sheep should happen to
swallow one of these encysted forms, the limy covering is dissolved
by the action of the gastric juice, and the young fluke, escaping,

PLANARIAN WORMS 445

passes on into the small intestine, whence it travels up the bile-
duct into the liver, there becoming adult.

CLASS III.— PLANARIAN WORMS (TURBELLARIA)

These are widely -distributed flat -worms, differing from the
members of the two preceding classes in being as a rule free-living,
though a few are parasitic. All are carnivorous. Planarians are
found abundantly in the shallower parts of the sea and in fresh
water, while others are terres-
trial, and are to be seen on
damp earth and vegetation.
One of the commonest British
species is a fresh-water form,

(fiV. 274). It Fi&- 2?4-— Planaria lactea (enlarged), with pharynx

l • • 1 protruded to exterior

is a flattened whitish creature

of elongated oval shape, and an inch or less in length, which
may be found gliding along over water-plants and stones. These
movements are effected by a uniform covering of cilia with which
the soft skin is clothed, and the presence of which constitutes one
of the important differences from the Tape-Worms and Flukes.
The ciliary action sets up minute currents and whirlpools in the
surrounding water, which can easily be seen with a lens, and have
given rise to the name Turbellaria, from the Latin turbellcz, a
word vulgarly employed to denote the bustling of a crowd.

The front end of the body is comparatively broad, while the
hind end is pointed. Upon the under side of the body, nearer
the hind than the front end, is situated the mouth, which leads
into a muscular pharynx continuous with a three-branched intestine,
of which one section is directed forwards and the others backwards,
while all three are beset with numerous branches that end blindly,
reminding one of the condition of the digestive organs in a Liver-
Fluke (p. 444). When the animal is in pursuit of prey, the pharynx
is protruded from the mouth and used as an organ of capture,
the food being taken directly into it, so, paradoxical as it seems, the
mouth is not used as a mouth. As in Flukes and Tape-Worms there
are muscular layers beneath the skin, and the spaces between the
complex internal organs are filled up with packing material. As,
too, in those forms there are no circulatory or respiratory organs,
and the excretory structures are much of the same kind. The

446

CHARACTERS OF INVERTEBRATE ANIMALS

BRAIN

central nervous system consists of a pair of brain-ganglia in the
front part of the body, giving off various nerves, of which the two
longest take a backward course. Upon the upper side of the head
region are two simple eyes, each of which is provided with a lens
and supplied by a special optic nerve.

The Land Planarians largely agree with the preceding in
structure, but are much more worm-like in appearance, though
equally unsegmented. Some of the tropical forms attain a large
size (up to 1 8 inches when extended), and are brilliantly marked
with longitudinal streaks of colour, but our few native species are
dull in colour and insignificant in size. • An example is Rhyn-
chodemus terrestris, which is under an inch in length, whitish in
colour below and dark-grey above.

The commonest example of the marine Turbellaria native to
Britain is probably Leptoplana tremellaris (fig. 275), which is
frequently to be found adhering to the under surface of stones
between tide-marks. It belongs to a group which is exclusively

marine, and differs in a number of
important particulars from Planaria
lactea. The flattened oval body is
rounded at either end but consider-
ably broader in front. Not only is
the animal able to progress in the
gliding manner already described,
TOMACH but it can also swim with consider-
able rapidity by flapping the sides
of the body up and down. It then
looks curiously like a minute Skate.
The mouth is situated about the
middle of the under surface, and
the pharynx, when protruded from

Fig. ^-LeptoplanTtremellaris (enlarged), as it, resembles a wide funnel with

rxlerit°mundersurface'withpharynxprotmdedt° plaited sides. This region of the

gut leads into an elongated oval

stomach from which branching, blindly-ending tubes radiate in all
directions. The bilobed brain is situated fairly far back, and gives
origin to a number of radiating nerves. There are four groups
of simple eyes situated on the upper side of the body, in the
neighbourhood of the brain.

Certain Turbellarians belonging to the same group as the

EYE

PHARYNX

THREAD-WORMS 447

species just described are brightly coloured, and some of them
attain a large size, one recorded specimen being six inches long
and four broad.

Speaking generally, the class of Turbellaria is one of extreme
interest, for it contains the simplest animals exhibiting bilateral
symmetry, and this is obviously associated with the creeping habit
(p. 22).

THREAD-WORMS (NEMATHELMIA)

This phylum comprises an enormous number of unsegmented
worm-like forms in which both ends of the body are usually
pointed. The large majority of them are parasitic in the bodies
of plants or animals, and various modifications in structure have
taken place in consequence of this habit. A common and large
type is the Round- Worm, one species of which (Ascaris lumbri-
coides) infests the human intestine, while a much larger species
{Ascaris megalocephald] often abounds in the corresponding part
of the horse.

The body of a Round- Worm (fig. 276), though cylindrical,
exhibits bilateral symmetry, but not in so marked a manner as
in the higher worms. Jaws and paired appendages of all kinds
are entirely absent. The mouth is placed at the front end, and
is guarded by three flap-like lips, of which one is dorsal, while
the interspace between the other two is in the mid-ventral line.
Each side-lip bears a small papilla which probably has to do
with touch, and two of these are to be found on the dorsal lip.
The intestine opens on the under side, not far from the hinder
end, which in the male is sharply bent round. Close examination
shows the presence of four streaks running longitudinally along
the body and corresponding to special modifications of the body-
wall. Two of these streaks are the lateral lines, one on each
side, and the others are dorsal and ventral lines, running in the
median plane above and below respectively.

The body-wall resembles that of a Nereis (p. 426) in so far
that it consists of external cuticle with underlying epidermis and
muscle, but the differences in detail are very considerable. The
cuticle is very firm and strong, probably serving to protect the
animal from the digestive juices of its host, while the epidermis
is indistinct and projects inwards to form the lateral and median

448

CHARACTERS OF INVERTEBRATE ANIMALS

lines. As a result of this, the underlying muscle, which constitutes
a sort of longitudinal layer, is broken up into four sections.

The digestive organs (fig. 276) consist of a narrow tube
running straight through the body, beginning with a thick-walled
gullet which passes into a thin-walled intestine.
Between the gut and body-wall is a body-cavity
containing fluid, but there is nothing else to which
the name of circulatory system can be applied.
There are no special breathing organs, and the
excretory organs are quite unlike those of a seg-
mented worm (p. 428), consisting as they do of
a narrow tube running in each lateral line, and
joining with its fellow in front to open by a minute
pore on the under surface. The neivous system
again is in many respects unlike that of a seg-
mented worm (p. 428). It is closely connected
with the skin, and its most conspicuous portions
consist of a ring not far behind the mouth, from
which nerves are given off both fore and aft.
Those in front supply the lips, and of the others
the largest are a dorsal and a ventral nerve which
traverse respectively the dorsal and ventral lines.

A common example of free -living Thread-
Fig. 276.-Round-worm Worms is the little Vinegar Eel (Anguillula aceti\

(Ascaris) . . fe -A

A side view B front common m paste and weak vinegar. Another
end of body, much en- curious creature, representing a group differing; in

larged to show mouth i ... .

and lips; INT. AP., in- many respects from the one containing Ascans and
Anguillula, is the Gordian-Worm (Gordius). This
is a very slender black worm, often to be seen in pools or puddles,
and the name has reference to the fact that a number of them
may often be found twined together into a complicated tangle.
In early life the Gordian-Worm is parasitic in an insect, leaving
its host, however, when it becomes adult, and as large numbers do
this about the same time, their appearance in public is somewhat
sudden. Hence the rustic belief, once prevalent, that they have
been rained down from the sky, a common explanation of the
rapid appearance in any quantity of all sorts of small animals.
Another curious popular idea, suggested by the appearance of the
worms, is that they are horse-hairs into which the breath of life
has mysteriously entered.

THREAD-WORMS 449

A small group, provisionally placed in the Thread- Worms, is
that represented by a curious parasite the Thorn-headed Worm
(Echinorhynchus), the front end of which is provided with a hook-
studded projection, by means of which attachment to the lining
of its host's intestine is effected. There are very many structural
differences from such a typical form as the Round -Worm, the
most remarkable being the absence of digestive organs, and it is
more than doubtful whether the group should be included in the
Thread- Worms at all. A typical species (Echinorhynchus gigas)
lives, when adult, within the digestive organs of the pig, while its
larval life is spent within the body of a Rose-Chafer or other
related beetle.

VOL. I.

CHAPTER XI
STRUCTURE AND CLASSIFICATION OF ECHINODERMS

(ECHINODERMATA)

This is one of the most sharply-marked phyla in the animal
kingdom, having specialized on very distinctive lines. Though
considered after the flat-worms, it must not be imagined that it
occupies a lower place in the scale. Indeed, as explained earlier
(p. 1 1 ), it is impossible to place the various groups of animals in
a linear series, each member of the series higher than the one next
below it, and lower than the one next above it. But it is clearly
necessary to describe the groups one after the other, just as if they
actually formed such a series, and this is apt to give misleading
ideas.

The phylum contains marine animals only, and probably the
most familiar of these is the Common Star- Fish (Uraster rubens),
found in abundance on many parts of our coast. It will serve
very well as our illustrative example.

The body of a Star-Fish (fig. 277) is built on a plan quite
different from those exhibited by the bilaterally symmetrical
animals with which we have so far been concerned. As in a very
large number of marine Invertebrates, there is a free-swimming
larva, known in this case as a Bipinnaria, which is markedly
bilateral. It is converted into the adult stage by a very com-
plicated series of changes, constituting a true metamorphosis, one
result of which is that the original bilateral symmetry becomes
obscured, being replaced by a simpler kind of regularity to which
the name of radial symmetry has been given, and which is well
exemplified by the flower of a primrose, a cart-wheel, or a regular
pentagon. In one kind of Star- Fish, indeed (Goniaster), the body
is a pentagonal disc, but in the form selected for description, the
corners of the pentagon have, so to speak, been drawn out so that
a distinction can be drawn between a central disc and five radiating
arms, which are continuations of it. In a Vertebrate, Arthropod,

450

ECHINODERMS 451

or Worm, the body can be divided into corresponding halves in one
way only, i.e. by a median plane which separates the right side
from the left, but here this can be done in five different ways.
Another way of putting it is to say that the body is regularly built
up around a series of axes which radiate from a common central
point. Any part or organ which is cut by one of these axes may
be said to occupy a radial position, while interradial structures

A.

TUBE-FOOT

PEDICELLARIA

Fig. 277. — Common Star-Fish ( Uraster rubens)
A and B, Upper and under surfaces, reduced ; c and D, much enlarged.

fall between adjacent axes. There is, it is true, a distinction
between upper and lower surfaces, as in vertebrates, &c., but it
is by no means clear that we are justified in considering these to
be truly dorsal and ventral.

Turning our attention to the pale under surface, we shall
see, in its centre, the mouth, entirely devoid of jaws and placed
in the middle of a soft area from which a deep groove runs along
each arm to its tip. Occupying these five grooves are a large
number of slender cylindrical structures which observation of a
living star-fish shows to be used in locomotion, and which have
consequently been termed tube-feet. Each groove has fancifully
been termed an ambulacrum^ because, together with its tube-feet,
it suggested to the giver of the name a little pleasure grove or
tree-lined avenue (Lat. ambulacrum]. The tips of the arms are
commonly seen to curl upwards, and each of them bears a reddish
eye-spot.

452 CHARACTERS OF INVERTEBRATE ANIMALS

The most conspicuous feature of the upper surface of the
disc is found in the presence of a rounded plate in one of the
interradial areas. The meandering grooves which mark its sur-
face look like the markings upon certain corals (madrepores),
and have suggested the name of madreporic plate or madre-
porite. As there is only one of these structures, the radial
symmetry of the body is interfered with, and, strictly speaking,
it is bilaterally symmetrical, the median plane passing through
the plate and along one arm, which is generally considered to
be the front or anterior one. So that the Star- Fish really presents
bilateral symmetry masked by radial symmetry, so to speak. The
minute aperture of the intestine is placed near the central point
of the upper surface, but is not absolutely in the centre.

The Star- Fish is firm to the touch, owing to the presence of
a well-developed system of hard calcareous plates, present in
the deeper part of the skin, and which are best seen in a dried
specimen. Those which support and strengthen the upper part
of the body are united together into an irregular net-work, but
in the neighbourhood of the ambulacra they are regular in shape
and have a definite arrangement. Of these plates the most con-
spicuous are a double series of ambulacral ossicles united together
so as to look, in the cross section of an arm, like an inverted V.
The edges of the ambulacra are fringed with spines, and it is
the presence of these in typical Echinoderms which has given
the name to the phylum (Gk. echinos, a hedgehog; derma, skin).
Spines, however, are much more numerous and conspicuous in
the members of some of the other sorts of Echinoderm, notably
in the Sea- Urchins, which is expressed in the name for " urchin ",
an old English word for a hedgehog. Among these spines,
especially in the neighbourhood of the mouth, are some which
have been peculiarly modified to form snapping jaw-like structures,
known as pedicellarice. One use of these is probably to assist in
cleaning the surface of the animal.

