PYCNOGONIDA, also known as Pantopoda or Podoso mata, a small group of marine Arthropoda characterized by a very small body and disproportionately long legs. The facts that there are usually four pairs of legs and, typically, a pair of chelate or pincer-like appendages in front of the mouth, have led to the Pycnogonida being associated with the Arachnida in most sys tems of classification, but the relationship is not close.
The body is composed of an unsegmented anterior head region followed by three, rarely four, somites which may be free or more or less coalesced, and a minute terminal piece which bears the anus. The legs, as a rule, are exactly alike, and are attached, the first pair to the head region and the others to the following three (or four) somites. The triangular mouth-opening is at the end of a proboscis which may be as long or longer than the rest of the body. In the more typical genera, such as Nymphon, the head-region carries, in front of the first legs, three pairs of ap pendages, the chelate chelophores above and in front of the pro boscis, followed by a pair of sensory palps and a pair of leg-like ovigers which get their name from the fact that they are nearly always used, in the male, to carry the eggs received from the female. Some or all of the appendages of the first three pairs may be reduced or absent in the adult, although they are probably always present in the young.
The internal structure is simple, with some peculiarities due to the reduced size of the body compared with the legs. The pro boscis contains a auctorial pharynx, but the greater part of the digestive cavity is formed by the immensely long diverticula which extend from the alimentary canal into each of the legs.
There is a tubular dorsal heart with two or three pairs of lateral ostia but no definite blood-vessels. The nervous system consists of a supra-oesophageal ganglion or "brain" and a ventral chain of six to eight ganglia. The eyes, when present, are four in number, simple in structure, and elevated on a tubercle on the dorsal sur face of the head. The gonads lie, to begin with, in the body, but send long diverticula into the legs, and the ripening eggs are lodged in the swollen fourth segments of these appendages. The genital apertures are on the second segments of the legs.
It is very characteristic of the Pycnogonida that the males take charge of the eggs as they are extruded by the females and carry them, cemented together in packets, on the ovigers. In a few genera, however, the egg-packets have never been seen and the mode of oviposition is unknown.
Not much is known of the habits of Pycnogonida. Most species are found clambering in ungainly fashion among the branches of hydroids or other coelenterates. Some are known to swim by beating the water with their long legs. Many species, perhaps all, feed upon the soft tissues of coelenterates which they suck into the mouth by dilating the pharynx. The distribution of the Pycno gonida extendg to all seas, and in arctic and antarctic waters they sometimes occur in vast numbers. The affinities of the Pycno gonida are very obscure. (W. T. C.) PYCNOSTYLE, the architectural term given by Vitruvius to the inter-columniation (q.v.) of a temple, when the distance between these columns was equal to times their diameter.