Exopodite

“ It is known from flattened specimens that Neolenus, Kootenia, Ptychoparia, Triarthrus, and Cryptolithus all have a single type of exopodite, consisting of a simple setiferous shaft. All these genera have been examined in a way that permits no doubt about the structure, and no trace of spiral arms has been detected. On the other hand, Walcott found spiral arms in three unrelated genera, Calymene, Ceraurus, and Acidaspis, all of the trilobites in which he found exopodites by the method of sectioning. ”

“ The coxopodite (cxp) and the basipodite (bp) are broad thin plates with setose cutting edges, while the endopodite (en) is short and only two-jointed, and the undivided portion of the exopodite (ex) is very long. The place of {167} the podobranchia is taken by a broad soft membranous plate entirely devoid of branchial filaments (ep) . Thus, in the series of the thoracic limbs, on passing forwards from the third maxillipede, we find that though the plan of the appendages remains the same; (1) the protopodite increases in relative size; (2) the endopodite diminishes; (3) the exopodite increases ”

“ The position of the exopodites in life seems to have been rather uniform in all the genera now known. I have set forth on a previous page my reasons for thinking that they took little part in swimming, and I look upon them as being, in effect, leaf-gills. It seems probable that in all genera the exopodites were held rather close to the test, the shaft more or less rigid, the filamentous setæ gracefully pendent, but pendent as a sheet and not individually, there having been some method by which adjoining setæ were connected laterally. ”

“ Every movement of one affected the other, and the motion of the endopodites in either swimming or crawling produced a movement of the exopodites which helped to keep up a circulation of water, thus insuring a constant supply of oxygen.
Although Neolenus is usually accounted a less primitive form than Ptychoparia or Triarthrus, it has much the most primitive type of exopodite yet known. It would appear that the exopodites were originally broad, thin, simple lamellæ, which became broken up, on the posterior side, into fine cylindrical setæ. ”

“ The first two of the appendages on the right are best preserved, and these are the ones figured. They belong to the second and third segments. The endopodites of each are ahead of the exopodites, and the proximal portion of each exopodite overlies portions of the first two segments (second and third) of the corresponding endopodite. The coxopodites are not visible, but very nearly the full length of the first segment of the endopodite (the basipodite) is exposed. ”

“ In Isopoda the antennæ are practically uniramous sensory organs. The second cephalic appendages of trilobites are capable of such development through reduction of the exopodite.
In the Isopoda the coxopodites are usually fused with the body, remaining as free, movably articulated segments only in a part of the thoracic legs of one suborder, the Asellota. Endobases are entirely absent. This is of course entirely unlike the condition in Trilobita, but a probable modification. ”

“ The biramous appendages of the first 9 segments of the body are formed of an endopodite of four or five broad, short joints and two slender distal joints. Each joint has one or more short, sharp spines curving forward from the outer end, and the terminal joint has two large and one small forward-curving spines not unlike the spines on the terminal joints of the cephalic appendages of Eurypterus. In other specimens the exopodite appears to be in the form of a lobe or lamellæ not unlike the branchial lobes of Pterogotus bilobus as illustrated by Dr. ”

“ It is also found in the earliest and most primitive form of larva, known as the Nauplius. As a rule the protopodite is composed of two segments, though one may be reduced or suppressed and occasionally three may be present. In many cases, one of the branches, generally the endopodite, is more strongly developed than the other. Thus, in the thoracic limbs of the Malacostraca, the endopodite generally forms a walking-leg while the exopodite becomes a swimming-branch or may disappear altogether. ”

“ This specimen shows three cephalic legs. The two anterior are slender (mandible and maxillula) , and the posterior maxilla is large and formed of short strong joints. There are eight pairs of thoracic legs. The distal portions of these are finely shown in figure 1. The broad, setiferous joints of the exopodite are also shown near the carapace. In other specimens they extend out over the legs so as nearly to conceal them. Traces of oval gills (epipodites) are shown for three legs on the outer side of what appears to be the second joint of the leg. ”