Internal Structure (fig. 278). — Several points of interest are
presented by the digestive organs. The mouth leads into a large
stomach with folded walls, capable of being protruded outside of
the body so as to engulf prey of comparatively large size. Star-
fishes are extremely rapacious animals, and are among the worst
enemies of the oyster. The stomach, after protrusion, is drawn
back into the body by special muscles. It is continued into a

ECHINODERMS 453

five-angled pyloric sac, and this into a short thin- walled intestine.
The cavity of each arm is largely occupied by a pair of large
saccular tubes which open into one of the corners of the pyloric
sac, and secrete a digestive juice. The circulatory organs
chiefly consist of an ill-developed blood-system devoid of heart,
and of a spacious body-cavity. Special respiratory organs are
evidently necessary in a creature furnished with a firm exo-
skeleton, and in this case the function is discharged in part by
delicate branched gills which can be protruded through the
interstices of the calcareous net- work which strengthens the upper

CAECUIV^|NTEST1NAL Ap< _JDIGESTIVE GLAND

MADREPORITE

GUI-LET-

AMBULACRAL-RING "TSjERVERING \ " RADIAC NEBVE

MOUTH RADIAL AMBULACRAL VESSEL

Fig. 278.— Common Star- Fish (Uraster rubens]; cut through disc and one arm to show structure. Diagrammatic

surface. The tube-feet most likely help in breathing, and some
of them in the neighbourhood of the mouth appear to be specially
concerned with this function.

Great interest attaches to what is known as the water -vascular
system, a set of organs to which the tube-feet belong, and which is
quite unlike anything existing in other subdivisions of the animal
kingdom. It consists of a tubular ring surrounding the beginning
of the food- tube, and sending a radial vessel along each arm, just
below the union of the two rows of ambulacral ossicles. The
tube-feet are branches of the radial vessels, but they are also
connected with little transparent bladders (ampullae) placed within
the cavities of the arms. The fluid filling the water -vascular
system is largely sea-water, which is able to get in from the
exterior by a stone-canal that runs from the madreporite to
the central ring, the grooves in the former being perforated by
numerous minute holes. The ampullae, by their contraction,
force fluid into the tube -feet, thus causing their protrusion.
This method of working a set of locomotor organs is quite
unlike anything we have so far met with, and it compensates
for the comparative feebleness of the muscular system, correlated,

454 CHARACTERS OF INVERTEBRATE ANIMALS

it would seem, with the marked development of a continuous
exoskeleton, between the parts of which there are not, as in
Arthropods, well-marked joints permitting of free movement.

The nervo^ls system is of very primitive type, and arranged,
like the other internal organs, in accordance with the radial
symmetry of the animal. It is intimately connected with the
skin, of which, in fact, it may simply be regarded as a thicken-
ing, and its best-marked portions consist of a ring round the
mouth, and five radial nerves, one of which runs along each arm.
But the entire body is invested in a nervous sheath, consisting
of a delicate net-work lying in the deeper part of the skin. The
chief sense organs are the tube- feet, which appear to be, among
other things, tactile structures — indeed there is an odd tube-foot
at the end of each arm which may definitely be called a tentacle, —
and the eye-spots already noted.

What are known as "comet" forms of Star-Fish are not
infrequently met with, i.e. specimens in which from one to four
arms are smaller than the remainder. The small arms are new
ones growing in place of others which have accidentally been
lost. Lower animals often possess considerable powers of re-
generating mutilated parts, and this is to a limited degree the
case even among some of the Vertebrates.

Living Echinoderms are arranged in five classes: — i. Star-
Fishes (Asteroidea) ; 2. Brittle- Stars (Ophiuroidea); 3. Sea-Urchins
(Echinoidea) ; 4. Sea- Lilies and Feather-Stars (Crinoidea); Sea-
Cucumbers (Holothuroidea).

CLASS I.— STAR-FISHES (ASTEROIDEA)

These conform in character to the described type in many
at least of the particulars. They are found in all seas, and
the majority are inhabitants of shallow water, though some,
including the largest species, are found at considerable depths.
There are generally five arms, but this is by no means a
universal rule. A common British species, for instance, the Sun-
Star {Solaster papposus}, possesses no less than thirteen. The
name has been given on account of its resemblance to the con-
ventional symbol by which the sun is usually represented, i.e. a
circle with a number of radiating lines.

ECHINODERMS

The animals here included constitute a well-marked group of
marine animals, in which the body is typically star-shaped or
spheroidal, or less commonly cylindrical. There is a more .or
less perfect calcareous skeleton developed in the skin, and part
of it may consist of fixed or movable spines, hence the name of
the group (Gk. echinos, a hedgehog ; derma, skin : i.e. hedgehog-
skinned). A peculiar system of tubes is present, containing sea-
water and communicating with the exterior. In three of the
constituent classes (star-fish, sea-urchins, and sea-cucumbers) part
of this "water- vascular" system consists of sucker-like tube-feet,
which are used for executing crawling movements.

Four common British forms are figured as types of the four
classes now dominant, i.e. Star-fish (Asteroidea), Brittle Stars
(Ophiuroidea), Sea-urchins (Echinoided), and Sea-cucumbers (Holo-
thuroidea).

1. Purple-tipped Sea-urchin {Echinus miliaris}. Body covered

by numerous spines attached by ball-and-socket joints to
little knobs. (" Urchin" is an old name for the hedge-
hog.)

2. Black Sea-cucumber (Holothtiria nigrd). Leathery skin, in

which scattered calcareous plates are embedded.

3. Brittle Star (Ophiura). Disc-like body with five flexible

limb-like arms.

4. Common Star-fish (Asterias rubens). With five arms which

are extensions of the central body. ('The large specimen
figured has more than the usual number of arms.)

ECHINODERMS

1. Sea-urchin.

2. Black Sea-cucumber.

3. Brittle Star.

4. Common Starfish.

BRITTLE-STARS 455

CLASS II.— BRITTLE-STARS (OPHIUROIDEA)

The members of this class, though to some extent they
resemble ordinary star-fishes, and are by some naturalists included
with them in a common section of the phylum, differ in a number
of important particulars. There are a number of British species,
of which one of the commonest (Amphiura squamatd] is a little
creature found on the under sides of stones near low-tide mark
(fig. 279). It is at once evident that the five arms are sharply
marked off from the central disc.
They are indeed more of the nature
of appendages, and are here the
organs of locomotion, for which their
extreme flexibility eminently fits
them. Upon the under side of the
disc, as in an ordinary star-fish, is
situated the mouth in the form of a
five-rayed slit. There are no ambu-

.. 1 , i • i r Fi&- 279. — Brittle-Star (Amphiura sguamata),

lacral grooves, and the under side of seen from above. Enlarged

each arm is covered by a regular

series of flat plates, at the sides of which the tube-feet project.
They are not, however, used as feet, but serve as organs of
touch and respiration. There are no eye-spots on the tips of
the arms. Turning to the upper surface, no trace of a madreporite
can be seen; it is represented by one of the plates situated inter-
radially on the under side. There is no intestinal opening at all.
Each arm is covered by a single series of plates, and a similar
series runs along each of its sides. The margins of the arms
are spiny, but there are no pedicellariae.

There are important differences as to internal structure between
a brittle-star and an ordinary star-fish. The mouth, for example,
is armed by a number of modified plates, and it leads into a
spacious stomach, which bulges out into ten very short pouches.
But there is no intestine and no digestive structures in the arms.
Indeed each of these is traversed by a special series of ossicles,
which have been called vertebral, because they are jointed together
something like the successive vertebrae making up a backbone.
Each of them is formed by the fusion of two ossicles, which are
equivalent to a pair of the ambulacral ossicles of an ordinary

456

CHARACTERS OF INVERTEBRATE ANIMALS

star-fish. The stomach is not capable of being protruded as in
one of the ordinary Stars.

CLASS III.— SEA-URCHINS (ECHINOIDEA)

Among the animals commonly brought up by the dredge in
British seas, and sometimes thrown up on the beach by storms
is the Edible Sea- Urchin (Echinus esculentus) (fig. 280), which,

A.

AMBU
AR.EA

AMBULACRAL

AREA

ADREPORITE
NTESTINAL

Fig. 280.— Edible Sea-Urchin (Echinus esculentus]

B, c, D, Enlarged to various scales. A, Diagram of upper surface, with most of spines removed. B, Plates of
small part of an ambulacral and an interambulacral area, with spines removed. Pores for tube-feet and knobs for
spines are shown, c, Diagrammatic section through spine and knob, to show mode of attachment. D, Pedicellariae.
E, Diagram of side-dissection, most of gut having been removed, w.v. is an abbreviation for " water vascular".

though it differs very much in appearance from a star-fish, is really
constructed on much the same type. The spheroidal body sug-
gests in appearance a curled-up hedgehog, whence, as previously
explained, the name " sea-urchin ", the appearance being due to
the presence of an enormous number of spines, movably jointed
to the underlying calcareous plates which, united firmly at their

SEA-URCHINS 457

edges, constitute a firm protective "test". The base of each
spine is hollowed out into a cup which fits over a corresponding
knob on the test, so that there is a ball-and-socket joint arrange-
ment, and by means of special strands of muscle the spine can be
moved in any direction. This is an adaptation to locomotion as
well as to protection. Effective tube- feet are, however, present,
and in a living sea-urchin these may be seen protruding between
the spines, and if we compare the animal to a globe with upper
and lower poles, their distribution is expressed by saying that they
are arranged in five meridional bands.

As in a star-fish, the mouth is situated in the centre of the
under surface, but in this case there are five pointed structures
to be seen projecting from it, these being the tips of hard jaws
shaped something like the front teeth of a rabbit. The opening of
the intestine is placed almost in the middle of the upper surface.

On scraping away the spines, the characters of the test may be
studied in detail, and it is found to be made up of twenty regularly-
arranged sets of plates arranged in ten strips, which are broad in
the middle but narrow at either end, like the " gores " which make
up a balloon, or are sewn together to cover a child's cloth-covered
ball. Two meridional rows of plates are united to make up each
of these bands. Five of these differ in one important particular
from the others which alternate with them, the distinction resting
in the presence of a series of pores arranged in pairs, and placed
near either edge of the band to which they belong, Each pair
of these pores belongs to a tube-foot, which is thereby placed in
communication with a radial water-vascular vessel, and with an
ampulla (see p. 453), both of these being situated within the test.
The presence of these pores enables us to distinguish between five
ambulacral areas, bearing tube -feet, and five intervening inter-
ambulacral areas. The former correspond to the radii about which
the body is symmetrically built, and the others are consequently
interradial. It may be useful to institute a comparison with the
star-fish, taking for the purpose, not the common form, but the kind
(Goniaster) shaped like a pentagonal disc. If we suppose the
body of such a form to gradually swell so as to become spheroidal,
while at the same time the original upper surface gets smaller
and smaller, the lower surface (bearing the tube-feet), becoming
correspondingly extended, the sea-urchin type will be produced.
For the five ambulacral grooves will be converted into the five

CHARACTERS OF INVERTEBRATE ANIMALS

Fig. 281. — Three-jawed Pedicellariae of Sea-
Urchins ; greatly enlarged

ambulacral areas, and the regions between them into the five
interambulacral areas.

There yet remain one or two other points to be noticed in
the sea-urchin test. ' The plates at the upper pole, in the neigh-
bourhood of the intestinal aperture,
are somewhat specialized, and consti-
tute an apical disc, the most important
elements in which are ten plates, of
which the five smaller ones are situated
radially, and therefore correspond with
the upper ends of the ambulacral areas.
Each of them is called an " ocular ",
because it bears an eye -spot. The
five interradial plates are somewhat
larger, and one of them is modified
into the madreporite, which has the
same function as in an ordinary star-
fish (see p. 452). Like that animal,
too, the sea-urchin possesses those
modified jaw-spines to which the name
of pedicellarice has been given, but in this case the jaws are three,
and not two in number (fig. 281). Some possess poison-glands
(fig. 281, right).

The digestive organs consist of gullet and intestine, the latter
taking a spiral course, while the former traverses a complicated
chewing apparatus fancifully called " Aristotle's lantern", and made
up of the five jaws, together with many other hard parts. By means
of special muscles the jaws can alternately be brought together
and separated. As in the other groups of Echinoderms so far
dealt with, there is a blood system of rather obscure nature, and
the rest of the circulatory organs consist of a particularly spacious
body -cavity in which the various internal organs are placed.
Breathing is effected by the tube-feet, and also by ten branching
gills placed in the neighbourhood of the mouth, and resembling
the structures which project between the interstices of the skeleton
in a star-fish (p. 453). This function is also possibly assisted by
a narrow tube (siphon) running parallel to the gut and opening
into it at either end. The water-vascular system is arranged on
the same general plan as in a star-fish (p. 453), consisting of a ring
round the gullet connected with the madreporite by a stone-canal,

SEA-LILIES AND FEATHER-STARS 459

and giving origin to five radial vessels of which the tube-feet are
branches, these vessels, however, running within the test. The
nervous system, too, is of the same general kind, but there are two
nervous net-works, one outside the test and the other within it.

Sea- Urchins are grouped in two divisions: I. Regular forms, of
which the just-described species is typical, and II. Irregular forms,
in which there is a strongly - marked bilateral symmetry, and the
body is often flattened, while the upper parts of the ambulacra
are broadened out so as to collectively make up a flower-shaped
figure, the tube-feet of which are definitely specialized into gills.
These irregular forms again are divided into Shield- Urchins and
Heart- Urchins.

