1902 Encyclopedia > Arachnida > Arachnida - Order II: Pycnogonidea - External & Internal Structure; General Observations

(Part 8)




The creatures of this order form a small group which it appears now necessary to receive among the Aracnida, though their true position has been hitherto (and to some extent still is) a matter of much difference of opinion among naturalists. They seem to connect the more typical Arachnida with the Crustacea, and also to form a passage from the Acaridea to the Phalangidea. It appears, however, when their peculiar structure, both external and internal, is considered, impossible to include them within either of those orders; it is therefore proposed here to constitute them a separate order between the acarids and phalangids. The characters of the order are --

EXTERNAL STRUCTURE. - Body, linear, composed of four segments; caput, tubular, in form of a beak or proboscis, and either simple, or ending with palpi and mouth organs; abdomen, rudimentary; legs, eight (each thoracic segment bearing a pair), very long, and consisting of eight to nine joints, terminating with a claw or claws. In the immature (so-called larval) state the legs are four only in number. The females have a supernumerary pair between those of the first pair, used for carrying the ova. Palpi, when present, filiform, and composed of five to nine joints terminating with a claw. Eyes, four.

With regard to their interval structure, the Pycnogonidea have a dorsal vessel (circulatory apparatus) divided into chambers, a stomach sending forth caecal prolongations into the legs and palpi, an abdominal intestine with dlated cloaca, and a nervous system, consisting of a cephalic and ophthalmic ganglia, closely united together and joined by the oesophagian fillet to the abdominal ganglia, which are four in number, or large size, sessile, and emitting from their lateral extremities nerves into the legs (Cuvier;s regn. An., edition cited post, pl. 22), but no organs of respiration (Huxley, l.c. and Cuvier's Regn. An., l.c.)

This order contains but one family, Pycnogonides, divided into thea. Some of the species are parasitic on cetaceous animals, others are found secreted among seaweed along the sea-shore, and feed on small marine animals. Some are British, and others exotic.

Fig. 7-- Pycnogonum littorale, Müller. a, parts of mouth, forming a beak; b, caput; c...c, thoracic segments; d, rudimentary abdomen; e, eyes.

Fig. 8 -- The same; under side. a...a, supernumerary pair of legs.

The genera may be thus shortly characterized.

Genus Pycnogonum (Brunnich). - Body short and thick; legs, short, strong; without either falces or palpi. The species of this genus are parasitic on the cetacea. Figs. 7, 8, P. littorale (Muller).

Genus Phoxichilus (Latr.) - Body narrow; legs of great length; falces present, but no palpi.

Fig. 7-- Nyphon coccineum, Johnston; under side. a, head; b, palpi.

Genus Nymphon (Fabr.) - Resembles Phoxichilus in the narrow body and long legs, which are also slender, as well as in having falces; but in the present genus there are also two palpi, each of five joints. Fig. 9 Nymphon coccineum (Johnston).

Genus Ammothea (Leach). Body short and rather broad; legs long; beak of great size and length; much longer than the falces, which are short. Palpi 9-jointed, the third joint much the longest; terminal tarsal claws double, and of unequal size. Supernumerary legs 9-jointed, and inserted under the first pair of legs behind the beak.

The latest authority in regard to their systematic position, i.e. whether Crustacean or Arachnidan, is Dr Anton Dohrn, who says in a memoir, "Ueber Entwicklung und Bau der Pycnogoniden" (contained in a publication, entitled Untersuchungen uber Bau und Entwicklung der Arthropoden, Leipsic, Engleman, 1870): "Die Pycnogoniden sind weder Arachniden noch Crustaceen." Dr Dohrn enters fully into the development and structure of Pycnogonum littorale, and on a species of another genus formed out of Phoxichillus, Achelia loevis. The present writer has not, however, had an opportunity of studying this memoir.

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