1911 Encyclopædia Britannica/Pycnogonida

by Reginald Innes Pocock

PYCNOGONIDA, or Pantopoda, marine Arachnida (q.v.) remarkable for the reduction of the opisthosoma or abdomen to an insignificant tubercular or rod-like process (whence their trivial name of “nobody crabs”), and for the development of the oral region into a relatively immense suctorial proboscis. They form a compact group, differing from all the other orders of Arachnida in certain structural characters of such morphological importance that it is impossible to affiliate them closely with any group of that class. For instance, in all typical existing Arachnida the ganglionic centres which innervate the ambulatory appendages are coalesced to form a single nervous mass, whereas in the Pycnogonida the ganglia supplying these limbs retain their original distinctness. More important still is the circumstance that in the Pycnogonida there may be as many as seven pairs of leg-like limbs behind the mouth; but in the typical Arachnida there are never more than five such pairs. Curiously enough, too, although the number of these appendages, in all the orders of typical Arachnida is, with the exception of some degenerate Acari, a quite constant character, the number in the Pycnogonida is very variable. In most cases there are four pairs of ambulatory limbs, but in two antarctic genera, namely Pentanymphon, belonging to the family Nymphonidae and Decalopoda, probably belonging to the Colossendeidoe, they are increased to five pairs. In front of these four or five pairs of ambulatory limbs there may be two pairs of longish post-oral limbs, called respectively the ovigerous legs and the palpi; but these may be totally absent. Finally, the single pair of pre-oral appendages may be well developed, three-jointed and chelate, or reduced in size and complexity, or altogether suppressed. As examples of this class exhibiting extremes of variation in the development and reduction of the appendages may be cited Decalopoda, which has the full complement of eight pairs of appendages, and the female of Pycnogonum littorale, in which all the appendages are aborted save four pairs of ambulatory limbs.

Fig. 1.—Male of Pycnogonum littorale, Müller.

a,	Parts of mouth forming a beak.
b,	Cephalic area.
c, c,	Thoracic segments.
d,	Rudimentary abdomen.
e,	Eyes.

Fig. 2.—The same; under size.

a, a, Ovigerous legs.

All the principal organs of the body are concentrated in that part which bears the appendages. The generative glands are lodged on each side, sending prolongations into the appendages, and their ducts open upon the second segments of more or fewer of them. The alimentary canal, beginning with the mouth at the extremity of the proboscis and terminating with the anus at the extremity of the tail-like opisthosoma, also sends long saccular prolongations into the limbs. Food is imbibed by means of the suctorial pharynx lodged in the proboscis, the sucking action being effected by means of muscles radiating from the wall of the pharynx to that of the inner surface of the exoskeleton of the proboscis. The circulatory system, where it has been observed, consists of a heart formed of about three chambers communicating with each other. In each chamber there is a pair of orifices for the entry of the blood; and the fluid is expelled through an orifice at the anterior extremity of the first chamber. No organs of respiration are known, the integument being the medium for the oxygenation of the blood.

The sexes are distinct, but commonly there is little external difference between the males and the females. Sometimes the female is considerably the larger of the two; and frequently the ovigerous legs are less well developed than in the male. Sometimes indeed these limbs are entirely wanting in the female, whereas this is never the case in the male. Finally, in the females the generative orifices are much more conspicuous than in the males, and the fourth joint of the legs is often swollen. The invariable presence of the ovigerous appendages in the males is correlated with the habit practised by this sex of carrying the fecundated eggs. The eggs are usually aggregated in two spherical masses round the middle of each of the ovigerous legs; sometimes, however, there are two such masses on each leg, or as many as four or five, whereas occasionally there is but one on the right or left side. More rarely, as in some species of Pallene, there are few very large eggs attached separately to the legs, or the eggs may be carried in a single mass attached to the underside of the body, as in some species of Pycnogonum. Cases have been recorded of the females carrying their own eggs, as has been observed in a specimen of Nymphon brevicaudatum, but this seems to be a rare phenomenon.

The newly-hatched young frequently differs greatly from the adult. The body, which is oval, subquadrate and unsegmented, has the proboscis well developed, but bears only three pairs of appendages; those of the first pair are large, three jointed and chelate, the basal segment containing a large so-called byssus gland, the duct of which opens at the tip of a spiniform or setiform process; these appendages are the mandibles of the adult. The appendages of the next two pairs are simple and small, and are generally held to be the palpi and ovigerous legs. This first larval stage, sometimes called the protonymphon, may be free living or may be retained within the egg-shell. In the second stage, which may also be contained in the egg, two or three of the remaining pairs of appendages have appeared, those representing the first pair of ambulatory limbs of the adult being as a rule better developed than the next. In the third stage the fourth pair of ambulatory limbs and the abdomen of the adult have begun to develop and gradually increase in size until the adult form is attained. But even within the limits of a single genus, e.g. Nymphon, the stage at which the young emerges from the egg is subject to considerable specific variation.

Pycnogonida vary greatly in size, the span of legs when extended ranging from about 2 in. in Pycnogonum littorale to 2 ft. in Colossendeis gigas. They are wholly marine and occur at depths varying from only a few fathoms to over 2500 fathoms. One of the best known British species is Pycnogonum littorale, a stoutly built form with only four pairs of appendages in the female. It occurs between tide marks on British coasts, but recedes to considerable depths, and on the Atlantic coast of America has been dredged at a depth of 430 fathoms. It is also wide-ranging, and has been recorded even from the coast of Chile. As a rule, but by no means an invariable rule, deep-water species have smoother bodies and much longer and thinner legs than shallow-water forms. The latter also commonly have four distinct eyes, whereas the former met with at a depth of over 400 fathoms not uncommonly have the eyes obsolete. There are many exceptions, however, to these rules. The habits of all Pycnogonida appear to be very similar. They are not swimmers, but crawl slowly over the bottom of the sea or amongst the fronds of seaweed, and they have been met with in polar, temperate and tropical seas.  (R. I. P.)