Shield -Urchins are sometimes very much flattened, as the
name implies, and though the mouth has the normal position, the
aperture of the intestine is shifted back into an interradial position.

Heart-Urchins are distinguished by the shape under which the
heart is conventionally represented, the notch of the heart being in
front. The opening of the intestine is displaced to the edge of the
disc, and the mouth, instead of being central, is shifted forwards.
The Heart -Urchins are devoid of the special chewing apparatus
known as " Aristotle's lantern ". A common British species is the
Purple Heart- Urchin (Spatangus purpiireus).

CLASS IV.— SEA-LILIES AND FEATHER-STARS

(CRINOIDEA)

This is a decadent group, once of great luxuriance, but of
which the most typical modern representatives are rare, and con-
fined to the very deep sea. Taking such a typical Sea- Lily, for
example, as Pentacrinus (fig. 282), it may roughly be compared to
a star-fish with branching arms placed on the top of a long stalk,
and having its mouth surface turned upwards. A certain vague
sort of resemblance to a flower upon its stem has suggested the
name.

Examining Pentacrinus more closely, there will be seen a
relatively small central disc, or cup, as it is usually termed, from
which five arms project. These are of the nature of appendages,
and begin almost at once to branch in a forking manner. Each
branch bears a double series of short filaments termed pinnules,
the name having reference to the arrangement seen in some plants,

460

CHARACTERS OF INVERTEBRATE ANIMALS

e.g. in many ferns, where a central axis supports a row of leaflets
on each side in a feather-like
way. The mouth is in the
centre of the upturned disc,
and from it radiate five ambu-
lacral grooves, corresponding
to the similarly -named struc-
tures in a star-fish (p. 451).
As the arms branch, so do
these grooves, and their small-
est subdivisions run along the
pinnules. Projecting from each
side of an ambulacral groove
and its branches are a series
of delicate pointed structures
representing the tube-feet of a
star-fish, but here no longer
having anything to do with
locomotion. The intestine

UT.ARM

NALAPERTURE

Fig. 282.— Sea-Lily (Pentacrinus)

A, Calyx and part of stalk (reduced). B, Upper side of
calyx, with arms cut off.

opens near the mouth upon a
projection situated in one of
the interradial spaces. The
ander side of the cup is sup-
ported by regularly-arranged
circlets of calcareous plates,
some of which are compar-
able, it would appear, with
the apical disc of a sea-urchin.
There is, however, no madre-
porite. The under side of
each arm, arm -branch, and
pinnule is also supported by
a series of hard parts, jointed
together, however, so as to
permit of a certain amount
of movement. The stalk
upon which the animal is supported is five-sided, and it is sup-
ported by a series of calcareous pieces, in the form of prisms

Fig. 283.— Feather-Star (Comatula), climbing

SEA-LILIES AND FEATHER-STARS

461

joined end to end. At regular intervals the stalk bears circlets
of jointed filaments (cirri) which also are strengthened by a
deposit of lime.

The internal structure is very complex, and it need only be
said that the gut does not extend into the arms, and that apart
from it the chief organs
have the radial arrange-
ment described for
other types.

Although Sea- Lilies
are distinctly scarce
animals, the same can
scarcely be said for the
Feather- Stars, of which
one kind (Antedon or
Comatula rosacea) (fig.
283) is not uncommon
in British seas. Its
life -history is one of
peculiar interest, for
the eggs develop into
a stalked larva which
is found attached to
sea -weed, and looks
like a minute Sea- Lily
(fig. 284). Later on,
the stalk disappears
and the animal takes
to a free life, though
it is possessed through-
out its existence of a circlet of cirri, corresponding to the top
circlet of those structures in a stalked form, and used to anchor
the creature, and also in climbing. The life-history is interesting,
chiefly because it is one of the best-known examples of a case
where the development of the individual is a compressed present-
ment of the genealogy of the species. If the Feather-Star were
the only known Crinoid there would be a strong presumption that
its remote ancestors were stalked, and this conclusion is fully
borne out by a study of recent and fossil forms.

Fig. 284.— Stages in development of Feather-Star (Comatula}, muck
enlarged. 3 is the stalked or pentacrinus stage

462 CHARACTERS OF INVERTEBRATE ANIMALS

CLASS V.— SEA-CUCUMBERS (HOLOTHUROIDEA)

These are for the most part leathery elongated forms, some
of which look uncommonly like cucumbers, a similarity which
has suggested the name of the group. They are regarded by
some authorities as representing most nearly of living animals the
ancestral stock from which all Echinoderms must be supposed
to have taken origin. This, however, is extremely doubtful ; but
as unfortunately the class, unlike the other ones included in the
phylum, is but poorly provided with hard parts, fossil remains are
infrequent, and throw but little light upon the question.

In a typical Holothurian (such as the genus Cucumaria) (fig.
285) the elongated body is somewhat angular, and the two open-
ings of the digestive organs are at opposite ends of the body,
the mouth being surrounded by a circlet of ten branching tentacles,
which can be retracted. The ambulacral areas are marked by five

double rows of tube-
feet protruding along
the side of the body.
No trace of a madre-
porite can be discerned.
The body- wall consists
of skin and underlying
muscles, the latter

Fig. 28S.-A Sea-Cucumber (Cucumaria}, reduced bemg aS Wel1 developed

here as they are scanty

in those forms which have a well-developed exoskeleton. The
hard parts of a Hoiothurian are comparatively insignificant, the
most important of them making up a calcareous ring round the
gullet ; and there are, besides, minute calcareous spicules scattered
through the skin.

The mouth, as already stated, is placed in the middle of the
crown of tentacles, and these are arranged in five pairs, one for
each ambulacral area. One pair is smaller than the rest, and
is used in shovelling food into the mouth. They correspond to
the under surface of the body, which in some other members of
the group is very sharply defined, though here the radial symmetry
is not much interfered with. The mouth leads into a mouth-
cavity, which opens into a gullet, and this again into a looped

SEA-CUCUMBERS 463

intestine, which dilates near its termination into a section which
is known as the cloaca, though it can scarcely be said to correspond
to the similarly-named cavity found in many of the vertebrate
animals (see pp. 69, 146, 200, 240, 261).

The circulatory organs consist, as in a star-fish or sea-urchin,
of a blood system and a lymph system. The former is chiefly
made up of a ring round the beginning of the gut, of five radial
vessels, and of branches to the digestive and some of the other
internal organs. The lymph system consists of the spacious
body-cavity filled with fluid, in which float both colourless and
red corpuscles, reminding us of the blood of a typical vertebrate
(see p. 38).

Breathing is no doubt partly effected, as in other cases, by the
tentacles and tube-feet, but there are also other structures which
probably have to do with the same function, in the form of two
large branching respiratory trees that open into the cloaca. The
branches of these organs are beset with innumerable minute ciliated
funnels, by which the body-cavity is placed in communication with
the exterior. It is extremely likely that these trees also have
to do with getting rid of the nitrogenous waste of the body, and
if so, they are excretory as well as respiratory organs.

The water -vascular system is constructed on the same plan
as in a star-fish. There is a ring round the beginning of the gut
from which a radial vessel runs along each ambulacrum to give off
the double row of tube-feet. Branches of these vessels supply
the tentacles, which are to be regarded as much-modified tube-feet,
and there is a stone-canal with a madreporite which, instead of
opening to the exterior, hangs down into the body-cavity. In
a very young Holothurian, the stone-canal opens directly to the
-exterior, as in a star-fish or sea-urchin, but this connection is lost
in the typical forms, though there are some deep-sea species
in which it persists throughout life.

The nervous system is of the type already described for other
subdivisions of the class (p. 454). Sense Organs are chiefly repre-
sented by the tentacles and tube-feet, which no doubt have to do
with the sense of touch.

Within the limits of the class there are numerous variations
in many respects. Certain deep-sea forms present a well-marked
bilateral symmetry, and the body is produced into pairs of pro-
jecting processes. In such forms, a well-marked flattened lower

464 CHARACTERS OF INVERTEBRATE ANIMALS

surface upon which the animal creeps presents a sharp contrast
with the curved upper surface. This is also the case with some of
the species inhabiting much shallower water, and in these bilateral
forms, three of the ambulacra are turned downwards, and bear
tube-feet used in locomotion, while the other two face upwards,
and their tube-feet are modified into pointed structures useful only
as organs of breathing and touch. The tentacles also present
a considerable variety in size and shape. Some common forms
have the tube-feet irregularly scattered over the body instead
of being arranged in rows, while in yet others the tube-feet, and
even the radial branches of the water- vascular system, are absent
altogether. This is the case, for example, in the genus Synaptay
species of which are found in British seas. Here the translucent
body is worm-like, and the small tentacles are feather-shaped, a
further character of interest being found in the shape of the
calcareous plates embedded in the skin, which are in the form of
anchors with associated oval perforated plates (anchor plates).
It also possesses ten minute auditory sacs in close connection
with the nerve-ring.

CHAPTER X
STRUCTURE AND CLASSIFICATION OF ZOOPHYTES

(CCELENTERATA), SPONGES (PORIFERA), AND ANIMALCULES

(PROTOZOA)

ZOOPHYTES (COELENTERATA)

The old expression " zoophyte " was applied to branching
colonies of organisms regarding the animal nature of which doubt
existed, and which were looked upon as being intermediate be-
tween animals and plants. Here were included the forms already
briefly dealt with under the heading of Polyzoa (p. 436), but these
are of much higher structure than the
bulk of zoophytes, which may con-
veniently be grouped together under
the phylum now to be considered.
This embraces the Corals, Jelly- Fish,
Sea -Anemones, and other creatures
less familiar to the ordinary observer.
As a simple and easily obtained type,
it is customary to take the Fresh-water
Polype (Hydra) (fig. 286), which is
often found attached to pond-weeds,
or sticking to the glass of a fresh-
water aquarium. By leaving such an
aquarium in a brightly-lit window for
some time, one may often secure a
supply of these animals, which collect
on the side next the light. As they
are not more than about -J- of an inch long, the naked eye alone
is not sufficient for their proper examination. Much can be made
out with a simple lens, but a compound microscope is necessary
to determine details. This is one of the numerous cases where
important results are to be obtained by means of cutting thin
slices with a razor, an operation which requires that the animal

Fig. 286.— Green Hydra (Hydra -viridis),
enlarged

VOL. I.

465

30

466 CHARACTERS OF INVERTEBRATE ANIMALS

should be killed, stained, and imbedded in paraffin wax or some
similar substance. Extremely delicate slices of known thickness
can be cut by means of the instrument known as a microtome, in
which a sharp razor is mechanically drawn across the imbedded
specimen, this being held firmly by a suitable device. The slices,
when prepared, have to be cleared from paraffin and mounted on
a glass slide, in some transparent medium, such as glycerine or
Canada balsam.

Specimens of Hydra are brown (e.g. Hydra fusca) or green
(Hydra viridis) (fig. 286) according to the species, but in any
case the form and structure are much the same. The colour of
the green kind is particularly interesting, since it is due to the
presence of a pigment (chlorophyll) which is characteristic of
ordinary plants, and, as we shall see later, plays a leading part
in their nutrition. The body of a fully -extended individual is
a hollow cylinder closed at one end, the foot, by which attach-
ment to a firm object is effected, while it is narrowed and open
at the other, the aperture being the mouth. A circlet of long
slender tentacles is situated quite close to the mouth, just where
the body begins to narrow. The large internal digestive cavity
has no other opening to the exterior. We find here a perfect
example of the star-like or radial symmetry which is exemplified
in a less complete manner by a star-fish or regular sea-urchin,
and there is absolutely no trace of the bilateral symmetry so
characteristic of higher forms (see p. 21). In other words, there
is no distinction between front and back ends, right and left sides,
or dorsal and ventral surfaces.

If a fully-extended Hydra be touched or shaken it will at once
become retracted, its body shortening into a little rounded lump,
while the tentacles become minute knobs. Such an arrangement
is obviously of a protective character. When in an extended
position the animal is on the look-out for prey, if such an ex-
pression may be used in the absence of visual organs. Its food
consists of small aquatic animals such as Water-Fleas (see p. 422),
which are much higher in the scale, and would at first sight appear
to have every chance of escaping capture. This, however, is
not the case, for if one of these active creatures happens to come
into contact with one of the tentacles of the Hydra it stops dead
as if paralysed, and is then by the help of the other tentacles
drawn down to the dilatable mouth and passes out of view into

ZOOPHYTES 467

the digestive cavity. The sudden arrest of activity on the part
of the unfortunate water-flea is brought about by means of minute
" thread -cells" or " nettling- cells ", which, as will be explained
elsewhere, are poisoned weapons of considerable virulence.
Numerous groups or " batteries " of these are present upon the
tentacles, on which they confer a roughened appearance, and
they are also present, though less abundantly, upon other parts
of the body. Nettling -cells are very characteristic of the
Ccelenterates, a fact which has been painfully brought home to
many bathers in the case of some of the larger jelly-fish, while
even the innocent-looking sea-anemones have been known to
seriously inconvenience persons who happened to touch them.
Even, therefore, if Hydra, like many of its brethren, were con-
demned to remain fixed in the same spot it would not necessarily
be starved, but, as already hinted, it possesses considerable powers
of locomotion. It can, for example, slowly shuffle along upon
the attached end of the body, and can also execute looping move-
ments by alternately attaching the two ends of its body to the
surface of a stone or water-weed, reminding one of what happens
in the case of a leech (p. 432) or a looper caterpillar (p. 364).
Specimens may also be found floating freely in the water, mouth
downwards, with the foot close to the surface and held in place
by what is known as surface tension.

