Fig. [263].—Dorsal view of Nymphon brevirostre, Hodge, × 6. Britain.
The common Pycnogonum is, by reason of the suppression of certain limbs, rather an outlying member than a typical representative of the Order, whose common characters are more strikingly and more perfectly shown in species, for instance, of Nymphon. Of this multiform genus we have many British species, some of the smaller being common below tide-marks, creeping among weeds or clinging like Caprellae with skeleton limbs to the branches of Zoophytes, where their slender forms are not easily seen. In contrast to the stouter body and limbs of Pycnogonum, the whole fabric of Nymphon tends to elongation; the body is drawn out so that the successive lateral processes stand far apart, and a slender neck intervenes between the oculiferous tubercle and the proboscis; the legs are produced to an amazing length and an extreme degree of attenuation: “mirum tam parvum corpus regere tam magnos pedes,” says Linnaeus. Above the base of the proboscis are a pair of three-jointed appendages, the two terminal joints of which compose a forcipate claw; below and behind these come a pair of delicate, palp-like limbs of five joints; and lastly, on the ventral side, some little way behind these, we find the ovigerous legs that we have already seen in the male Pycnogonum, but which are present in both sexes in the case of Nymphon. At the base of the claw which terminates each of the eight long ambulatory legs stands a pair of smaller accessory or “auxiliary” claws. The generative orifices are on the second joint of the legs as in Pycnogonum, but as a rule they are present on all the eight legs in the female sex, and on the two hindmost pairs in the male. One of the Antarctic Nymphonidae (Pentanymphon) and one other Antarctic genus less closely related (Decolopoda) have an extra pair of legs. No other Pycnogon, save these, exhibits a greater number of appendages than Nymphon nor a less number than Pycnogonum, nor are any other conspicuous organs to be discovered in other genera that are not represented in these two: within so narrow limits lie the varying characters of the group.
Fig. [264].—Nymphon brevirostre, Hodge. Head, from below, showing chelophores, palps, and ovigerous leg.
In framing a terminology for the parts and members of the body, we encounter an initial difficulty due to the ease with which terms seem applicable, that are used of more or less analogous parts in the Insect or the Crustacean, without warrant of homology. Thus the first two pairs of appendages in Nymphon have been commonly called, since Latreille’s time, the mandibles and the palps (Linnaeus had called them the palps and the antennae), though the comparison that Latreille intended to denote is long abandoned; or, by those who leaned, with Kröyer and Milne-Edwards, to the Crustacean analogy, mandibles and maxillae. Dohrn eludes the difficulty by denominating the appendages by simple numbers, I., II., III., ... VII., and this method has its own advantages; but it is better to frame, as Sars has done, a new nomenclature. With him we shall speak of the Pycnogon’s body as constituted of a trunk, whose first (composite) segment is the cephalic segment or head, better perhaps the cephalothorax, and which terminates in a caudal segment or abdomen; the “head” bears the proboscis, the first appendages or “chelophores,” the second or “palps,” the third, the false or “ovigerous” legs, and the first of the four pairs of “ambulatory” legs. The chelophores bear their chela, or “hand,” on a stalk or scape; the ambulatory legs are constituted of three coxal joints, a femur, two tibial joints, a tarsus, and a propodus, with its claws, and with or without auxiliary claws.
The Body.—The trunk with its lateral processes may be still more compact than in Pycnogonum, still more attenuated than in Nymphon.
In a few forms (e.g. Pallene, Ammothea, Tanystylum, Colossendeis) the last two, or even more, segments of the trunk are more or less coalescent. In Rhynchothorax the cephalic segment is produced into a sharp-pointed rostrum that juts forward over the base of the proboscis. The whole body and limbs may be smooth, tuberculated, furnished with scattered hairs, or sometimes densely hispid.
Fig. [265].—A, Colossendeis proboscidea, Sabine, Britain; B, Ammothea echinata, Hodge, Britain; C, Phoxichilus spinosus, Mont., Arctic Ocean. (The legs omitted.)
The proboscis varies much in shape and size. It may be much longer or much shorter than the body, cylindrical or tumid, blunt or pointed, straight or (e.g. Decolopoda) decurved; usually firmly affixed to the head and pointing straight forwards; sometimes (Eurycide, Ascorhynchus) articulated on a mobile stalk and borne deflexed beneath the body.