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ARAOHNIDA

Order 2. Ascorhynchomorpha, Pocock (nov. )• Appendages of the 2nd and 3rd pairs retained and developed, as ^ yie .m0ie primitive types of Nymplionomorpha ; but those of the 1st pair are either rudimentary, as in the Ascorhynchidse, or atrophied, as in the Colossendeidae. In the latter a further specialization is shown in the fusion of the body segments. ,, Two families: 1. Ascorhynchidse (genera Ascorhynchus and Ammothea); 2. Colossendeidie (genera Colossendeis and Disco-

swimming, none of the prosomatic appendages modified to act as paddles ; segments of the mesosoma and metasoma (= opisthosoma) not more than ten in number, distinct or coalesced. Family—Limulidse {Limulus). Belinuridse {Belinurus, Aglaspis, Prestwichia). ” Hemiaspidse {Hemiaspis, Bunodes). Remarks.—The Xiphosura are marine in habit, frequenting the shore. They are represented at the present day by the single genus Limulus (Figs. 44 and 45 ; also Figs. 7, 9, 11, to 15 and 20), which

Order 3 Pycnogonomorpha, Pocock (nov.).—Derivative forms in which the reduction in number of the anterior appendages is carried farther than in the other orders, reaching its extreme m the Pycnogonidse, where the 1st and 2nd pairs are absent m both sexes and the 3rd pair also are absent in the female. In the Hannoniidse, however, which resemble the Pycnogonida m the absence of the 3rd pair in the female and of the 2nd pair m both sexes, the 1st pair are retained in both sexes. Two families: 1. Hannoniid* (genus Hannonia); 2. Pycnogonidae (genera Pycnogonum and Phoxichilus). _ _. Remarks.—The Pantopoda are not known in the fossil condition. They are entirely marine, and are not uncommon in the coralline zone of the sea-coast. The species are few, not more than fifty (23). Some large species of peculiar genera are taken at great depths. Their movements are extremely sluggish. They are especially remarkable for the small size of the body and the extension of viscera into the legs. Their structure is eminently that of degenerate forms. Many frequent growths of coralline Algae and Hydroid polyps, upon the juices of which they feed, and in some cases a species of gall is produced in Hydroids by the penetration of the larval Pantopod into the tissues of the polyp. Sub-Class II (of the Nomomeristic Arachnida). EU-ARACHNIDA.—These start from highly developed and specialized aquatic branchiferous forms, exhibiting prosoma with six pediform pairs of appendages, an intermediate prae-genital somite, a niesosoma of six somites bearing lamelliform pairs of appendages, and a metasoma of six somites devoid of appendages, and the last provided with a post - anal spine. Median eyes are present, which are monomeniscous, with distinct retinal and corneagenous cell-layers, and placed centrally on the prosoma. Lateral eyes also may be present, arranged in lateral groups, and having a single or double celllayer beneath the lens. The first pair of limbs is often chelate or prehensile, rarely antenniform ; whilst the second, third, and fourth may also be chelate, or may be simple palps or walking Fig. 44.—Dorsal view of Limulus polyplumus, Latr. One-fourth the natural legs. size, linear. (From Parker and Haswell, Text-look of Zoology, after An internal skeletal plate, the so-called “ entosternite of Leuckart.) fibro-cartilaginous tissue, to which many muscles are attached, is placed between the nerve-cords and the alimentary tract in the prosoma of the larger forms (Limulus, Scorpio, Mygale). In the same and other leading forms a pair of much-coiled glandular tubes, the coxal glands (ccelomoccels in origin), is found with a duct opening on the coxa of the fifth pair of appendages of the prosoma. The vascular system is highly developed (in the non-degenerate forms); large arterial branches closely accompany or envelop the chief nerves ; capillaries are well developed. The blood-corpuscles are large amcebiform cells, and the blood-plasma is coloured blue by haemocyanin. The alimentary canal is uncoiled and cylindrical, and gives rise laterally to large gastric glands, which are more than a single pair in number (two to six pairs), and may assume the form of simple caeca. The mouth is minute and the pharynx is always suctorial, never gizzard-like. The gonadial tubes (gonoccels or gonadial ccelom) are originally reticular and paired, though they may be reduced to a simpler condition. They open on the first somite of the mesosoma. In the numerous degenerate forms simplification occurs by obliteration of the demarcations of somites and the fusion of body-regions, together with a gradual suppression of the lamelliferous respiratory organs and the substitution for them of tracheae, which, in their turn, in the smaller and most reduced members of the group, may also disappear. The Eu-arachnida are divided into two grades with reference to the condition of the respiratory organs as adapted to aquatic or terrestrial life. Grade a (of the Eu-araclmida). delobranchia (Hydropneustea). Mesosomatic segments furnished with large plate-like appendages, the 1st pair acting as the genital operculum, the remaining pairs being provided with branchial lamellae fitted for breathing oxygen dissolved in water. The prae-genital somite partially or wholly Fid. 45.—Ventral view of Limulus polyphemus, one-fourth the natural size, linear. 1 to 6, the six prosomatic pairs of appendages; aid, the solid obliterated in the adult. The mouth lying far back, so that the opisthosomatic carapace; tels, the post-anal spine (not the telson as the basal segments of all the prosomatic appendages, excepting those lettering would seem to imply, but only its post - anal portion); operc, of the 1st pair, are capable of acting as masticatory organs. Lateral the fused first pair of mesosomatic appendages forming the genital operculum. (From Parker and Haswell, Text-look of Zoology, after Leuckart.) eyes consisting of a densely-packed group of eye-units (‘ ‘ compound ” eyes). Order 1. Xiphosura. — The prse - genital somite fuses in the occurs on the American coast of the Atlantic Ocean, but not on its embryo with the prosoma and disappears (see Fig. 19). Not free- eastern coasts, and on the Asiatic coast of the Pacific. The Atlantic