Stritcture of the Body (fig. 287). --Most of the external
characters of Hydra have now been dealt with, and it remains
to consider the minute structure of the body as determined by
means of the microscope. Much can be learnt by examination
of a thin cross-section, which may instructively be compared with
a similar section through the body of an earth-worm. There is
a central space corresponding to the digestive cavity, and outside
this the body-wall, which is clearly made up of an outer layer
and a much thicker inner layer, the two being separated by a
thin membrane (supporting lamella). These two layers are
respectively known as the ectoderm (Gk. ektos, outside; derma, a
skin) and endoderm (Gk. endon, within; derma, a skin), and a
double body-wall made up in this way is eminently characteristic
of Ccelenterates and Sponges, especially in embryonic stages, on
which account these two phyla are often grouped together as
the " Diploblastica" (Gk. diplous, double; blastos, germ or bud).
Turning now to the section of earth-worm, we see in the middle

468

CHARACTERS OF INVERTEBRATE ANIMALS

a space corresponding to the digestive cavity or gut, which,
however, is not bounded directly by the wall of the body but by
a wall of its own separated by a considerable space, the body-cavity,
from the body- wall. Hydra is absolutely devoid of a body-cavity
in this sense. If we attempt a comparison between the two

animals, the epidermis of the
worm may be looked upon
as ectoderm, and the inner-
most layer of the wall of the
gut would appear to be equi-
valent to endoderm. But the
greater part of body -wall
and gut -wall in the worm,
together making up what is
called the middle layer or
mesoderm, has no distinct
representative in Hydra,
unless, perhaps, it is repre-
sented in an inadequate
manner by the thin mem-
brane between ectoderm and
endoderm. The presence of
a distinct third layer coming
between ectoderm and endo-
derm is characteristic of all
the phyla above the Zoo-
phytes, and this comes out
very clearly and simply in
embryo stages. Hence all
these phyla are collectively
grouped together as Trip lob las tica, i.e. three-layered animals, as
contrasted with the two-layered forms or Diploblastica. The
body-cavity is simply a split in the mesoderm, dividing it into an
outer sheet lining the ectoderm and an inner sheet investing the
endoderm.

Allusion has already been made to the fact that the body of
a higher animal is made up of a number of different sorts of
material having specific purposes and known as tissues, which are
in effect the building materials from which the various organs are
constructed. They are as follows: — i. Epithelium, consisting of

Fig. 287.— Diagrams, enlarged to various scales, illustrating
the structure of Hydra

A, Longitudinal section: ectoderm left white, endoderm re-
presented in black. B, Small part of longitudinal section through
body- wall (the black dots are nuclei); ECT., three large cells and
two packing cells are seen ; SUP. LAYER, supporting layer or
lamella; END., endoderm, showing four cells with flagella, and
one with pseudopodia. c, A nettling-cell, with thread pro-
truded— note barbs at the base of this, and the trigger hair on
the right side ; the protoplasm investing cell is shaded, and the
nucleus represented in black.

ZOOPHYTES 469

membranes which line internal cavities and cover the external
surface, having in the latter case the name of epidermis; 2. Sup-
porting tissues, such as bone, gristle (cartilage), and connective
tissue which makes up tendons and ligaments, while it is also
found as a sort of supporting framework in almost all parts of
the body ; 3. Muscle, constituting the flesh or meat, and also
entering largely into the construction of the walls of various
internal organs, such as heart and stomach; 4. Nervous tissue,
constituting the essential part of the nervous system; and 5.
Blood and Lymph, which must be looked upon as liquid tissues,
serving as media of exchange. In a higher animal these various
tissues are highly specialized so as to fit them for particular kinds
of work; e.g., muscle has as its province the bringing about of active
movements. Indeed it may be said that animals are "high"
or " low " in proportion as the division of physiological work is
completely or incompletely carried out. In a high animal there
is great complexity of structure associated with this perfect
division of function, while exactly the reverse is true for a low
animal. And in degenerate forms, such as the Ascidians (p. 297),
animals derived from relatively complex ancestors have become
simplified so as to suit them for simpler conditions of life. The
loss of digestive organs by tape- worms (p. 441) is a further
example of the principle.

Careful examination of any one kind of tissue shows that it
Is entirely or largely composed of structural units known as cells,
comparable to the bricks and stones, &c., which make up the
building materials of a house, if we pursue a comparison which
has elsewhere been made use of. It is these cells that consist
of the actual living substance, protoplasm, with which vital actions
are associated. Each cell contains a particle of specially modified
protoplasm known as the nucleus, which appears to be a regu-
lative centre. These cells differ largely in shape according to
the nature of the tissue. A simple case is that of lymph (p. 41),
which consists of a clear fluid (plasma) in which are suspended
innumerable irregular lymph corpuscles, which in this instance
are the constituent cells. Epithelium (fig. 288) again consists of
one or more layers of cells closely packed together and possessing
various shapes in different cases. There is, for example, simple
scaly epithelium, made up of flattened cells united by their edges
and only one cell thick; this is comparable to a tessellated pave-

47°

CHARACTERS OF INVERTEBRATE ANIMALS

ment, each separate bit corresponding to a cell. The lining of
heart and blood-vessels is of this nature. Stratified scaly epithe-
lium resembles the preceding, but is several cells thick. Examples
are to be found in the lining of the human mouth, and the epi-
dermis of Frog. Simple columnar epithelium consists of prismatic
cells packed together in a single layer, and is very characteristic
of stomach and intestines. Other examples will be noted else-
where. In many cases the surface cells of epithelium are provided

Fig. 288.— Different kinds of Epithelium, enlarged to various scales. All except c are in vertical
section. Nuclei represented in black

A, Simple columnar; B, simple glandular; c, simple scaly; c', simple scaly, surface view; D, stratified
scaly; E, simple columnar, with flagella; F, simple columnar, with cilia; G, stratified, with cilia.

with numerous slender threads of protoplasm, cilia, which by their
united action can set up currents. An individual cilium possesses
the power of alternately bending and straightening itself. Various
examples have already been given of the presence of such struc-
tures (pp. 49, 428, 445).

The ectoderm and endoderm of Hydra are to be regarded
as tissues, but do not exhibit the high degree of specialization
found in the phyla so far dealt with. They are more of the nature
of epithelium than anything else, and this form of tissue is justly
regarded as a comparatively primitive one. Beginning with the
ectoderm (fig. 287), we find that the largest elements contained
in this are what may be termed tailed-cells, since each of them
is drawn out at its inner end into a fibre which appears to be of
muscular nature. These tails all take a longitudinal direction, and
collectively constitute a muscular layer by means of which the
body can be shortened, supposing the fibres to contract or shorten

ZOOPHYTES 471

simultaneously. Filling up the spaces between these large cells
are much smaller packing-cells with indistinct outlines, some of
which become transformed into the nettling-cells already noticed,
and force their way to the surface. Deep down in the ectoderm
are also to be found scattered nerve-cells of star-shaped outline,
collectively representing an extremely primitive form of nervous
system.

The endoderm cells of Hydra are much larger than those of
the ectoderm, and are provided with muscular tails running in this
case transversely, so that collectively they make up a circular
muscle layer, by the contraction of which the body can be ex-
tended. These cells also have digestive functions, and their free
ends, which are directed towards the digestive cavity, can be
protruded into lobes by which food particles are bodily engulfed,
or, to speak more technically, ingested. Many of the cells are
also provided with groups of protoplasmic filaments, which execute
lashing movements by means of which the contents of the digestive
cavity are kept circulating. These are called flagella (L. fla-
gellum, a whip), and though they are in some respects allied to
cilia, differ from them in the relative complexity of their move-
ments, their larger size, and the fact that but a small number
of them are to be found on the same cell. In the green Hydra,
the endoderm cells contain numerous spheres in which the charac-
teristic pigment is contained, while in other species similar spheres
are present, differing, however, in the nature of the colouring
matter.

From what has been said concerning Hydra it will be seen
that in this animal there is but little specialization or division of
the work of the body. As to digestive organs, the animal is little
more than an animated stomach, food being procured by the action
of the tentacles, and digested by the endoderm. Circulatory organs
are entirely absent, and, indeed, they are not necessary, for the
digested food can easily diffuse to all parts of the body. Waste
products are similarly easily got rid of, and there are therefore
no special organs of respiration or excretion. Both nervous system
and sense organs are in a very undifferentiated condition, the latter
being chiefly represented by sensitive "trigger hairs", with one
of which each thread-cell is provided.

Development (fig. 289). — New Hydrae are produced either by a
process of budding or from eggs. In the former case, a little knob

472

CHARACTERS OF INVERTEBRATE ANIMALS

is seen to make its appearance on the body-wall, and, becoming
gradually larger and larger, is shaped by degrees into a fresh indi-
vidual, which ultimately becomes detached. This process goes on
very vigorously in summer, and a Hydra may often be found
bearing several mature buds, which, in their turn, are giving rise
to a third generation. A temporary form of branching tree-like
form is thus produced. Late in the summer, when external con-
ditions as regards temperature and food become unfavourable, one
or more rounded projections make their appearance near the foot,
each of them containing an egg. After developing to a certain

ENDODERM

Fig. 289. — Development of Hydra, greatly magnified

i, Ovum. 2, Ovum, projecting from body-wall of parent. 3, Section through young
embryo, to show protective egg-shell. 4, Cross-section of older stage. 5, Longitudinal
section through embryo after rupture of egg-shell.

extent, the egg surrounds itself with a firm horny coating, serving
as an efficient protection, and, invested in this, falls from the
parent animal into the mud at the bottom of the pond or stream,
where, in a dormant state, it is able to survive the winter, which is
not usually the case with the Hydra itself. The development is
completed in the following spring, when the egg-shell splits and
the young animal makes its way out. In many groups of the
animal kingdom, it is a common phenomenon for " winter eggs "
of this kind to be produced, and the extinction of the species
during the cold season is thereby prevented. A comparison may
well be drawn with the seeds of higher plants, in which dormant
embryos are contained that are able to resume growth with the
advent of warmer weather.

An interesting property possessed by Hydra is its power of
regenerating parts which have been injured or removed. To such

ZOOPHYTES 473

an extent is this carried, that if an individual is cut into several
pieces, each of them becomes a fresh Hydra. Powers of this kind
are common among lower animals, and, as might be expected, are
greatest where the body is but little specialized, as in the case now
under consideration. Another case was mentioned when dealing
with the Star-Fish (p. 454). The absence of specialization in Hydra
is not so great, however, as supposed by the older zoologists, who
imagined that the animal was so little discomposed when turned
inside out that it lived on with the functions of the layers reversed.
Zoophytes are conveniently divided into the following three
classes: — i. Sea-Flowers (Anthozoa or Actinozoa); 2. Hydroids
(Hydrozoa); and 3. Comb Jelly-Fish (Ctenophora),

CLASS I.— SEA-FLOWERS (ANTHOZOA or ACTINOZOA)

The brilliantly-coloured Sea- Anemones, Corals, Sea- Pens, and
the like, which are grouped together in this class, justify its name
(Gk. anthos, a flower; zoon, an animal), for, when expanded, they
look not unlike chrysanthemums or double dahlias, to the indi-
vidual florets of which the numerous tentacles bear a certain
resemblance. Many of them are colonial, while others live singly,
and of these we may take the common Beadlet (Actinia mesem-
bryanthemum) as an example (compare fig. 290). It is abundant
on rocks between tide-marks all round our coasts, looking, when
expanded, like a scarlet flower, but shrinking to a rounded jelly-
like mass when contracted. In the former condition, it may be
compared to a short broad Hydra, but the mouth is in the centre
of a wide disc, and is surrounded by several rows of comparatively
short pointed tentacles. There are, however, important differences
in structure. The mouth leads, not into a widely -continuous
digestive cavity, but into a tubular gullet, which hangs down within
the body, and ends abruptly below. This may be roughly repre-
sented by taking a piece of wide india-rubber tubing, and tucking
in one end of it. The space between the lower end of the gullet
and the base is called the stomach, and performs the function of
a digestive organ. But this is not all, for running across in a
radiating way from the body-wall to or towards the gullet are
a large number of fleshy partitions, which divide the space external
to gullet and stomach into a number of compartments. These
partitions, or mesenteries, are, however, perforated above, so as to

474

CHARACTERS OF INVERTEBRATE ANIMALS

A.

put adjoining compartments into communication. The stomach,

which is bounded by the thickened free edges of the septa, is

of necessity continuous with the
various compartments, except when
digestion is going on, at which time
the edges of the septa are brought
close together, a digestive juice being
poured out from them upon the
food. These points will be made
clear by examination of the accom-
panying diagram. It may further be
noted that the sides of the gullet are
furrowed by two grooves placed
opposite to each, and this organ caa
be collapsed in such a way as to con-
vert these grooves into narrow tubes,
these being lined by long cilia, which
work in such a way that a current of
water runs inwards in one tube, and
outwards in the other.

