The following table shows in parallel columns the post-oral somites and their appendages in the Crustacea, Arachnida, Myriapoda, and Insecta. The antennae of the two latter classes are not included as being apparently processes of the pro-cephalic lobes. The post-oral appendages of Crustacea and Arachnida which become prae-oral are printed in a different type, and the first antennae of the former Class are marked with a note of interrogation as being somewhat doubtfully homologous with the succeeding appendages.

See, for remarks on the descent of Arthropoda, Balfour, Comparative Embryology, i. p. 451; for a discussion on the relations of the Arachnida and Crustacea, Kingsley, Notes on the Embryology of Limulus, Q. J. M. xxv. 1885, p. 556.

Table Of Post-Oral Somites And Their Appendages In Arthropoda

Number of Segment.

Crustacea.

Arachnida.

Myriapoda.

Insecta.

I.

First Antenna (?) uniramose in Nau-plius and adult En-tomotraca, secondarily biramose in Decapoda, etc.

Chelicerae.

Mandible: with a small palp in Chi-lopoda: non-pal-pate in Diplopoda.

Mandible: non-pal-pate.

II.

Second Antenna: biramose in Nau-plius as a rule, but uniramose in many adult forms.

Chelae in Scorpio and Thelyphonus.

Pedipalpi in Spiders.

Maxilla: palpate in Chilopoda: 4-lobed plate in some adult Diplopoda, but simple appendage in embryo.

Maxilla: palpate.

Number of

Segment.

Crustacea.

Arachnida.

Myriapoda.

Insecta.

III.

Mandible: biramose in Nauplius: not palpate in

Zoaea, nor in Phyllopoda, terrestrial

Isopoda and Amphipoda, nor Cumacea, etc.

A3-articulate'palp' in adult Decapoda.

1st pair of limbs bearing outer element of so-called

'labium' in Scorpion.

Limb - like with basal parts in contact, Chilopoda: short, 4 - jointed with claw in lulus

(Diplopodd), in $ of some species hook-like limb.

Labium: palpate.

IV.

1st Maxilla.

2nd pair of limbs bearing inner element of 'labium' in Scorpion.

Poison claws of

Chilopoda: 1st

(single) pair of 5jointed limbs with claw in lulus (Diplopoda).

1st pair of limbs: prothorax.

V.

and Maxilla: aborted in adult Cladocera.

3rd pair of limbs.

1st pair of limbs in

Chilopoda sometimes lost:apodous in lulus: with ♂ and ♀ genital apertures.

2nd pair of limbs: mesothorax.

VI.

1st pair of thoracic limbs, Phyllopoda,

Nebalia, Schizopoda.

Maxilliped in many forms.

4th pair of limbs.

2nd pair of limbs in

Chilopoda: single pair in lulus.

3rd pair of limbs: metathorax.

VII.

1st pair of limbs in

Arthrostraca.

2nd pair of maxillipeds in Decapoda.

Genital operculum in Scorpion and

Limulus. An embryonic limb in

Scorpion and Spider: somite included in abdomen of latter.

1st double pair of limbs in lulus.

'Due probably to an imperfect division of the primitive segments.'

Balfour, Comp.

Embryology, i. p. 324-

1st abdominal somite: rudiments of embryonic limbs on this and 9 succeeding somites in Sphinx populi.

VIII.

3rd pair of maxillipeds in Decapoda.

(Last somite in

Ostracoda.)

Pectines of Scorpion.

2nd pair of embryonic limbs in Scorpion and Spider.

Do. in lulus.

2nd do.: pair of cylindrical appendages, Machilis

( Thysanuran ), and on 8 succeeding somites, 1st pair of prolegs in many Tenthre-dinid larvae.

IX.

1st ambulatory limb in Decapoda or

Forceps.

4th pair of maxillipeds in Squilla.

1st pulmoniferous somite in Scorpion.

3rd pair of embryonic limbs in do., and do.

Copulatory organ and single pair of limbs in ♂ lulus: double pair in ♀.

3rd do.; 1st pair of prolegs in many

Lepidopteran larvae.

Number of Segment.

Crustacea.

Arachnida.

Myriapoda.

Insecta.

X.