“ The narrow abdomen or tail is very flexible, and can be doubled up under the body or stretched out behind; it ends in a forked telson. There are two pairs of swimming limbs, each with endopodite and exopodite, and the short antennules and antennæ are seen on either side of the rostrum. There are a pair of very large compound eyes, which are not set on movable stalks, but are under the front part of the carapace. ”

“ A protopodite is generally formed by the fusion of coxopodite and basipodite, accompanied by a transference of the proximal end of the exopodite to the distal end of the basipodite. A new segment, not known in the trilobites (precoxal) , is sometimes added at the inner end.
Among modern Crustacea, the anterior cephalic appendages and thoracic appendages of the Copepoda and the thoracic appendages of certain Malacostraca, Syncarida especially, are most nearly like those of the trilobite. ”

“ The carapace widens, the fore part of the head is modified so as to bring about the formation of the characteristic metope: and the abdomen, losing more or fewer of its posterior appendages, takes up its final position under the thorax.
In the Zoæa state, those thoracic limbs which give rise to the maxillipedes are provided with well-developed exopodites, and in the free Mysis state all these limbs have exopodites. In the Opossum-shrimps these persist throughout life; in Penæus, the rudiments of them only remain; in the lobster, they disappear altogether. ”

“ The eyes are marginal and pedunculate. The antennules are imperfectly known, but apparently short, while the antennas are long and slender, with relatively few, long, segments. The mandibles appear to be like endopodites of trilobites and show at least six segments. As so often happens in these specimens from British Columbia, [109] the preservation of the other appendages is unsatisfactory. As illustrated (Walcott, 1912 A, pl. 27, fig. 5) , both endopodites and exopodites appear to be present, and the shaft of the exopodite seems to be segmented as in Triarthrus. ”

“ In the young Lobster, when just hatched from the egg, however, each of the legs has a large exopodite like that of the third maxilliped. These exopodites, which are used in swimming, are afterwards lost as the animal grows; but their presence in the young is interesting as confirming the conclusion that the legs, like the maxillipeds, are built on the same plan as the swimmerets. The large claws, and also the first and second pairs of walking legs, end in pincers, or chelæ, the penultimate segment projecting in a thumb-like process against which the last segment works. ”

“ As in other specimens, the endopodites of the thorax turn backward at the distal end of the carpopodite, which is broad and curved, and bears a tuft of spines on the posterior margin. The dactylopodites seem to preserve their natural shape, and are very nearly cylindrical in form. Under the pygidium are several sets of overlapping fringes of setæ of exopodites, and along the edge of the dorsal furrow, a number of fragments of segments of what may be coxopodites while with them are a number of fragmentary shaft of exopodites. ”

“ Carapace strong, small, subquadrangular and with two postero-lateral spines comparable with the lateral lobes of the carapace of Apodidæ. Eyes sessile. Head with five pairs of appendages. Thorax with 24 pairs of appendages. Abdomen a single plate-like telson. Thoracic leg with jointed leg-like endopodite, a jointed setiferous exopodite, and expanded gill-like epipodite.
One genus, Marrella.
Observations.—This family is instituted to include Marrella splendens, a species that, despite its remarkable carapace and cephalic appendages, recalls Apus and Lepidurus. ”

“ These latter bodies Professor Beecher took to be leaf-like exopodites, which they certainly resemble, and as they lie beyond the line of endopodites they probably do belong to the outer halves of the appendages.
The exopodites under the thorax are long, the shaft shows numerous short segments, and is in each case bent backward, though not through a right angle. They extend considerably beyond the endopodites. The setæ do not diverge from the shaft at a right angle as on the dorsal side of this same specimen, but at an acute angle, indicating that they were not rigid. ”