The minute structure corresponds
in many ways to that of Hydra,
there being ectoderm, endoderm, and
a supporting lamella between them.
Nettling-cells are abundantly present,,
but are more complex in pattern
than those possessed by Hydra.
There is further a greater amount of

specialization, as seen especially in the muscular and nervous

systems.

The Anthozoa include two orders: — i. Six-rayed Sea-Flowers

(Hexactinia); and 2. Eight-rayed Sea-Flowers (Octactinia).

Fig. 290. — Sea-Anemone

A, External view of a Sea- Anemone (Anemonia).
B, Diagrammatic cross-section; MES., mesenteries:
only one of the two food -grooves is lettered,
c, Diagrammatic longitudinal section, showing a
mesentery (MES.), with mesenteric filaments (MES.
KILT.) on the right, and one of the spaces between
two mesenteries on the left ; EX., EX'., excretory
pores: the arrows indicate the course of currents
into and out of the gullet along the food-grooves.

Order i. — SIX-RAYED SEA-FLOWERS (Hexactinia)

These are distinguished by the fact that the tentacles are
simple, while they and the mesenteries are usually arranged in
multiples of six, and though this is often difficult to easily make
out in the adult, it is clearly shown during the development. The
Sea- Anemone just described is a good type of the order. Among

SEA-FLOWERS

475

Fig. 291. — Skeletons of Arabian Corals, all reduced

i, Tuft-Coral {Pocilloporafavosa]\ 2, Clove-Coral (Trachyphyllia Geoffroyi]', 3, Antler-Coral (Seriatoportt
subulata]', 4> Madrepore-Coral (Madrepora laxci}; 5, Shrub-Coral (Heteropora H entprichii]; 6, Hedgehog-
Coral (Echinopora gentmacea,} ; 7, Sun-Coral {Heliastr&a Forskaliand] ; 8, Mushroom- Coral (Fungia
scutaria}; 9, Brain-Coral (Ccelaria labyrintkiformis}.

476 CHARACTERS OF INVERTEBRATE ANIMALS

other British anemones may be mentioned: Tealia crassicornis,
a large red form in which the outside of the body is studded
with warts, to which fragments of sand or shells may be found
adhering; and the brownish or greenish Opelet (Anthea cereus),
which can only partially draw in its tentacles.

The majority of forms known as "corals" (fig. 291) are also
members of the six-rayed order, and they include both simple
and colonial species. Of the former may be mentioned a British
species (Gary ophy Ilia Smithii) found in the English Channel.
In structure it is comparable to a sea -anemone, but a hard
calcareous skeleton has been secreted by the ectoderm in the
basal part of the animal. When the soft parts are removed
this is seen to consist of a conical cup, from which calcareous
plates project inward. A simple coral attaining a much larger
size is the familiar Mushroom- Coral (Fungia), the flattened cup
of which has some resemblance to the top of a mushroom, the
gills being represented by the calcareous partitions or septa.
Most corals, however, form colonies by budding (gemmation)
or splitting (fission), and it is these which chiefly build up the
coral-reefs so characteristic of the warmer parts of the ocean
where the water is free from sand or mud. All sorts of different
shapes may be assumed by the colonies, according to the species,
some being encrusting or massive, while others branch in a tree-
like way (fig. 291). The individual polypes may be separated
by a larger or smaller interspace, so that their separate cups can
be clearly made out, either imbedded in or else projecting from a
common skeleton secreted by the common or colonial body (cceno-
sarc) which unites the different members together. In other cases
the polypes are closely packed together, and it even happens
in some cases that there is no proper division between them, it
being only possible to determine the number of individuals by
counting the mouths which are present.

Order 2. — EIGHT-RAYED SEA-FLOWERS (Octactinia)

Although these forms agree in general plan of structure with
corals and sea-anemones they also present striking differences,
among which the most obvious are the possession of not more
than eight feather-shaped tentacles and the same number of
mesenteries. Nearly all the included species are colonial.

SEA-FLOWERS

477

One of the commonest British members of the group is the
organism to which the unpleasant name of Dead-Mans Fingers
(Alcyonium digitatum) is applied, a name justified to some extent
by the thick branches of the colony that look something like
the swollen fingers of a clumsy hand (fig. 292). In a specimen
cast up on the beach the individual polypes will have been drawn
back into the fleshy substance of the colony, their position being
indicated only by small depressions situated at a little distance
from one another. Quite

another appearance is pre- ^ - C.

sented by a living specimen
with all the polypes pro-
truded, these having a dis-
tinctly flower-like appear-
ance, with eight feathery
tentacles suggesting petals.
As in an anemone, the
mouth leads into a gullet,
provided in this case, how-
ever (as also in some of the
anemones), with but one
ciliated groove, and united

°
tO the body -Wall by Only

Pl'crht QPntP TVlP Hicrp^tivP

C1& ** ' U *& C^

Cavities Q|" the OOlvDeS are
• i i I • i

continuous with canals which
traverse the common flesh. At first there appears to be no
skeleton; but microscopic examination reveals the presence of
numerous calcareous spicules of characteristic shape, scattered
through the comparatively thick supporting layer which comes
between the ectoderm and endoderm (see p. 474). There is
reason to believe, however, that the spicules have been formed
by the activity of ectodermic cells which have become detached
from their own layer.

Another familiar example of the group is the Organ-pipe
Coral (Tubipora musica), in a dried specimen of which may be
seen numerous red tubes connected together by " platforms" of
similar material at different levels. From these platforms new
individuals grow up, and so the coral increases in breadth as it
gets older. As in colonial corals generally the individuals are

jFig. 292. — Dead-Man s Fingers (Alcyonium)

A, A colony, reduced. B, Tip of a branch, magnified, showing
Polypes in different stages of expansion and retraction; MES., a
mesentery, c, Cross-section through body of a polype, magnified ;
MES., mesenteries (8 in all are present): note the single food-groove
in lower side of gullet.

478 CHARACTERS OF INVERTEBRATE ANIMALS

limited to the surface-layer, while the deeper part of the mass
is cut off by partitions and merely consists of dead material.
In a living specimen of Tubipora the upper end of each tube is
occupied by a polype of similar character to those found in
Alcyonium, and adjacent members of the colony are connected
by living matter which extends along the uppermost platforms.
Microscopic examination of the skeleton shows that it is formed
by the intimate union of innumerable minute spicules comparable
in nature though not in shape to those of Dead- Man's Fingers.

Other examples are Red Coral (Corallium rubrum) (fig. 2),
where there is a compact branching skeleton (composed of united
spicules) covered by the common flesh (ccenosarc) of the colony
from which the polypes project at intervals; Sea- Pens (Pennatula,
&c.), where the colony is feather - shaped with a series of indi-
viduals on each side, while the axis is supported by a firm rod;
and the Sea-Mats (Gorgonia, &c.), where the flat upright colony
is variously branched and supported by a horny skeleton, which
is covered by common flesh with polypes much as in the Red
Coral.

CLASS II.— HYDROIDS (HYDROZOA)

The Fresh- water Polype, Hydra, has already been described
as a simple type of this class (pp. 465-471), which is pre-
dominatingly marine. The apparent simplicity of Hydra, it
should be noted, is in all probability the result of reduction, and
a much better idea of the group is to be obtained by briefly
considering one of those marine forms to which at different
stages of the life - history the terms "hydroid zoophyte" and
"medusa" are applied. The horny skeletons of zoophytes of
this sort are cast up on the shore in abundance by storms, and
some may be found growing near low-tide mark in rock-pools.
They are often confused with sea-weeds in amateur collections,
in which their branching skeletons are frequently to be found. A
very common British genus is Obelia (fig. 293), for which there
is no popular name. It will be remembered that in dealing with
Hydra it was pointed out (p. 472) that during summer, when
that animal is actively budding, two or three generations of indi-
viduals may be temporarily connected together. If these buds
were to remain united, and the budding process were carried

HYDROIDS

479

further, a permanent branching colony would be the result.
Obelia is a colony of this kind, and for its support some sort of
skeleton is necessary, as is the case in the corals already described.
Here, however, it is in the form of a horny investment, which
covers the common body (ccenosarc) and expands at the tip of
•each branch into a little cup in which is lodged a hydra -like
polype. There is,

B.

A.

TENTACLES

however, a further
arrangement in the
form of much larger
cups (gonangia)
within which are
produced groups of
special buds, the
function of which
is to produce eggs.
When these buds
are mature they are
liberated in the
form of small jelly-
fish or medusce,
which lead an in-
dependent life for
some time and pos-
sess active powers
of movement. The
jelly - like consis-
tence in cases of
the kind is due to
the excessive de-
velopment of the lamella between ectoderm and endoderm, which
becomes thick and gelatinous, while cells from the two layers in
question make their way into it. The little medusa may be
compared to an umbrella with a very short handle (manubrium),
and around its margin is a fringe of tentacles, eight of which
have minute auditory vesicles at their bases, one to each. The
mouth is situated at the end of the handle, and leads into a
stomach, from which four tubes radiate to the edge of the umbrella,
where they are continuous with a circular ring -canal. It may
further be added that a little shelf or velum projects inwards from

Fig. 293.— Fixed and Free-swimming Stages of a Hydroid Zoophyte (Obelia}.
A, Natural size ; B-D, enlarged

A, A colony of the fixed (hydroid) stage, attached to a piece of sea-weed.
B, End of a branch of same, c, Upper side, and D, under side of the free-
swimming stage (jelly-fish or medusa).

480 CHARACTERS OF INVERTEBRATE ANIMALS

the margin of the umbrella. The eggs give rise to hydra-like
individuals, each of which develops a fresh colony by means of
budding.

The Hydrozoa may conveniently be divided into two orders:
i. Budding Hydroids (Hydromedusae) ; and 2. Splitting Hydroids
(Scyphomedusae).

Order i. — BUDDING HYDROIDS (Hydromedusae)

Obelia is a good typical example of the order, but in by no
means all cases are egg-producing bud» set free as medusae,
and a complete series of examples may be selected which range
from that condition down to the state of things found in Hydra,
where each egg is produced in a little swelling which has no
resemblance whatever to a medusa. The intermediate stages
are represented by cases where the egg-producing buds resemble
medusae but remain attached to the colony, and other cases where
such buds may be compared to medusae in which some of the
typical features have been, as it were, suppressed.

Obelia is a type of one large division in which the fixed
stage is distinguished by the possession of cups into which the
individuals can be withdrawn, while if free-swimming jelly-fish
are produced their sense organs are usually auditory vesicles.
Other common British genera are Sertularia and Plumularia.

In another large group the investing skeleton ends abruptly
at the base of each polype and does not expand into a cup, while
the medusae, if liberated as free-swimming individuals, usually
possess eye-spots instead of auditory vesicles round the margin
of the umbrella. Tubularia is a typical British genus, in which
the polypes are comparatively large, and free-swimming medusae
are not developed. It is also customary to place Hydra in this
group, as well as a very interesting marine genus, Protohydra, of
somewhat similar character, though it has no tentacles, and is to-
be regarded as the simplest known member of the Hydrozoa.

Although most corals belong to the Anthozoa, there are a
few cases of species belonging to the Hydrozoa which, instead
of secreting a horny investment, develop a firm calcareous
skeleton, and superficially resemble the true corals, though in
reality sharply marked off from them by the structure of the
soft parts. Representative genera are Millepora and Stylaster*

HYDRO1DS

481

in which there are hydra-like nutritive individuals, without well-

developed tentacles, and reduced prehensile individuals devoid

of mouths, and looking like large tentacles richly provided with

nettle-cells, and branched in the

case of Millepora. Each nutritive

polype is surrounded by a num-

ber of these modified individuals,

the function of which is to secure

food. There are also egg -pro-

ducing members of the colony,

comparable in function, though

not in form, to the medusa-stage

of Obelia.

It will have been gathered
from the preceding that there is
often a division of physiological
work between the different mem-
bers of a hydrozoan colony, just
as in the complex body of a higher
animal there is a similar division
between the various tissues. This
phenomenon is carried to an
extreme in the free -swimming
marine forms which are grouped
together under the name of Com-
pound Jelly -Fish (Siphonophora)

(fig. 294). Each COlony COm-

prises individuals of the most ™s "^ regarded M a med.usa F(°f whjch .tjc Iarge

bell or umbrella is seen at upper end of figure) with a very

VariOUS nature, and jUSt aS a lo"g mouth-stalk (manubrium), on which smaller indi-
. viduals are formed as buds.

hydroid zoophyte arises by the

budding of a fixed hydra-like individual, so here we must suppose
that buds of different kinds have been produced on a modified
jelly-fish or medusa, in some cases upon the under side of the
umbrella and in others upon the elongated mouth-stalk, which
has been compared to the handle of the umbrella.

Fig. 294- -A Compound Jelly-Fish (Sarsia)

Order 2. — SPLITTING HYDROIDS (Scyphomedusae)

To this order belong the large jelly-fish which are often
seen in great numbers in British seas during the warmer parts

VOL. I.