2nd ambulatory limb in Decapoda.

5th pair of maxilli-peds in Squilla.

2nd pulmoniferous somite in Scorpion.

4th pair of embryonic limbs in Scorpion and Spider.

Double pair of limbs in both ♂ and ♀, lulus: and last somite in Pauro-pus (Diplopoda).

4th abdominal somite.

XL

3rd ambulatory limb in Decapoda. 1st in Squilla.

3rd do., Scorpion, and 5th embryonic limb in do.: limbless somite in embryo Spider.

5th do.

XII.

4th do., in Decapoda. Lost in Lucifer.

4th do. in Scorpion, and 6th embryonic limb in do.: limbless somite in embryo Spider.

6th do.

XIII.

5th do., in Decapoda. Lost in Lucifer.

Somite without lungs in Scorpion. Limbless somite in embryo Scorpion and Spider. Embryonic somite in Limulus.

7th do.: no proleg in caterpillar nor in two next somites.

XIV.

1st abdominal somite with pleo-pod in a Malacos-tracan.

(Last somite in Daphnia (Clado-cera).)

1st caudal somite in Scorpion. Limbless somite in embryo Spider. Embryonic somite in Limulus.

8th do.: 1st pair of ♀ genital processes.

♀ genital aperture.

Anal somite, some Diptera.

XV.

2nd do.

(Last somite in a typical Copepod.)

2nd do., Scorpion. Limbless somite in embryo Spider. Telson in Limulus:

Anus in front of it.

Last somite in Lithobius and Scu-tigera (Chilopoda).

9th do.: 2nd pair of ♀ genital processes.

Anal somite, some Diptera.

XVI.

3rd do.

3rd do., Scorpion. Azygos terminal piece in embryo

Spider.

The total number of somites may be very great in some Iulidae: and in the Geophilidae (Chilopoda) may amount to more than 200.

10th do.: anal somite in Thysa-nura, Coleoptera, and Lepidoptera.

Anal proleg in caterpillar and Tenthre-dinid larva.

3 processes in Ma-chilis.

XVII.

4th do.

4th do., Scorpion.

nth do.: anal somite in many Or-thoptera, etc.

? Cerci anales limbs.

Number of Segment.

Crustacea.

Arachnida.

Myriapoda.

Insecta.

XVIII.

5th abdominal somite with pleo-pod in a Malacos-tracan.

5th caudal somite in Scorpion. Anus behind it.

XIX.

6th do.

Telson in Scorpion. A jointed filament in Thelyphonus.

XX.

Telson.

7th abdominal somite, Nebalia.

XXI.

8th abdominal somite in Nebalia, but there are only 6 abdominal ganglia in this animal.

In Apus numidicus (Phyllopod) the total number of somites is said to reach 46.

The following notes of the differences observable between the Lobster (Ho-marus) and the Crayfish, with which it is sometimes grouped in the family Astacidae, may be useful to the student.

The squame of the second antenna is relatively small; the fifth thoracic sternum is fixed; the first abdominal appendage in the male is two-jointed, and the terminal joint is lamellate but only curved to form a gutter, not rolled up as in Astacus; the second abdominal appendage in the same sex has a small plate to represent the rolled up lamina of Astacus; in the female the first abdominal somite bears appendages well developed; the telson has no transverse suture. The number of podo-branchiae is as in Astacus, but the stem is completely split into a plume and lamina, and the branchial filaments are stiff and close set; the single arthrobranch of the second maxilliped in Astacus is absent, and the total number of these organs is hence reduced to ten, but there are four pleurobranchiae. The infra-oesophageal ganglion is small relatively to the thoracic ganglia, and more distinctly constricted at the sides than in Astacus. The caecum of the mesenteron is small and bilobed: the caeca of the liver are short and the anterior portion of the gland large.

The first part of the intestine is smooth, the terminal portion plicated, and at the junction of the two parts there is a dorsal caecum as in Amphipoda and most Decapoda. The testicular lobes are long and only joined by a commissure, and the vasa deferentia are shortened. The largest Gregarine known, Porospora gigantea, inhabits the alimentary canal: see Schneider, A. Z. Expt. iv. 1875; E. van Beneden Q. J. M. xii. 1872.

