The Phoxichilidae and Pycnogonidae (Agnathonia, Leach; Achelata, Sars), though differing greatly in aspect, are not improbably allied to one another; and whether this be so or not, the complete absence of chelophores and of palps affords an arbitrary character by which they are conveniently separated from all the rest.
The following table epitomises the chief characters of the several families:—
| Pycnogonida. | Proboscis. | Chelophores. | Palps. | Ovigerous legs. | Teeth on do. | Legs. | Trunk-segments. | Genital Openings. | |
|---|---|---|---|---|---|---|---|---|---|
| ♂ | ♀ | ||||||||
| (Cryptochelata, Sars)— | |||||||||
| Decolopodidae | Fixed, decurved | Complete, small, scape 2–jointed | 10 joints | 10 joints ♂, ♀ | Four rows, simple | 5 | Condensed, coalescent | 1, 2, 3, 4, 5, | 1, 2, 3, 4, 5 |
| Colossendeidae | Somewhat mobile, sometimes decurved | 0 | 10 | 10 ♂, ♀ | Many rows, simple | 4 | Coalescent | 1, 2, 3, 4 | 1, 2, 3, 4 |
| Eurycididae | Mobile, stalked, deflexed | Scape 2–jointed, chelae rudimentary | 10 | 10 ♂, ♀ | More than one row, serrate | 4 | Well segmented | 3, 4 | 1, 2, 3, 4 |
| Hannonia | „ | Rudimentary | 0 | 10 ♂, ♀ | Scattered spines | 4 | „ | „ | „ |
| Ammotheidae | Mobile, deflexed | „ | 4–9 | 10 (or less) ♂, ♀ | Few, scattered, serrate or smooth | 4 | Condensed, segmented | „ | „ |
| ? Rhynchothoracidae | Large, fixed, aberrant | 0 | 8 (5) | 10 ♂, ♀ | Toothed tubercles | 4 | „ | 4 | 4 |
| (Euchelata, Sars)— | |||||||||
| Nymphonidae | Large, fixed | Large, scape 1–jointed | 5 (7) | 8–10 ♂, ♀ | One row, serrate | 4–5 | Well segmented | 2, 3, 4 (5) | 1, 2, 3, 4 (5) |
| Pallenidae | „ | „ | 0 or rudimentary | 10 ♂, ♀ | „ | 4 | „ | (1, 2), 3, 4 | „ |
| Phoxichilidiidae | „ | „ | 0 | 5–6 ♂ | One row, simple | 4 | „ | 1, 2, 3, 4 | „ |
| (Achelata, Sars)— | |||||||||
| Phoxichilidae | Large, fixed | 0 | 0 | 7 ♂ | Scattered, simple | 4 | Well segmented | 2, 3, 4 | 1, 2, 3, 4 |
| Pycnogonidae | „ | 0 | 0 | 9 ♂ | Small, irregular | 4 | Segmented, condensed | 4 | 4 |
CLASS PYCNOGONIDA.[[429]]
Marine Arthropoda, with typically seven (and very exceptionally eight) pairs of appendages, of which none have their basal joints subservient to mastication, the first three are subject to suppression, the first (when present) are chelate, the second palpiform, the third ovigerous, and the rest form ambulatory limbs, usually very slender and long; with a suctorial proboscis, a limbless, unsegmented abdomen, and no manifest respiratory organs.
Fig. [282].—Decolopoda australis, Eights. A, × 1: from a specimen obtained at the South Shetlands by the Scotia Expedition. B, First appendage, or chelophore. (A, original; B, after Hodgson.)
Fam. 1. Decolopodidae.—Appendage I. dwarfed, but complete and chelate, scape with two joints; II. 9–10–jointed; III. well developed in both sexes, 10–jointed, the terminal joints with about four rows of teeth; five pairs of legs, destitute of accessory claws; genital apertures on all the legs (Bouvier).
Decolopoda australis, Eights[[430]] (1834), a remarkable form from the South Shetlands, recently re-discovered by the Scotia expedition. The animal is large, seven inches or more in total span, in colour scarlet; it was found in abundance in shallow water and cast upon the shore. The body is greatly condensed, the proboscis is “clavate, arcuated downwards,” and beset with small spines. A second Antarctic species, D. antarctica, has been described by Bouvier. The presence of a fifth pair of legs distinguishes Decolopoda from all known Pycnogons, except Pentanymphon. Stebbing would ally Decolopoda with, or even include it in, the Nymphonidae; but the presence of a second joint in the chelophoral scape, the number of joints in, and the armature on, the ovigerous legs, and the deflexed proboscis, are all characters either agreeing with or tending towards those of the Eurycididae; while the Colossendeidae would be very like Decolopoda were it not for the complete suppression of the chelophores. It seems convenient to constitute a new family for this remarkable form.
Fam. 2. Colossendeidae (Pasithoidae, Sars).—Appendage I. absent in adult; appendage II. very long, 10–jointed; appendage III. 10–jointed, clawed, with many rows of teeth; auxiliary claws absent; segments of trunk fused; proboscis very large, somewhat mobile; genital apertures, in at least some cases, on all the legs.