31

482

CHARACTERS OF INVERTEBRATE ANIMALS

Fig. 295.— Development of Aurelia, enlarged

a, The fixed stage (hydra-tuba), b and c, Transverse splitting of a to form
medusae, d and e, Young medusa, seen from the side and from below.

of the year. The edge of the umbrella is lobed, the notches
between the lobes sheltering peculiar sense organs formed by the
modification of tentacles and covered by little lappets. Hence

the term " covered-
eyed medusae " some-
times applied to these
forms, to distinguish
them from the' 'naked-
eyed medusae " of the
preceding class, in
which the marginal
sense organs are not
covered by such lap-
pets. These terms,

hoWCVCr, WCrC not
Uq nnil V rllOQf^n fnr if

is only in certain cases
that the sense organs have to do with sight. A negative charac-
teristic of the jelly-fish included in this division is to be found in
the absence of the true velum (see p. 479). The most typical

members of the group present two stages in
the life-history, as in Obelia (p. 480), i.e. (i)
a fixed hydroid stage, and (2) the free-swim-
ming medusa. The common British form
Aurelia may be taken as an example (fig.
295). Here the fixed stage is what is called
a Hydra-tuba, somewhat resembling a short,
broad Hydra in shape, though internally it
presents a difference in the presence of four
longitudinal folds which project into the diges-
tive cavity. Medusae are developed from it,
not by budding, but by a process of trans-
verse splitting, and when the process is far
advanced, the incipient medusae may be com-
pared to a pile of saucers. Sooner or later
these become detached and grow into the mature jelly-fish, by
which eggs are produced which complete the cycle of development
by becoming hydra-tubae.

Some of the medusae belonging to the order have no fixed
stage, and in the case of certain other species (Haliclystus,

Fig. 296.— Lucernaria, enlarged

COMB JELLY-FISH 483

Lucemaria, &c.) there is, on the contrary, no medusa stage
(fig. 296).

CLASS III.— COMB JELLY-FISH (CTENOPHORA)

These are small, transparent, free-swimming forms occurring
in vast numbers in the open sea. When alive they are of
extreme beauty. A common British genus, Cydippe, typically
exhibits the features of the class (fig.
297).

Here the body may be compared in
shape to a minute melon, with the mouth-
opening at one pole and a complex sense
organ at the other. Locomotion is
brought about by eight longitudinal
bands of little paddles which suggest
the teeth of a comb. Each paddle is
apparently made up of a number of cilia

\\ ! , J f i T- Fig. 297—Cydippe

which have fused together, rrom each

side of the body a long feathery tentacle can be protruded, which
is used for the capture of food. These tentacles, when not in use,
are drawn back into pouches. The mouth leads into a gullet, and
that into a complex system of canals, which communicate with
the exterior by two pores at the opposite end of the body.
Nettling -cells are absent, but the tentacles are provided with
glutinous adhesive cells by which particles of food are secured.
There is no fixed stage in the life-history.

Among other genera may be mentioned Venus s Girdle (Cestus),
in which the body is band -shaped and may be as much as a
foot in length; and Beroe, a cap-shaped form devoid of tentacles.
There are also two very interesting creeping genera (Ctenoplana
and Cozloplana), which, as a result of their mode of life (see
p. 23), present an approach to bilateral symmetry, and have
been compared to some of the Turbellarian worms (p. 445), it
even being suggested that the Turbellaria have been derived
from ancestral forms closely related to the Ctenophores.

484 CHARACTERS OF INVERTEBRATE ANIMALS

SPONGES (PORIFERA)

Sponges were long considered to be of vegetable nature, an
idea that was only finally upset by the study of their minute
structure. Except to a naturalist the word "sponge" merely
suggests a bath-sponge, which is in reality the horny skeleton
of a colonial species. To gain a clear idea of the structure of
the group it is necessary to consider the simpler cases presented
by some of the solitary forms. In the simplest of these the
body is shaped like a cup or vase, fixed at one end and open
at the other. It is tempting to draw a comparison with Hydra,
or better with Protohydra (p. 480), which is simply a tube open
at one end. Such a comparison, however, is beset with diffi-
culties, for while in Protohydra the aperture is clearly a mouth
through which food is introduced, it will be found by watching
the living sponge that currents of water are constantly flowing
out of the corresponding opening, here technically known as
the osculum (L. for little mouth). And, further, the body- wall of
the sponge is perforated by numerous small holes through which
water-currents set into the central digestive cavity. These cur-
rents are the result of ciliary action, and by their means the
animal is provided with the food and oxygen it requires, while
the various products of waste are swept out to the exterior
through the osculum. This mode of life is associated with the
sedentary or fixed habit of sponges, which is the chief reason
for the old mistake of considering these creatures of vegetable
nature.

Microscopic examination shows that the thin body-wall of
the simple sponge consists of an external ectoderm made up
of a layer of flattened cells, a middle supporting layer, and an
internal layer of entoderm composed of collar-cells. Each of
these cells is provided with a whip-like projection of protoplasm
{flagellum) (see p. 471), at the base of which is a collar-like
projection. By the lashing movements which these threads
execute the water-currents upon which the life of the animal
depends are produced. The middle supporting layer is com-
parable to the similarly -placed gelatinous layer of a jelly-fish,
and it contains numerous cells of various kinds which have
been derived from the ectoderm and entoderm. Some of these

SPONGES

485

cells produce three-rayed calcareous spicules which form a very
characteristic skeleton (fig. 298).

The majority of sponges, like corals (p. 476), are able to
produce colonies of the most varied shape by processes of budding
or splitting, and in such cases the boundaries between the indi-
viduals are usually very ill-marked, though the number of these

\

Fig. 298. — Sponge Spicules, enlarged

is generally to be told by counting the number of larger openings
or oscula which are present. There is also a very large amount
of variation in the nature of the skeleton and the extent to which
it is developed. It may, as in the simple case described, be made
up of scattered spicules, and these may either be calcareous or
siliceous. Such spicules are of many different shapes (fig. 298),
and they may be compacted into a firm, continuous mass, while
they may further be associated with, or replaced by, a complex
horny net -work. It should further be remarked that in the large
majority of cases the body- wall is much more complicated than

486

CHARACTERS OF INVERTEBRATE ANIMALS

in the case described, and the collar-cells are restricted to small
rounded chambers situated in the thickness of the wall and
communicating by narrow canals with the exterior on the one

hand and the central
cavity on the other.
The canal system
may be exceedingly
complex, as may be
realized by looking
at the labyrinth of
spaces present in
an ordinary bath-
sponge.

It will be suffi-

Fig. 299. -Bath-Sponge (Em&mfia), reduced cient for the prCSCttt

purpose to consider

the Sponges as divided into two groups according to the nature of
the skeleton; i.e.: i. Calcareous Sponges, and 2. Siliceous Sponges.

1. Calcareous Sponges, when simple, correspond to the type
described. A more complex example is afforded by the common
British species Grantia compressa, a small white flattened sponge
about an inch long, frequently found attached to stones and other
objects between tide-marks. Other forms are represented in fig.
300.

2. Siliceous Sponges include a great variety of forms in which
the supporting spicules, when present, are of siliceous or flinty
nature. One of the commonest British species is the Bread-
crumb Sponge (Halichondrid), which, in the form of yellowish-
brown or orange-coloured masses, may be found encrusting rocks
near low-tide mark. On the surface of the sponge a number
of little conical elevations may be seen, and on the end of each
an osculum is situated. The pores by which currents enter are
placed in the part of the sponge between these projections. For
beauty of form nothing can surpass Venus s Flower - Basket
(Euplectella), a deep-water form in which the skeleton may be
compared to a cornucopia with a wall of lace -like appearance
and a perforated lid. Another very interesting kind is the
Glass-rope Sponge (Hyalonema) dredged in deep water off Japan.
Here the ovoid body is anchored in the mud by bundles of long
twisted spicules which have suggested the name.

ANIMALCULES

487

The Bath-Sponge (Euspongia) possesses a horny skeleton, the
texture of which makes it useful for a variety of purposes.

One small group of sponges is found in fresh water, and of

Fig. 300. — Group of Calcareous Sponges i, Leucandra aspera. 2, Sycandra raphanus

these the commonest British genus (Spongilla) is found as a
greenish crust upon various objects.

ANIMALCULES (PROTOZOA)

The innumerable host of simple animals which constitute
this phylum are nearly all minute, while many of them can only
be studied by the highest powers of the microscope, so that
every improvement in the construction of that instrument has
been followed by the acquisition of fresh knowledge concerning
this group. Very great interest attaches to the study of the
Protozoa, for here we have to deal with life under the simplest
conditions, and find the actual living substance, protoplasm, which

488 CHARACTERS OF INVERTEBRATE ANIMALS

\

constitutes the essential part of all organisms, in a comparatively
pure form, and not obscured to the same extent as in higher
animals by the products of its own activity. It has become a
tradition to take as a first type one of the simplest members of
the group, i.e. the Proteus Animalcule or Amoeba, which is
commonly found creeping on the mud of ponds.

Reference has frequently been made to the microscopic
bodies known as white or colourless corpuscles which abound
in the blood and lymph of higher animals (see p. 41). These
crawl through the body in all directions and perform various
functions of no mean importance, one being that of destroying
disease germs which have entered the organism from the exterior.
These corpuscles may almost be said to lead an existence inde-
pendent of the rest of the body, and indeed it is possible to keep
them alive outside the organism to which they belong for some
time, especially in the case of cold-blooded creatures such as the
Frog. An Amoeba resembles in many essential respects one of
these corpuscles, so much so that when it cannot be obtained
for study in the laboratory a white corpuscle is often taken as
the best substitute. The body (fig. 301) consists of a particle of
semi-fluid protoplasm possessing the power of active locomotion,
employed in the search for food, which consists of microscopic
plants and other solid bodies of organic nature. The complex
and solid nature of the food, or part of it, is, as will be elsewhere
shown, a characteristic of average animals as compared with
average plants, and the powers of locomotion with which most
animals are endowed has an obvious relation to this. In such
a fixed animal as a sponge there is, as we have seen, a special
arrangement by which food is brought to the body, compen-
sating for the absence of locomotor powers. An ordinary green
plant, feeding as it does upon gaseous and liquid food extracted
from air and soil, has no need for powers of locomotion, and its
branching form gives a very large surface through which the
simple food can diffuse. An active animal, on the other hand,
such as Amceba, has a compact body which is clearly more con-
venient for locomotor purposes and less exposed than a branching
form would be to the attacks of enemies. When tree-like organ-
isms, such as zoophytes, are of animal nature, they usually represent
fixed colonies to which food is brought by currents.

The body of a living Amceba is seen to be constantly changing

R

Fig. 301. — Protozoa, enlarged to various scales. Reference letters: — F, Food; FL., flagellum; G, gullet;

M, mouth; MAC., macronucleus ; MIC., micronucleus ; N, nucleus; P.V., pulsating vacuole
A, Cockroach Gregarine (Clepsidrina blattarum}. B, Bell-Animalcule (Vorticella], extended and re-
tracted, c, Slipper-Animalcule (P aratncecium}. D, Volvox. E. Euglena. F, Noctiluca. G, Codonocladium.
H, Monads: i and 2, Springing Monad (Heteromita}', 3, Chilomonas; 4, Hexamita. J and j', Skeletons of
Radiolaria {Heliosph&ra and Riicyrtidiutn\ K, A Sun-Animalcule {Actinospharium}. L and L', Small piece
of a Mycetozoon and two capsules (one ruptured) of same. M, A Foraminifer (Rotalia), with protruded threads
of protoplasm. N, o, P, Shells of Foraminifera (Globigerina, Nodosaria, and Miliola). Q, A shell-bearing
Rhizopod (Difflng ia] , allied to Amoeba. R, Proteus Animalcule {Amoeba}.

48.)

490 CHARACTERS OF INVERTEBRATE ANIMALS

its shape, blunt lobes of the protoplasm (pseudopods) being pro-
truded from time to time. They are not constant structures,.
but can be formed and again obliterated at any part of the body —
at least in the commonest species of Amoeba. Not only do these
lobes enable the animal to crawl about, but they also serve to
secure food particles, engulfing them, so to speak. No breach
is formed thereby, for any hole temporarily made in the semi-
fluid protoplasm at once closes up without leaving any trace.
Once within the protoplasm the food is digested, and the un-
digested portions of it are cast out from any part of the body.
In the case of Hydra some of the endoderm cells take in and
digest solid particles in much the same way (p. 471).

The simple body of Amoeba presents but little distinction of
parts, but the outer part of the protoplasm is clearer and perhaps
somewhat denser than the inner part into which the food is taken,
and which also contains, as a rule, numerous granules of various,
nature. As in a colourless blood corpuscle a small rounded
particle of specialized protoplasm, the nucleus, can be distinguished,
and this appears to have a great deal to do with regulating and
controlling the nutrition and other functions. Its presence shows
that Amoeba is a single cell or unit of structure, i.e. is unicellular.
The body of Hydra, and the same thing is true for all animals
higher than the Protozoa, has been compared (p. 469) to a house
composed of various building materials of which the units are
bricks, blocks of stone, and the like, these materials being likened
to the tissues of the animal in which cells of differing kind are
the constituent units. An unicellular creature like Amceba may
therefore be compared to a house built of one brick, if such a
metaphor can be regarded as thinkable. The PROTOZOA indeed
are defined as animals in which the body is made up of one cell
only, or at most of an aggregate of cells which are not specialized
into tissues, so that each member of the aggregate has to perform
all the functions of life. The remaining thirteen phyla of animals
are often grouped together as METAZOA, in which the body is
made up of more or less numerous cells specialized to form tissues.
It may be as well to express this in tabular form, advantage also
being taken of the opportunity to show the limits of Vertebrata
and Invertebrata, of Diploblastica and Triploblastica.

