The Pycnogons, or some of them, can swim by “treading water,” and Pallene is said by Cole to swim especially well; they more often progress half by swimming, half by kicking on the bottom. They move promptly towards the light, unless they have Hydroids to cling to, and Cole points out that when they crawl with all their legs on the bottom they move forwards towards the light,[[421]] but backwards when they swim in part or whole. The legs move mostly in a vertical plane, horizontal movements taking place chiefly between the first and second joints. Tanystylum is uncommonly sluggish and inert; it sinks to the bottom, draws its legs over its back and remains quiet, while Pallene, by vigorous kicks, remains suspended.
The long legs of the Pycnogons are easily injured or lost, and easily repaired or regenerated. This observation, often repeated, is as old as Fabricius: “Mutilatur etiam in libertate sua, redintegrandum tamen; vidi enim in quo pedes brevissimi juxta longiores enascentes, velut in asteriis cancris aliisque redintegratis.” In such cases of redintegration of a leg, the reproductive organ, the genital orifice, and the cement-gland are not restored until the next moult.[[422]]
Systematic Position.—To bring this little group into closer accord with one or other of the greater groups of Arthropods is a problem seemingly simple but really full of difficulty.
The larval Pycnogon, with its three pairs of appendages, resembles the Crustacean Nauplius in no single feature save this unimportant numerical coincidence; nor is there any significance in the apparent outward resemblance to isolated forms (e.g. Cyamus) that induced some of the older writers, from Fabricius downwards and including Kröyer and the elder Milne-Edwards, to connect the Pycnogons with the Crustacea. To refer them, or to approximate them to the Arachnids, has been a stronger and a more lasting tendency.[[423]] Linnaeus (1767) included the two species of which he was cognisant in the genus Phalangium, together with P. opilio. Lamarck, who first formulated the group Arachnida (1802), let it embrace the Pycnogons; and Latreille (1804, 1810), who immediately followed him, defined more clearly the Pycnogonida as a subdivision of the greater group, side by side with the subdivision that corresponds to our modern Arachnida (“Arachnides acères”), and together with a medley of lower Crustacea, Myriapoda, Thysanura, and Parasitic Insects; he was so cautious as to add “j’observerai seulement, que je ne connais pas encore bien la place naturelle des Pycnogonides et des Parasites,” and Cuvier, setting them in a similar position, adds a similar qualification.[[424]]
Leach (1814), whose great service it was to dissociate the Edriophthalmata and the Myriapoda from the Latreillian medley, left the group Arachnida as we still have it (save for the inclusion of the Dipterous Insect Nycteribia), and divided the group (with the same exception) into four Orders of which the Podosomata, i.e. the Pycnogonida, are one. Savigny (1816), less philosophical in this case than was his wont, assumed the Crustacean type to pass to the Arachnidan by a loss of several anterior pairs of appendages, and appears to set the Pycnogons in an intermediate grade, marking the pathway of the change. He considered the seven pairs of limbs of the Pycnogons to represent thoracic limbs of a Malacostracan, and, like so many of his contemporaries, was much biased by the apparent resemblance of Cyamus to Pycnogonum. The reader may find in Dohrn’s Monograph a guide to many other opinions and judgments, some of them of no small morphological interest and historical value[[425]]; but it behoves us to pass them by, and to inspect, in brief, the case as it stands at present. The obvious features in which a Pycnogon resembles a Spider or other typical Arachnid, are the possession of four pairs of walking legs, and the pre-oral position and chelate form of the first pair of appendages; we may perhaps also add, as a more general feature of resemblance, the imperfect subservience of limbs to the mouth as compared with any of the Crustacea. The resemblance would still be striking, in spite of the presence of an additional pair of legs in a few Pycnogons, were it not for the presence of the third pair of appendages or ovigerous legs of the Pycnogon, whose intercalation spoils the apparent harmony. We are neither at liberty to suppose, with Claus, that these members, so important in the larva, have been interpolated, as it were, anew in the Pycnogon; nor that they have arisen by subdivision of the second pair, as Schimkewitsch is inclined to suppose; nor that they have dropped out of the series in the Arachnid, whose body presents no trace of them in embryo or adult. In a