Phrynus (Pedipalpi) and the Scorpionidae are viviparous 1. The Tardigrada lay their ova in their cast-off skin; the Araneidae as a rule and the Psendoscorpionidae carry their ova about attached to the abdomen. The ovum of the Scorpions is telolecithal; of others centrolecithal. The mode of segmentation in the latter case varies. The young Arachnid is as a rule hatched in a form resembling the adult. The Acarina however are often at first hexapodous; and in some instances the young octopod animal differs from the adult in colour, presence of hairs, etc, and passes through an inert stage 2. The embryo Pentastomum taenioides lives encysted in the liver or lungs of the rabbit: it moults and acquires two pairs of hooks and a ringed abdomen, and then wanders about in its host, and may become encysted a second time. When devoured by the dog or wolf it becomes adult in the nasal fossae and frontal sinuses.

Certain Acarina, e. g. Demodex, are parasitic, as are the Linguattdina. The Phalangidae and Solifugae are nocturnal. The Araneidae are terrestrial for the most part. Some Acarina are aquatic, Limtdus marine, as were the extinct Eurypterina and Trilobita. A Scorpion has been found in Silurian strata, and a Phalangid in the Solenhofen slates (Meso-zoic); Spiders occur in amber. A Xiphosuran is found in the upper Silurian, Limtdus in the Solenhofen slates: the Eurypterma extend from the upper Silurian to the Lower Carboniferous period; the Trilobita from Cambrian to the same period, but are chiefly Cambrian and Silurian.

1 In Sphaerogyna ventricosa (Acarina) development is intra-uterine, and the female gives birth to adult males and females, which are fecundated at birth; Laboulbene and Megnin, Journal de l'Anat. et Physiol, xxi. 1885.

2 This stage known as Hypopus appears to be the heteromorphous nymph or immature octopod form of Tyroglyphus and some allied genera. It has typically a large covering carapace; the fourth pair of limbs armed with long setae; mouth-parts rudimentary; the ventral integument soft and furnished with suckers, especially at the posterior end of the body. The special use of this stage, which passes by an ecdysis into the ordinary adult, appears to be the transport and dispersal of the species. See Michael, J. L. S. xvii. 1884, and Journal Roy. Micr. Soc. (2), v. 1885, p. 22 et seqq.

The Arachnida are classified as follows: A. Tracheate groups.

I. Linguatulina: parasitic, worm-like, elongated, body ringed; Pentastomum.

2. Acarina: abdomen united to cephalothorax; oral appendages adapted to biting, or piercing and sucking; mostly tracheate; minute. Demodex, Sarcoptes, Tyroglyphus, Hydrachna, etc.

3. Tardigrada: no abdomen; four pairs of short limbs; oral appendages adapted for piercing and sucking. Hermaphrodite. Minute, aquatic. Arctiscon, Macrobiotus, etc.

4. Araneidae: abdomen soft, unsegmented; falces with a poison-gland; 4-6 spinnerets; 2-4 lung-sacs. Mygale, Cteniza, Tegenaria, etc.

5. Phalangidae: abdomen as a rule with six somites, broad and applied to cephalothorax; falces with a didactyle claw or chela; limbs long and slender; tracheate. Phalangium, etc.

6. Pedipalpi\ abdomen with 11-12 somites; falces clawed; pedipalpi clawed (Phrynus) or chelate (Thelyphonus); first pair of limbs slender and elongate; four lung-sacs. Phrynus, Thelyphonus.

7. Scorpionidae: abdomen composed of a seven-segmented mesosoma and five-segmented metasoma, with telson armed with a poison-gland; both oral appendages chelate; second pair large; four lung-sacs. Androctonus, Buthus, Scorpio, etc.

8. Pseudo-scorpionidae: abdomen broad and flat, with 10-11 somites; pedipalpi chelate; tracheate. Chelifer, Obisium, Chthonius, etc.

9. Solifugae: head and thorax separate; thorax of three separate somites; abdomen with nine somites; falces chelate; palpi limb-like, tracheate. Galeodes (= Solpugd).