There can be no doubt that the Metazoa are descended from
unicellular forms, which must have resembled some of the

ANIMALCULES

491

INVERTEBRATA ^

I. VERTEBRATA

II. Nemertea

III. Mollusca

IV. Arthropoda
V. Annelida

VI. Gephyrea

VII. Rotifera

VIII. Molluscoida
IX. Platyhelmia

X. Nemathelmia

XL Echinodermata

XII. Coelenterata

Kill. Porifera
PROTOZOA

METAZOA.

Triploblastica

| Diploblastica

Protozoa, and consequently great interest attaches to the study
of those members of the latter group which are aggregates of
cells, since some of these may, in some respects, be like the
transitional forms.

Returning to the consideration of Amceba, we find that this
differs from a colourless corpuscle in possessing a liquid -filled
space within the protoplasm which is constantly changing its size
in a regular manner, and is therefore termed a pulsating vacuole.
If carefully watched in a living specimen placed under the micro-
scope, this will be found to slowly expand to a certain size and
then to suddenly contract so as to be entirely lost to view, soon
reappearing, however, at the same spot. It appears to communi-
cate with the exterior, and probably has to do with getting rid
of waste products (excretion) and perhaps also with the introduction
of dissolved oxygen into the body for the purpose of respiration.

Amceba is sensitive to external influences, being affected, for
example, by changes of temperature, but it also appears to be
able to execute movemepts, apart from the direct action of such
influences. Otherwise expressed, the minute fragment of proto-
plasm which constitutes the body of this animal, performs those
functions which, in higher forms, are relegated to a nervous system,
and its connected sense organs.

Careful observation has shown that Amceba multiplies itself by
a process of splitting or fission, as observed in some animals higher
in the scale (pp. 476). The nucleus elongates, and becomes
divided into two parts, and at the same time the rest of the body
is halved, the result being that the parent animal disappears
altogether as an individual, being replaced by two new animals
which subsequently increase in size.

492 CHARACTERS OF INVERTEBRATE ANIMALS

It therefore appears that all the functions of life can be, and
are, performed by a single cell, this having to discharge the duties
which in higher organisms are distributed between different com-
plex organs and tissues.

It is convenient to divide the Protozoa into three groups:—
i. Infusorians (Infusoria); 2. Amceba-like Protozoa (Rhizopoda);
and 3. Gregarines (Sporozoa).

Group i. — INFUSORIANS (Infusoria)

If boiling water is added to chopped ha^ or other vegetable
matter, and the infusion so procured allowed to stand for some
time, it will begin to putrefy, and a large number of minute active
creatures will make their appearance in it. The same thing would
happen if the infusion were of animal nature. The term Infusoria
was first applied to the minute forms noted in various decomposing
substances of the kind just mentioned, but they are by no means
invariably associated with putridity, and abound both in salt and
fresh water, or even on damp soil and vegetation.

A very common form is the Slipper Animalcule (Paramcecium)
(fig. 301), which is readily obtained by making an infusion of hay
as described. It is an active whitish creature, just visible to the
naked eye. Placed under the microscope, it will be found to
possess an elongated body, which, unlike Amoeba, has a definite
shape owing to the fact that the outer layer of the protoplasm
(ectosarc) is firmer than the rest (endosarc), and covered by a thin
elastic membrane or cuticle. As another consequence of this, food
cannot be taken into, or solid waste ejected from, any part of the
body, but there is on one side a depression which leads to a mouth,
and this again into a very short gullet, that ends abruptly in the
soft internal protoplasm (endosarc). Not far from this depression
there is a spot where the cuticle is absent, and the solid refuse
from the food can pass out to the exterior.

The presence of a cuticle, and the firm nature of the outer
protoplasm, prevent the formation of the blunt lobes (pseudopods),
which, as we have seen, serve in Amoeba as organs for obtaining
food and effecting locomotion. Their place is here taken by a
uniform covering of cilia, which are protruded through minute
holes in the cuticle. By them the animalcule is rowed about from
place to place, and they also set up currents by which food particles

ANIMALCULES 493

are conducted into the mouth. Within the body two pulsating
vacuoles can be seen, one at each end, and there are also two
bodies of nuclear nature, which are placed close together. One of
them is large (macronucleus), and the other small (micronucleus).
They pass through a series of very complicated changes before
the animal undergoes transverse splitting or fission, which is here
the characteristic method of multiplication.

A very large section of the Infusoria are characterized by the
presence of cilia, the arrangement of which differs in different
species, and these constitute the group of Ciliata. We may
take as a second example of this division the Bell Animalcule
(Vorticelld) (fig. 301), which is commonly found attached to water-
weeds, or it may be to aquatic animals, such as the little red worm
Tubifex (p. 431). Here the body is of bluntly conical form, with
the narrow end drawn out into a long stalk, firmly fixed to the
object upon which the animal lives. When fully expanded,
it can be seen that the cilia are limited to the broad end of the
body, on which they are arranged in a short spiral, while smaller
cilia are continued down into a depression, which may be compared
to that seen in the Slipper Animalcule, for it leads into a short
gullet of the same kind. Within this depression there is a soft
spot through which the undigested remains of the food are ejected.
The wreath of cilia produces currents, which set into and out
of the mouth - depression, carrying food and dissolved oxygen
inwards and waste matters outwards. As in a Slipper Animalcule,
the protoplasm is divided into a softer central endosarc and a firm
external ectosarc, the latter being continued into a fibre which runs
in a wavy manner through the stalk. If Vorticella is alarmed in
any way, the free end of the body is folded in, and, at the same
time, the elastic stalk is thrown into the form of a spiral by the
shortening or contraction of the fibre it encloses.

Within the body one pulsating vacuole can be seen, and also
two nuclei, the big one having a very characteristic horse-shoe
form. Multiplication is effected by longitudinal splitting, one of
the two new individuals remaining on the stalk, while the other
becomes detached, being rowed about by the cilia till a suitable
spot is reached, when it becomes attached, and develops a new
stalk.

Some of the near relatives of the Bell Animalcule form colonies
by means of fission, the new individuals remaining attached instead

494 CHARACTERS OF INVERTEBRATE ANIMALS

of at once becoming separated, as in Vorticella. All the members
of the colony are of exactly similar nature.

Another large section of Infusorians, the Flagellata, is charac-
terized by the presence of those protoplasmic threads known as
flagella (p. 471), which, though allied to cilia, are capable of
executing much more complex movements, and are not present
in large numbers on the same cell. A common example is
Euglena (fig. 301), a minute green form with worm-like body,
found, sometimes very abundantly, in stagnant water. At the
front end there is one long flagellum, which acts as a swimming
organ, pulling the body after it through the* water. At its base
there is a minute mouth leading into an exceedingly short gullet,
near which is a pulsating vacuole, and a red pigment -spot. The
nucleus is central, and the green colouring matter (chlorophyll),
which is of the same nature as that found in ordinary plants, is
contained in specialized parts of the protoplasm. The body is
constantly altering its shape in a peculiar manner, but the presence
of a firm cuticle prevents the formation of pseudopods.

Putrefying infusions contain vast numbers of very minute
flagellates, commonly known as monads (fig. 301). A well-known
form is the Springing Monad (Heteromita), so named from the
character of its movements. It is shaped like a bean, and pos-
sesses two long flagella, attached near the notched side. One
of these is extended forward during locomotion like the single
flagellum of Euglena, while the other is trailed behind.

A very interesting section of the present group contains simple
and colonial forms, in which the individuals closely resemble the
collar-cells of sponges (see p. 484). Codonocladium (fig. 301) may
be taken as an example. The interest chiefly lies in the fact that
such forms perhaps give a hint as to the ancestry of the sponges.

It has been mentioned that Euglena is like a green plant,
inasmuch as it contains chlorophyll, but the presence of a mouth
proves it to be an animal. There are, however, certain flagellates
which are coloured green by this pigment, and at the same time
are devoid of any mouth-opening. Indeed, many botanists claim
them as plants. Some are simple, others colonial, and of the
latter, one genus is particularly well known on account of its great
beauty as a microscopic object. This is Volvox, a form shaped
like a hollow sphere (fig. 301), and about the size of a very small
pin's head. At regular intervals in the wall of the sphere are

ANIMALCULES 495

imbedded the green pear-shaped individuals, each of which is
provided with a pair of flagella, and is connected by threads of
protoplasm with its neighbours. The colony swims about by
means of the flagella, slowly revolving as it does so. We have
here an example of the most perfect type of symmetry known,
that of the sphere, a shape which is only possible when external
influences act on all parts alike, as can only be the case with a
revolving aquatic form. At times, however, the symmetry of
Volvox is disturbed by the specialization of certain cells for the
purpose of propagation.

A small marine group of flagellates is represented by the
remarkable form Noctiluca (L. nox, night; lux, light), countless
myriads of which are sometimes found floating together in the
sea (fig. 301), and are one of the causes of the phosphorescence
common in the summer months. Each individual is of com-
paratively large size (^ of an inch in diameter) and is shaped
like a peach, a very large flagellum being attached at one end of
the groove. The mouth is situated at the base of this structure
and leads into a short gullet into which a second but smaller
flagellum projects.

Group 2. — AMCEBA-LIKE PROTOZOA (Rhizopoda)

A description has already been given of the Proteus Animalcule
(Amoeba), which may be taken as a type of the group, all the
members of which are provided with those projections of the
body which are called pseudopods, though it is only in some
cases that these are broad lobes, as in many species of Amoeba.

There are a number of common fresh-water members of
this group (fig. 301), which essentially resemble Amoeba except
that they possess a shell of varied nature and shape. Sometimes
it consists of foreign particles cemented together, and at other
times it is entirely made up of material presumably of horny
nature formed by the activity of the protoplasm.

Cases like those just mentioned lead on to a large and
important group found both in salt and fresh water, and char-
acterized by the presence of a shell that is often riddled with
minute pores or foramina, on which account the name of Fora-
minifera has been given (fig. 301). The shell may be either
tough and membranous, composed of foreign particles agglutinated

496 CHARACTERS OF INVERTEBRATE ANIMALS

together, or, and usually, of calcareous nature. It either con-
sists of a single chamber or of a number of chambers associated
in various manners and presenting the utmost variety in form.
One of the most abundant forms is Globigerina, the shell of
which consists of several rounded chambers united together,
and which occurs in such profusion in a calcareous deposit
covering large tracts of the ocean floor that this has received
the name of Globigerina ooze. Globigerina and other forms
have also lived in earlier epochs of the earth's history, playing
a very important part as rock-builders. The pure form of lime-
stone called chalk is, for example, mainly composed of such
remains. Most of the Foraminifera are of small size, but
Nummulina, so called from its coin-like shape (L. nummus, a
coin), reaches the size of a shilling piece. Its shells chiefly
make up the important nummulitic limestone by which mountain
ranges in the Mediterranean region are to a large extent built up.

The pseudopods of the Foraminifera are slender threads which
often unite together into a sort of net-work. More than one
nucleus is commonly present in the protoplasm of which the soft
body consists.

The Sun- Animalcules (Heliozoa) (fig. 301) are a group of
mainly fresh-water forms in which the body is spherical, and
stiff pointed pseudopods radiate from it in a way which suggests
the rays seen in the conventional representation of the sun;
hence the name. There may be more than one nucleus and
pulsating vacuole. Actinophrys and Actinosphcerium are typical
genera. In some members of the group the surface of the
body is covered by loose flinty spicules, and in the stalked
genus Clathrulina there is a continuous shell in the form of a
perforated hollow sphere. This leads on to the condition found
in most of the marine group of Ray -Animalcules (Radiolaria)
(fig. 301), where there is a siliceous shell of extreme beauty
and of the most varied shape. Some parts of the sea-floor are
covered by Radiolarian ooze, mainly composed of such shells;
and there are also fossil forms making up certain rocks, notably
in the Barbados. Almost every microscopic cabinet contains
a slide of this " Barbados earth ".

The Amoeba group may be reckoned to include certain pro-
blematic organisms called Myxomycetes or Mycetozoa, the latter
name indicating the doubt which has existed as to whether

ANIMALCULES

497

Fig. 302. — Stages in Life-History of Protomyxa, enlarged

i, Spherical resting stage ; 2, the same invested with firm coat; 3, contents of same dividing into
spores ; 4, escape of spores (5), which assume amoeba-like shape, feed (g and 10), and fuse together
(13, 14) to form adult stage, which is shown in hungry stage (n), and feeding (12). This ultimately
contracts (15). and becomes round (16), after which the life-cycle recommences as before.
VOL. I. 32

498 CHARACTERS OF INVERTEBRATE ANIMALS

they are plants or animals (Gk. mykes, a fungus ; zoon, an animal).
Indeed, like Volvox and its allies, they will be found described
in most text-books, both of botany and zoology. A good example
is the so-called "flowers of tan " (/Ethaliuni] (fig. 301), which is
to be found in the form of sulphur-coloured net-works of proto-
plasm creeping slowly over heaps of spent tan, and attaining
a considerable size. At a certain time this organism produces
little capsules in which a number of small hard-coated repro-
ductive bodies known as spores are formed. From these escape
little fragments of protoplasm, which are at first something like
flagellate protozoans, for each of them possesses a single
flagellum, while later on they assume the shape of minute
amoebae. A number of these fuse together to form a creeping
net-work.