“ The thoracic limbs never have any trace of exopodites. The first pair are maxillipeds, and if they carry an epipodite it is never enclosed in a gill cavity, as in Tanaidacea. The swimmerets form one of the most characteristic features of the Isopoda, for they are always flattened into thin plates, which act as gills. In the Woodlice, which breathe air, certain curious modifications of the swimmerets are found, which will be described in a later chapter. In some Isopods that live as parasites on fish or on other Crustacea, each individual is at first a male, and later becomes a female. ”

“ As expected, there are five pairs on the pygidium. The cephalon is unfortunately so exfoliated that the appendifers there are not preserved. The doublure of the pygidium is extremely narrow.
The cast of the interior shows, rather faintly, the exopodites of the right side of the thorax and of the left side of the cephalon, and, still more faintly, the caudal rami and a few pygidial endopodites. The exopodites on the right side are in what seems to be the customary position, directed obliquely forward and outward, and the tips of their distal lobes project slightly beyond the edge of the test. ”

“ In Cryptolithus the shaft does not seem to be made up of small segments, and is narrow, with a decided backward curve. The setæ are considerably longer and much more flattened than in Triarthrus. In Calymene the state of preservation does not allow a very full knowledge of the exopodites, but they appear to have a slender, unjointed shaft and short and delicate setæ. The coiled branches of the exopodites as described by Walcott seem to me to be only ordinary Triarthrus-like organs, and this, as I understand from Schuchert, was also the view of Beecher. ”

“ In each genus there is a pair of appendages for each segment of the thorax. When the axial lobe is narrow, the endobases of the coxopodites are small and short (Cryptolithus, Ceraurus, Calymene) . When the axial lobe is wide, the endobases are long and stout (Isotelus, Triarthrus) . The exopodites always lie above and in front of the corresponding endopodites. In Triarthrus the two branches are of practically equal length. In Cryptolithus the exopodites are much the longer. In Neolenus, Calymene, Ceraurus, Kootenia, and Ptychoparia, the exopodites are shorter than the endopodites. ”

“ The produced angle is that which is turned downwards when the limb is fully extended (fig. 46) . In the forceps, the great chela is formed in just the same way; the only important difference lies in the fact that, as in the external maxillipede, the basipodite and the ischiopodite are immoveably united. Thus, {170} the limbs of the thorax are all reducible to the same type as those of the abdomen, if we suppose that, in the posterior five pair, the exopodites are suppressed ”

“ Fig. 1.—Appendages of Crustacea (A-G) and Trilobita (H) . A, First maxillipede of Astacus; B, second maxilla of Astacus; C, second walking leg of Astacus; D, thoracic limb of Branchipus; E, first maxillipede of Mysis; F, first maxillipede of Gnathophausia; G, thoracic limb of Nebalia; H, thoracic limb of Triarthrus. bp, basipodite; br, bract; cp, carpopodite; cxp, coxopodite; cx.s, coxopoditic setae; dp, dactylopodite; end, endopodite; ep, epipodite; ex, exopodite; ip, ischiopodite; mp, meropodite; pp, propodite; 1–6, the six endites. ”

“ It is possible to make out remains of eight pairs of appendages on the thorax, some of them represented by coxopodites only, but most with more or less poorly preserved endopodites as well. No exopodites are visible. The coxopodites of the thorax seem to be of the same form as the last one on the cephalon, but slightly less curved. All are long and heavy, and there seems to be no decrease in size toward the pygidium. The endopodites are very imperfectly shown. ”

“ The primitive character of the Schizopods is also indicated by the fact that most of the Decapoda with uniramous limbs on the five hinder thoracic segments pass through what is known as the “Mysis stage” in development, when these limbs are biramous, the exopodites being subsequently lost in most cases. ”

“ In Isopoda there is a distinct grouping of the appendages, with specialization of function. The trilobites show a beginning of tagmata, and such development would be expected if evolution were progressive.
In both groups, development from the embryo is direct. Rudiments of exopodites of thoracic legs have been seen in the young of one genus. ”