A B

FIG. 42. — So-called " trilobite stage " of Limulus polypkemus. A, Dorsal; B, ventral view. (From KoncMt and Heider. after Leuckart.) Peripatus, Diplopoda, Hexapoda and Chilopoda. Probably tracheae have developed independently by the same process in several groups of tracheate Arachnids. The nomomeristic Arachnids comprise two sub-classes—one a very small degenerate offshoot from early ancestors; the other, the great bulk of the class. Sub- Class L (of the Nomomeristica). PANTO PODA.— Nomomeristic Arachnids, in which the somites corresponding to mesosoma and metasoma have entirely aborted. The seventh, and sometimes the eighth, leg-bearing somite is present and has its leg-like appendages fully developed. Monomeniscous eyes with a double (really triple) cell-layer formed by invagination, as in the Eu-arachnida, are present. The Pantopoda stand in the same relation to Limului and Scorpio that Cyamus holds to the thoracostracous Crustacea. The reduction at the organism to seven leg -bearing somite*, of which the first pair, an in so many Eu-araehnida. are chelate. is a form of degeneration connected with a peculiar quasi -para* tic habit resembling that of the crustacean Laemodipoda.

The genital pore* are situate at the base of the 7th pair of limbs, and may be repeated From Parker and HaflreU'i Tat^ock ./ Zxlciy. after Hodu FIG. 43. — One of the Nymphonomorphous Pantopoda, Nympkon hispidum, showing the seven pairs of appendages I to 7; ab, the rudimentary opisthosoma ; s. the mouth-bearing proboscis. on the 4th, 5th, and 6th. In all known Pantopoda the size of the body is quite minute as compared with that of the limbs: the alimentary canal sends a long caecum into each leg (cf. the Araneae) and the genital products are developed in gonocoels also placed in the legs. The Pantopoda are divided into three orders, the characters of which are dependent on variation in the presence of the full number of legs. Order 1 (of the Pantopoda). Nymphonomorpha, Pocock (nov.) (fig. 43). — In primitive forms belonging to the family Nymphotiidae the full complement of appendages is retained — the 1st (mandibular), the 2nd (palpiform), and the jrd (ovigerous) pairs being well developed in both sexes. In certain derivative forms constituting the family PaUenidae, however, the appendages of the 2nd pair are either rudimentary or atrophied altogether. Two families: I. Nymphonidae (genus Nymphon), and 2. Pallenidae (genus Patient).

Order 2. Ascorhynchomorpha, Pocock ( nov.).— Appendages of the 2nd and 3rd pairs retained and developed, as in the more primitive types ol Nymphonomorpha ; but those of the 1st pair are either rudimentary, as in the Ascorhynchidae, or atrophied, as in the Colosstndtidae. In the latter a further specialization is shown in the fusion of the body segments. Two families: I. Ascorhynchidae (genera Ascorhynchus and Ammothra); 2. Colossendeidae (genera Colossendeis and Discoarachne). Orders. Pycnogonomorpha, Pocock (noy.). — Derivative forms in which the reduction in number of the anterior appendages is carried farther than in the other orders, reaching its extreme in the Pycnogonidae, where the 1st and 2nd pairs are absent in both sexes, and the 3rd pair also are absent in the female. In the Hannoniidae, however, which resemble the Pycnogonidoe in the absence of the 3rd pair in the female and of the 2nd pair in both sexes, the 1st pair are retained in both sexes. Two families: I. Hannoniidae (genus Hannotiia); 2. Pycnogonidae (genera Pycnogonum and Phoxidtilus). Remarks. — The Pantopoda are not known in the fossil condition. They are entirely marine, and are not uncommon in tne 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 specie* of gall is produced in hydroids by the penetration of the larval Pantopod into the tissues of the polyp. Sub-Class n. (of the Nomomeristic Arachnida). EU-ARACH-