word, their presence precludes us from assuming a direct homology between the apparently similar limbs of the two groups,[[426]] and at best leaves it only open to us to compare the last legs of the Pycnogon with the first abdominal, or genital, appendages of the Scorpion and the Spider. On the other hand, if we admit the seventh (as we must admit the occasional eighth) pair of appendages of Pycnogons to be unrepresented in the prosoma of the Arachnids, then, in the cephalothorax of the former, with its four pairs of appendages, we may find the homologue of the more or less free and separate part of the cephalothorax in Koenenia, Galeodes, and the Tartaridae. There is a resemblance between the two groups in the presence of intestinal diverticula that run towards or into the limbs, as in Spiders and some Mites, and there are certain histological and embryological resemblances that have been in part referred to above; but these, such as they are, are not adequate guides to morphological classification. We must bear in mind that such resemblances as the Pycnogons seem to show are not with the lower Arachnids but with the higher; they are either degenerates from very advanced and specialised Arachnida, or they are lower than the lowest. Confronted with such an issue, we cannot but conclude to let the Pycnogons stand apart, an independent group of Arthropods[[427]]; and I am inclined to think that they conserve primitive features in the usual presence of generative apertures on several pairs of limbs, and probably also in the non-development of any special respiratory organs. But inasmuch as the weight of evidence goes to show that subservience of limbs to mouth is a primitive Arthropodan character, the fact that the basal elements of the anterior appendages have here (as in Koenenia) no such relation to the mouth must be taken as evidence, not of antiquity, but of specialisation. In like manner the suctorial proboscis cannot be deemed a primitive character, and the much reduced abdomen also is obviously secondary and not primitive.
Classification.—No single genus more than another shows signs of affinity with other groups, and no single organ gives us, within the group, a clear picture of advancing stages of complexity. On the contrary, the differences between one genus and another depend very much on degrees of degeneration of the anterior appendages, and we have no reason to suppose that these stages of degeneration form a single continuous series, but have rather reason to believe that degeneration has set in independently in various ways and at various points in the series. But while we are unable at present to form a natural classification[[428]] of the Pycnogons, yet at the same time a purely arbitrary or artificial classification, conveniently based on the presence or absence of certain limbs, would run counter to such natural relationships as we can already discern.
The classification here adopted is a compromise between a natural system, so far as we can detect it, and an artificial one.
Two forms, separated from one another by many differences, show a minimum of degeneration, namely Decolopoda on the one hand, and the Nymphonidae on the other. The former genus has five pairs of legs, and this peculiarity is shared by Pentanymphon. In both groups the three anterior limbs are all present and well formed, save only that the ovigerous legs, which have ten joints in Decolopoda, are reduced to five joints in the Nymphons, and their denticulate spines, of which several rows are present in the former, are reduced to one row in the latter; on the other hand, a greater or a less degeneration of these limbs marks each and all of the other families.
Decolopoda is very probably the most primitive form known, though it has characters which seem to be the reverse of primitive in the dwarfish size of its chelophores and the crowded coalescent segmentation of the trunk. Colossendeis, in spite of its vanished chelophores, is probably closely allied: the shape and segmentation of the body and the several rows of smooth denticles on the ovigerous legs are points in common. The Eurycydidae are closely allied to Colossendeidae; they agree with Decolopoda in the two-jointed scape of the chelophore, and with Ammotheidae in the deflexed mobile proboscis. The true position of Rhynchothorax is very doubtful.
The Nymphonidae and Pallenidae are closely allied, and the Phoxichilidiidae have points of resemblance, especially with the latter. Nymphon compares with Decolopoda in the completeness of its parts, and is more typical in its long well-segmented body, and in its highly-developed chelae; but it already shows reduction in the scape of the chelophore, in the palps, and in the armature of the ovigerous legs.