The Lobster quits the egg in the Mysis-stage, cf. Rathke, A. N. H., 1841; Sars, Vidensk. Selsk Forh. Christiana, 1874; Smith, Trans. Connecticut Acad, of Arts and Sciences, 1873.

The anatomy of a Crab (Callinectes) and its developmental forms or Zoaea is given in W. K. Brooks's Handbook of Invertebrate Zoology, Boston, 1882, p. 168 et seqq. For the Zoaea of the Brachyura, see also Balfour, Comp. Embryology, i. p. 398.

Crustacea.Woodward, Encyclopaedia Britannica (ed. ix.) vi; Gerstaecker, Bronn's Klass. und Ordn. des Thierreichs, v. 1, pt. 2 in progress. Histoire Naturelle des Crustaces, M. Milne-Edwards, 3 vols., Paris, 1834-40. British Stalk-eyed Crustacea, Bell, London, 1856. British Sessile-eyed Crustacea, Spence Bate and Westwood, 2 vols., London, 1863-68.

The Crayfish.Huxley, International series, xxviii. 1880. Classification and Distribution of Crayfish. Id. P. Z. S. 1878. Revision of Astacidae. Faxon, Mem. Harvard Mus. x. No. 4, 1885. Figures, Atlas of Practical Elementary Biology, Howes, 1885.

Limbs, etc, and Phylogeny.Claus, Arb. Zool. Inst. Wien. vi. 1885; Boas, M. J. viii. 1883. Homologies of Limbs. Ray Lankester on Apus, Q. J. M. xxi. 1881; Packard, Amer. Naturalist, xvi. 1882.

Tegumentary Skeleton.Milne-Edwards, A. Sc. N. (3), xvi. 1851.

Integument.Vitzou, A. Z. Expt. x. 1882; Braun, Arb. Zool. Zoot. Inst. Wurzburg, ii. 1875. Moult of Astacus. Chantran, C. R. lxxi. 1870; Vitzou, op. cit. supra. Change of Tendon of Adductor mandibulae. Baur, Muller's Archiv (Archiv f. Anat. und Phys.), 1860.

Self-Amputation of limbs.Dewitz, Biol. Centralbl. iv. 1884-5; in Crab, Fredericq, Arch. de Biologie, iii. 1882; Id. A. Z. Expt. (2) i. 1883. Regeneration of limbs in Astacus. Chantran, C. R. lxxiii. 1871.

Glands of branchial cavity and cement glands. Braun, op. cit. supra. Cf. Nebeski on Amphipoda, Arb. Zool. Inst. Wien, iii. 1881.

Organs of Special Sense.Eye. Grenacher, Sehorgane der Arthropoden, Got-tingen, 1879; Carriere, Sehorgane der Thiere, Munchen und Leipzig, 1885. Cope-podan (azygos) eye. Hartog, A. N. H. (5), x. 1882. Functions of facetted eye. Exner, Biol. Centralbl. i. 1881-2; Notthaft, Senckenberg, Abhandl. xii. 1881. Regeneration of Eyes in Astacus. Chantran, C. R. lxxvi. 1873. Ear. Hensen, Z. W. Z. xiii. 1863. Olfactory hairs. Jourdain, Journal de l'Anatomie, xvii. 1881; Leydig, Muller's Archiv (Archiv f. Anat. u. Phys.), 1860.

Development of Astacus.Rathke, Ueber die Bildung, etc, des Flusskrebses, Leipzig, 1829; Lereboullet, Mem. de l'lnstitut (Savans Etrangers), xvii. 1862; Bobretzky, cf. Hofmann und Schwalbe's Jahresbericht, ii. 1873; Reichenbach, Z. W. Z. xxix. 1877, and Q. J. M. xviii. 1878; Schimkewitsch, Z. A. viii. 1885.

On various larval forms, see Balfour, Comp. Embryology, i. 1880, p. 380, etc., and lit. cited. For Penaeus, add Brooks, A. N. H. (5), xi. 1883, and Faxon, Amer. Naturalist, xvii. 1883; see also Conn, on Significance of larval skin in Decapoda, Studies from Biological Laboratory, Johns Hopkins University, iii. 1884.