B. Branchiate groups srPoecilopoda of Wallcott,

1. Xiphosura: cephalothorax shield-shaped; meso- and meta-soma fused; first pair of oral appendages chelate; coxae of the five pairs of cephalothoracic limbs masticatory; six pairs of mesosomic limbs; a telson. Limulus, Belinurus (fossil).

2. Eurypterina s. Merostomata: extinct; abdomen with twelve somites and a broad telson; first pair of oral appendages sometimes non-chelate; four pairs of cephalothoracic limbs surround mouth; no abdominal limbs. Eurypterus, Ptery-gotus, Hemtaspis, etc.

3. Trilobita: extinct; a cephalothorax; a segmented mesosoma and unsegmented metasoma (pygidium), both bearing limbs; no prae-orally placed appendages, but four pair of cephalothoracic limbs surround the mouth. Calymene, Phacops, Asaphus, etc.

'Arachnida,' P. Cambridge, Encyclopaedia Britannica (ed. ix.), ii. 1875; Packard, Study of Insects, Salem, 1872; Murray, Economic Entomology, S. Kensington Handbooks, 1877; Blanchard, 'Arachnides,' L'organisation du Regne animal, Paris, 1860; Zoological position of Class, Kingsley, Q. J. M. xxv. p. 556.

Linguatulina, P. J. Van Beneden, A. Sc. N. (3), xi. 1849; Leuckart, Bau der Pentastomen, Leipzig, i860.

Acarina, see pp. 496-7, ante. Classification, cf. Michael, Oribatidae, Ray Soc. i. cap. 4; Demodex, Megnin, Journal de FAnat. et Physiol, xii. 1877; Sarcoptidae, Fiirstenberg, 'Kratzmilben der Menschen,' etc. Leipzig, 1861; Sarcop tides psoriques, Me'gnin, Revue etMagasin de Zoologie (3), v. 1877 (some plates in vi. 1878). Sar-coptides plumicoles, Trouessart, Paris, 1885; Robin et Me'gnin, Journal de l'Anat. et Physiol, xiii. 1877; Haller, Z. W. Z. xxxvi. 1881. Tyrogiyphidae, Michael, Journal Roy. Micr. Soc. v. 1885 j Nalepa, SB. Akad. Wien, xc. pt. 1. 1884; Haller, Z. W. Z. xxxiv. 1880. Glyciphagus, Michael, J. L. S. xix. Gamasidae, Kramer, A. N. 48. (1), 1882; Michael, J. L. S. xv. 1881. Argas (Lxodidae), Laboulbene and Me'gnin, Journal de l'Anat. et Physiol, xviii. 1882. Phytoptidae: Phytoptus, Thomas, Zeitschrift f. Ges. Natw. (Giebel), 33. 1869; Galls, Id. op. cit. 49. 1877; Nova Acta, xxxviii. 1876; Schlechtendal, Zeit. Natw., 55, 1882; 56, 1883. Trombidiidae: Trombidium (and lit.), Henking, Z. W. Z. xxxvii. 1882; Donnadieu, Recherches pour servir a l'histoire des Te'traniques, Paris, 1875 (Ann. Soc. Lin. Lyons, xxii). Hydrachnidae, Kramer, A. N. 41. (1), 1875. British Oribatidae, Michael, Ray Soc. i. 1884.

Tardigrada, Greeff, A. M. A. ii. 1866; Echiniscus, Schultze, ibid. i. 1865.

Araneidae, see pp. 303, 304. Classification, see Thorell, A. N. H. (5), xvii. 1886 (with lit.). Spiders of Great Britain, P. Cambridge (systematic list), Tr. L. S. xxx. 1875. Trap-door spiders, Moggridge, 1873, and Suppl. 1874. Mouth-parts, Croneberg, A. N. 46 (1), 1880; Bertkau, A. N. 36 (1), 1870. Spinnerets of Epeira, Oeffinger, A. M. A. ii. 1866. Respiratory organs, Bertkau, A. N. 38 (1), 1872. Generative organs, Id. op. cit. 41 (1), 1875; male palps, see also Emerton, Proc. Boston Soc. xvii. Seasonal dimorphism, Karsch, Bertkau, Z. A. viii. 1885. Development, Balfour, Q. J. M. xx. 1880. Regeneration of lost parts, Blackwall, Brit. Assoc. Reports, 1844. Stridu la ting organs, Campbell, J. L. S. xv. 1881.

