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PYCNOGONIDA, or PANTOPODA

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Originally appearing in Volume V22, Page 677 of the 1911 Encyclopedia Britannica.
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PYCNOGONIDA, or PANTOPODA , marine See also:Arachnida (q.v.) remarkable for the reduction of the opisthosoma or See also:abdomen to an insignificant tubercular or See also:rod-like See also:process (whence their trivial name of " nobody crabs "), and for the development of the oral region into a relatively immense suctorial See also:proboscis. They See also:form a compact See also: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 See also:ambulatory appendages are coalesced to form a single See also:nervous See also:mass, whereas in the Pycnogonida the ganglia supplying these limbs retain their See also:original distinctness. More important still is the circumstance that in the Pycnogonida there may be as many as seven pairs of See also: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 See also:constant See also:character, the number in the Pycnogonida is very variable. In most cases there are four pairs of ambulatory limbs, but in two See also:antarctic genera, namely Pentanymphon, belonging to the See also:family Nymphonidae and Decalopoda, probably belonging to the Colossendeidoe, theyare increased to five pairs. In front of these four or five pairs of ambulatory limbs there may be two pairs of longish See also: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 See also:developed, three-jointed and chelate, or reduced in See also:size and complexity, or altogether suppressed. FIG. 1.—Male of Pycnogonum littorale, See also:Muller. a, Parts of mouth forming a c, c, Thoracic segments. See also:beak.

d, Rudimentary abdomen. b, Cephalic See also:

area. e, Eyes. 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 See also:complement of eight pairs of appendages, and the See also:female of Pycnogonum littorals, in which all the appendages are aborted See also:save four pairs of ambulatory limbs. All the See also:principal See also:organs of the See also:body are concentrated in that See also:part which bears the appendages. The generative glands are lodged on each See also:side, sending prolongations into the appendages, and their ducts open upon the second segments of more or fewer of them. The alimentary See also: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 See also:long saccular prolongations into the limbs. See also:Food is imbibed by means of the suctorial pharynx lodgec in the proboscis, the sucking See also:action being effected by means of muscles radiating from the See also:wall of the pharynx to that of the inner See also:surface of the exoskeleton of the proboscis. The circulatory See also:system, where it has been observed, consists of a See also:heart formed of about three See also:chambers communicating with each other. In each chamber there is a pair of orifices for the entry of the See also: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 See also:medium for the oxygenation of the blood. The sexes are distinct, but commonly there is little See also:external difference between the See also:males and the See also: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 See also:

case in the male. Finally, in the females the generative orifices are much more conspicuous than in the males, and the See also:fourth See also:joint of the legs is often swollen. The invariable presence of the ovigerous appendages in the males is correlated with the See also:habit practised by this See also:sex of carrying the fecundated eggs. The eggs are usually aggregated in two spherical masses See also:round the See also:middle of each of the ovigerous legs; sometimes, however, there are two such masses on d each leg, or as many as four or five, whereas occasionally there is but one on the right or See also:left side. More rarely, as in some See also:species cf 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 See also:young frequently differs greatly from the adult. The body, which is See also: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 See also:simple and small, and are generally held to be the palpi and ovigerous legs. This first larval See also:stage, sometimes called the protonymphon, may be See also:free living or may be retained within the See also:egg-See also: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 See also: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 in-crease 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 ex-tended 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 See also:

British species is Pycnogonum littorale, a stoutly built form with only four pairs of appendages in the female. It occurs between See also:tide marks on British coasts, but recedes to considerable depths, and on the See also:Atlantic See also:coast of See also:America has been dredged at a See also:depth of 430 fathoms. It is also wide-ranging, and has been recorded even from the coast of See also:Chile. As a rule, but by no means an invariable rule, deep-See also: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 See also:

sea or amongst the fronds of seaweed, and they have been met with in polar, temperate and tropical seas. (R. I.

End of Article: PYCNOGONIDA, or PANTOPODA

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