In the Amoeba group we may also include one of the simplest
known kinds of Protozoa, a minute marine animal — Protomyxa
aurantiaca (fig. 302) — found on the coast of the Canary Islands.
In colour and appearance it resembles a small individual of the
tan-flower organism, but no trace of a nucleus was observed by its
discoverer. After leading an active life for some time the body
contracts into a spherical form and becomes surrounded by a
firm investment. The protoplasm then divides up into a number
of fragments, each of which possesses a single flagellum. These
are liberated by the rupture of the protecting investment, soon
assuming the shape of little amoebae, of which numbers fuse
together to form an adult.

Group 3. — GREGARINES (S.porozoa)

This third and last group of the Protozoa has been modified
by the parasitic habit. The body is covered by a firm cuticle,
but cilia as well as pseudopods are absent. A good example is a
form (Clepsidrina blattarum) (fig. 301), found within the alimentary
canal of the Cockroach. The elongated body, when very young,
is attached by a hooked narrower end to the lining of the cock-
roach's intestine. Later on, the hooked end is shed, the animal
becoming free. A nucleus is discernible within the protoplasm,
but no pulsating vacuole. As the name indicates, the group
is characterized by the presence of those specialized reproductive
cells called spores, which are more commonly met with in plants

ANIMALCULES 499

than in animals. Before these can be formed two individuals
must together be surrounded by a firm coating or cyst, and
they then break up into a multitude of little spores surrounded
by firm coats. The spores become free by a somewhat complex
process, the firm coat of each then ruptures, and the contained
protoplasm emerges, ultimately growing into an adult Gregarine,
which is at first imbedded in and nourished by one of the cells
lining the intestine of the cockroach.

The Sporozoa not only include many forms more or less
similar to the one described, but also a large number of much
simpler species, parasitic in the cells of Vertebrates and other
higher animals.

END OF VOL. I

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The following is a list of the volumes in the Imperial Edition

The Pickwick Papers.
Oliver Twist.
Nicholas Nickleby.
Martin Chuzzlewit.
The Old Curiosity Shop
Barnaby Rudge.
David Copperfield.
Bleak House.
Sketches by Boz.

Hard Times and Master Humphrey's Clock.
Christmas Books.
Dombey and Son.
Little Dorrit.
A Tale of Two Cities.
Charles Dickens: A Critical Study.
By GEORGE GISSING.

Prospectus of any Book post free.

The Gresham Publishing Company.

NEW EDITION, REVISED AND GREATLY AUGMENTED.

°F ™E ENGLISH LANGUAGE. A complete Encyclopedic
Lexicon, Literary, Etymological, Scientific, Technological, and Pro-
nouncin^ Edited by CHARLES ANNAN-

DALE) M>A>j LLD inustrated by above

three thousand engravings on wood, besides a series of finely engraved and coloured plates.
This edition of the IMPERIAL DICTIONARY is beautifully printed on paper of imperial size,
specially made for the Work. It is issued in eight Divisional Volumes of a handy size for
reference, bound in cloth, with a fine design on side, at los. net each volume; also in
20 Divisions, paper cover, at 3-y. each, net.

The reception accorded by the press and the public to*this new edition of the IMPERIAL
DICTIONARY has been such as to show that the care and labour bestowed upon it have met
with due recognition, and to prove that it will continue fully to maintain its established
position as a standard lexicon of the English language, and as a work of the highest utility
for the purposes of general reference and everyday requirement.

DISTINCTIVE POINTS.
To sum up the chief points of this edition—

I. It is the latest revised dictionary, and has a supplement of many thousand new words.

II. It contains more words, exclusive of compound and obsolete words, than any other English
dictionary.

III. The pronunciation is explained on a plan which is simplicity itself.

IV. It gives the pronunciation and the meaning of the word as recognized to-day.
V. It has more illustrations than any other English dictionary.

VI. It has full-page plates (coloured and otherwise), which are an outstanding feature of the work.
No other English dictionary contains full-page plates.

VII. It has clear type, beautifully printed on fine paper, and is substantially and elegantly bound.

VIII. It has a specially prepared Supplement issued with each volume, and not, as is usually the
case, relegated to the end of the complete work.

IX. It has a very full Appendix, probably the best and finest given with any dictionary in the world.

X. It is sold on a plan at once acceptable and convenient, within the reach of all, and the price is
very moderate.

"The IMPERIAL DICTIONARY", says the St. James1 Gazette, "is a work which fairly
deserves the epithet of monumental. It is really what it professes to be — 'a complete
encyclopaedic lexicon, literary, scientific, and technological'. In other words, it is the best
dictionary of its kind in the English language, and its kind is the best."

"We have no hesitation in saying", writes the Spectator, "that it will prove a most
thorough piece of workmanship, and that among reference-books of its class it will hold
the first place, both as an authority and a source of instruction and entertainment."

"The encyclopedic method of treatment which has been adopted", remarks the
Athenceum, "will be found of the greatest service, affording as it does to the reader the
advantages of the ordinary dictionary combined with those of the encyclopedia."

The St. James1 Gazette says : — " The encyclopaedic part is executed with great skill
and accuracy ; and the genius of the editor has been exercised with the power and precision
of a hydraulic press upon the enormous masses of facts with which he has had to deal ".

Bookbinding a Speciality.

The Gresham Publishing Company.

AN- ENCYCLOPAEDIA OF ALL MATTERS RELATING TO THE
HOUSE AND HOUSEHOLD MANAGEMENT. Produced under
the general editorsnip of H. C. DAVIDSON, assisted by over
one hundred specialists. Copiously illustrated by coloured
and black-and-white plates and engravings in the text. In 4 volumes, super- royal 8vo, cloth,
with artistic design, price ^2, 2s. net. Also in 8 divisional volumes, cloth, price $s. net each.

THE BOOK OF THE HOME is intended to form a complete work of reference on all subjects connected
with household management. No efforts have been spared to ensure that every matter bearing upon the
Home and Home Life shall receive full and sufficient treatment, and that the information given shall be
reliable and in the best sense of the phrase up-to-date.

A few among over one hundred specialists who have contributed to the work :

Mrs. ADA S. BALLIN, Editor of Baby— the Mother's

Magazine, and of Womanhood.
Miss BERTHA BANNER, Training Teacher of Sewing

and Dressmaking at the Liverpool Technical

College for Women.

Mr. A. BLACK C E Architect, Author of First ^ Qther icultural ers>

Principles of Building.
Mrs. DAVIDSON, Author of Dainties, What our Mrs" C S' PEEL' Dress and Household Editor of

Daughters can do for themselves, &c.
Miss J. FORSTER, Principal of the Cheshire County

Mrs. H. R. HAWEIS (the late), Author of The Art
of Decoration, The Art of Beauty, &c.

Miss HELENA HEAD, Principal of the Liverpool
Girls' School for Secondary Education in
Domestic Science, and Author of the Manual of
Housewifery.

Mrs. A. HODGSON, Home Decorator to The Lady.

Mr. R. KEITH JOHNSTON, Author of Household
Difficulties and How to overcome Them.

Miss GERTRUDE J. KING, Secretary to the Society
for Promoting the Employment of Women.

Miss E. E. MANN, Head Teacher at the Liverpool

Training School of Cookery.
Colonel M. MOORE-LANE, Contributor to the Field

Hearth and Home, and Author of The New
Home.

Council Dairy Institute. Miss- B" SIBTHORPE PoOLEY, Lecturer to the Liver-

pool Ladies' Sanitary Association.

Miss RANKIN, Head Teacher of Laundry Work at
the Liverpool Technical College for Women.

Miss FLORENCE STACPOOLE, Lecturer to the National
Health Society and the Councils of Technical
Education, and Author of Handbook of House-
keeping for Small Incomes, &c.

Mr. DAVID TOLLEMACHE, late editor of The Chef
and Connoisseur.

The contents of THE BOOK OF THE HOME may be grouped under four heads. The first deals with
all matters concerning the House — from the choice of its site to the least of its internal decorations. The
householder is instructed in the laws regarding landlord and tenant, and counselled in the important
matters of sanitation and ventilation, heating and lighting, and the stocking and management of
the garden. The housekeeper is advised as to furnishing, everything necessary for the comfort
and adornment of a well-equipped house being described in detail, hints being also given regarding
removals, painting and papering, artistic decoration, arrangement of linen and store cupboards, &c.

In the second the daily routine of the Household is considered — the duties oi the servants, their
wages, their leisure and pleasures, the management of the kitchen, laundry, and store-room. Plain and
fancy cooking receive due attention, recipes being given of a large variety of dishes, and suggestions
made for breakfast, lunch, afternoon-tea, dinner, and supper. A number of menus are added suitable
for the different seasons. Invalid cookery also has its special section.

In the third are discussed the legal and customary duties, and the occupations and pastimes,
of Master and Mistress, the former being instructed as regards insurance and the making of a will,
and the smaller matters of carving, the care of the wine-cellar, and the inspection of garden and stables,
while the latter is advised as to account-keeping, payments, shopping, and innumerable other matters
connected with her duties as Mistress. Other subjects treated under this head are dress, home
occupations, visiting and entertaining, and indoor and outdoor amusements.

In the fourth sound, systematic, and practical advice is given as to the management, in health
and sickness, and the education, of children, and also on such important subjects as occupations
for boys and girls, the ceremonies necessary on the coming out of a daughter, and the preparations
and formalities necessary before and after a marriage.

THE BOOK OF THE HOME will thus be at once an indispensable ally to the young bride and the
novice in housekeeping, and a valuable work of reference to the more experienced.

Prospectus of any Book post free.

The Gresham Publishing Company,

Tni GRtsHAM LIBRARY

or STANDARD FICTION

The Gresham Library
of Standard Fiction.

A SERIES OF EIGHTEEN STAN-
DARD WORKS OF FICTION by
the most eminent authors, each volume
illustrated with six full-page drawings,
and containing a biographical sketch of the author. Large crown 8vo. Price 3^. 6d. net each.

The novels of the Gresham Library of Standard Fiction are all "old, tried, and valued friends": books
to read, books to keep and read again, books to hand down from generation to generation. Breezy
historical tales, recalling, like Westward Ho! the stirring days and valiant deeds of old; witty social
satire like Pride and Prejudice ; tales of quiet heroism like John Halifax ; weird tales of mystery like
After Dark ; intimate studies of the human soul like Jane Eyre — here are books for all tastes.

CHARLOTTE BRONTE

Jane Eyre. Illustrated by JOHN H. BACON.
WILLIAM MAKEPEACE THACKERAY

The Neweomes. Illustrated by WILLIAM RAINEY,R.I.
CHARLES KINGSLEY

Westward Ho! Illustrated by WILLIAM RAINEY, R.I.
EDWARD BULWER. Lord Lytton

The Caxtons. Illustrated by JOHN H. BACON.
CHARLES DICKENS

David Copperfield. Illustrated by WILLIAM

RAINEY. R.I.
SIR WALTER SCOTT, Bart.

The Heart of Midlothian. Illustrated by CLAUDE

A. SHEPPERSON.
JANE AUSTEN

Pride and Prejudice. Illustrated by CHRIS. HAM-
MOND, R.I.
CHARLES LEVER

Harry Lorrequer. Illustrated by CARTON MOORE

PARK.

MRS. CRAIK

John Halifax, Gentleman. Illustrated by JOHN
H. BACON.

BENJAMIN DISRAELI, Lord Beaconsfield

Coningsby. Illustrated by CLAUDE A. SHEPPER-
SON.
WILKIE COLLINS

After Dark. Illustrated by GORDON BROWNE, R.I.
NATHANIEL HAWTHORNE

The Scarlet Letter, and The House with the

Seven Gables. Illustrated by JOHN H. BACON.
GEORGE BORROW

Lavengro. Illustrated by CLAUDE A. SHEPPERSON.
MRS. GASKELL

Cranford, and Mary Barton. Illustrated by

CHRIS. HAMMOND, R.I.
MARIA EDGEWORTH

Ormond. Illustrated by FRED. PEGRAM.
JAMES MORIER

Hajji Baba. Illustrated by H. R. MILLAR.
ANTHONY TROLLOPE

Barehester Towers. Illustrated by L. LESLIE

BROOKE.
GEORGE ELIOT

Scenes of Clerical Life. Illustrated by CHRIS.

HAMMOND, R.I.

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GENERAL LIBRARY
UNIVERSITY OF CALIFORNIA— BERKELEY

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Renewed books are subject to immediate recall.

MAY 2 8 1955

DEC 1 i I960

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UNIVERSITY OF CALIFORNIA LIBRARY