PYCNOGONIDA

BY

D’ARCY W. THOMPSON, C.B., M.A. Trinity College

Professor of Natural History in University College, Dundee

CHAPTER XXI
PYCNOGONIDA[[393]]

Remote, so far as we at present see, from all other Arthropods, while yet manifesting the most patent features of the Arthropod type, the Pycnogons constitute a little group, easily recognised and characterised, abundant and omnipresent in the sea. The student of the foreshore finds few species and seldom many individuals, but the dredger in deep waters meets at times with prodigious numbers, lending a character to the fauna over great areas.

Fig. [262].—Pycnogonum littorale, Ström, × 2.

The commonest of our native species, or that at least which we find the oftenest, is Pycnogonum littorale (Phalangium littorale, Ström, 1762). We find it under stones near low water, or often clinging louse-like to a large Anemone. The squat segmented trunk carries, on four pairs of strong lateral processes, as many legs, long, robust, eight-jointed, furnished each with a sharp terminal claw. In front the trunk bears a long, stout, tubular proboscis, at the apex of which is the mouth, suctorial, devoid of jaws; the body terminates in a narrow, limbless, unsegmented process, the so-called “abdomen,” at the end of which is the anal orifice. The body-ring to which is attached the first pair of legs, bears a tubercle carrying four eye-spots; and below, it carries, in the male sex, a pair of small limbs, whose function is to grasp and hold the eggs, of which the male animal assumes the burden, carrying them beneath his body in a flattened coherent mass. In either sex a pair of sexual apertures open on the second joints of the last pair of legs. The integument of body and limbs is very strongly chitinised, brown in colour, and raised into strong bosses or tubercles along the middle line of the back, over the lateral processes, and from joint to joint of the limbs. The whole animal has a singular likeness to the Whale-louse, Cyamus mysticeti (well described by Fr. Martins in 1675), that clings to the skin of the Greenland Whale as does Pycnogonum to the Anemone, a resemblance close enough to mislead some of the older naturalists, and so close that Linnaeus, though in no way misled thereby, named it Phalangium balaenarum. The substance of the above account, and the perplexity attending the classification of the animal, are all included in Linnaeus’s short description:[[394]] “Simillimus Onisco Ceti, sed pedes omnes pluribus articulis, omnes perfecti, nec plures quam octo. Dorsum rubrum, pluribus segmentis; singulis tribus mucronibus. Cauda cylindrica, brevissima, truncata. Rostrum membranaceum, subsubulatum, longitudine pedum. Genus dubium, facie Onisci ceti; rostro a reliquis diversum. Cum solo rostro absque maxillis sit forte aptius Acaris aut proprio generi subjiciendum.... Habitat in mari norvegico sub lapidibus.”[[395]]