In Pycnogonum, Phoxichilus, Phoxichilidium, and their immediate allies they are absent in the female; in all the rest they are alike present in both sexes, though often somewhat smaller in the female than in the male. They are always turned towards the lower side of the body, and in many cases even their point of origin is wholly ventral. The number of joints varies: in Phoxichilidium five, Anoplodactylus six, Phoxichilus seven; in Paranymphon eight; in Pycnogonum nine, with, in addition, a terminal claw; in the Ammotheidae from seven (Trygaeus) to ten, without a claw; in Pallenidae ten, with or without a claw; in Rhynchothorax, Colossendeis, Eurycide, Ascorhynchus, Nymphon, ten and a claw. The appendage, especially when long, is apt to be wound towards its extremity into a spiral, and its last four joints usually possess a peculiar armature. In Rhynchothorax this takes the form of a stout toothed tubercle on each joint; in Colossendeis of several rows of small imbricated denticles; in Nymphon and Pallene of a single row of curious serrate and pointed spines, each set in a little membranous socket.
Fig. [273].—Nymphon strömii, Kr. Male carrying egg-masses on his ovigerous legs.
Fig. [274].—Terminal joints (tarsus and propodus) of legs. 1, Chaetonymphon hirtum, Fabr.; 2, N. strömii, Kr.; 3, Nymphon brevirostre, Hodge; 4, Ammothea echinata, Hodge; 5, Ascorhynchus abyssi, G.O.S. (All after Sars.)
Legs.—The four pairs of ambulatory legs are composed, in all cases without exception, of eight joints if we exclude, or nine if we include, the terminal claw. They vary from a length about equal to that of the body (Pycnogonum, Rhynchothorax, Ammothea) to six or seven times as much, perhaps more, in Nymphon and Colossendeis, the fourth, fifth, and sixth joints being those that suffer the greatest elongation. The seventh joint, or tarsus, is usually short, but in some Nymphonidae is much elongated; the eighth, or propodus, is usually somewhat curved, and usually possesses a special armature of simple or serrate spines. The auxiliary claws, sometimes large, sometimes small, lie at the base of the terminal claw in Ammotheidae, Phoxichilidae, in Phoxichilidium, in most Pallenidae, in nearly all Nymphonidae. Their presence or absence is often used as a generic character, helping to separate, e.g., Pallene from Pseudopallene and Pallenopsis, and Phoxichilidium from Anoplodactylus; nevertheless they may often be detected in a rudimentary state when apparently absent. The legs are smooth or hirsute as the body may happen to be.
Fig. [275].—Legs of A, Pallene brevirostris, Johnston; B, Anoplodactylus petiolatus, Kr.; C, Phoxichilus spinosus, Mont.; D, Colossendeis proboscidea, Sabine; E, Ammothea echinata, Hodge, ♂.
Fig. [276].—Boreonymphon robustum, Bell. Male with young, slightly enlarged. Faeroe Channel.