Phalangidae: Classification (with lit.), Simon, 'Arachnides de France,' vii. 1879; cf. Id. Ann. Soc. Entomol. Belg. xxii. 1879. British sp., Meade, A. N. H. (2), xv. 1855. Anatomy, Rossler, Z. W. Z. xxxvi. 1882; Loman, Bijdrage tot de Anat. der Phalangiden (Diss.), Amsterdam, 1881. Genital organs, H. de Graaf, Sur la structure des organes genitaux des Phalangiens, Leiden, 1882 (Z. A. iii. 1880).

Pedipalpi, Karsch, A. N. 45 (1), 1879; 46 (I), 1880; Butler, A. N. H. (5), iv. 1879. Phrynus, Butler, A. N. H. (4), xii. 1873; Thelyphonus, Id. (4), x. 1872, and xii. 1873.

Scorpionidae: Classification, Thorell, A. N. H. (4), xvii. 1876; Atti Soc. Ital. xix. 1876. Anatomy, Ray Lankester, Q. J. M. xxi. 1881 (under Limulus); xxiii. 1883; xxiv. 1884; Tr. Z. S. xi. Poison apparatus, Joyeux-Laffuie, A. Z. Expt. (2), i. 1884; Dufour, 'Histoire anatomique' Me'm. prentees par divers savants a l'Acad. des Sciences, xiv. 1856. Lung-books, Ray Lankester, Q. J. M. xxv. 1885.

Pseudoscorpionidae, Stecker, A. N. 41 (I), 1875 (lit. and distribution); L. Koch, Ubersichtliche Darstellung des Europ. Chernetiden, Numberg, 1873.

Solifugae, Karsch, A. N. 46 (1), 1880. Poison-glands, Croneberg, Z. A. ii. 1879. Anatomy, Dufour, C. R. 46, 1858.

Xiphosura: Limulus, Ray Lankester, Q. J. M. xxi. 1881; xxiii. 1883; xxiv. 1884. Blood, Howell, Studies Biol. Lab. Johns Hopkins Univ. iii. (6), 1886. Testes, Benham, Tr. L. S. (2), ii. Zoological position, Lankester, op. cit. xxi; Packard, A. N. H. (5), ix. 1882; Moseley, ibid.; Lankester, A. N. H. (5), xvii; Id. and Claus, ibid, xviii. 1886; cf. also Kingsley, infra, p. 543, and Packard, infra. Embryology, Kingsley, Q. J. M. xxv. 1885; Packard, American Naturalist, xix. 1885; Osborne, Johns Hopkins Univ. Circulars, v. (Oct.) 1885. Moult, sexual characters, Koons, American Naturalist, xvii. 1883. For older lit. see Gerstacker, Bronn's Thierreich, v. pt. 1.

Eurypterina, Woodward, Monograph, Palaeont. Soc. 1866-78.

Trilobita, Wallcott, Bull. Mus. Harvard, viii. 1880-81; Id. Science, iii. 1884; Salter, Monograph, Palaeont. Soc. 1863-83; Woodward, Monograph, Palaeont. Soc. 1883-84.

The fossil Branchiate groups and other Arachnids are given (with lit.) in Zittel, Handbuch der Palaeontologie, Abth. 1, Palaeozologie, ii. 1881-85.

Heart in Gamasidae (Acarina), Claus; Michael, A. N. H. (5), xvii. 1886.

Respiratory organs of Arachnida, Macleod, Archives de Biologie, v. 1884.

Coxa/ gland of My gale, Pelseneer, P. Z. S. 1885; of other spiders, Bertkau, A. M. A. xxiv. 1884, p. 435; Id. Z. A. ix. 1886, p. 431; of Oribatidae, Michael, British Oribatidae, Ray Soc. i. 1884, p. 177, (cf. Journal Roy. Micr. Soc. (2), iii. 1883); of Phalangidae and Galeodes, MacLeod, Bull. Ac. Roy. Belg. (3), viii. 1884; ofLimulus, Gulland, Q. J. M. xxv. 1885, and Kingsley, ibid, in 'Notes,' etc.