Isopoda

“ 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. ”

“ Amphipoda.—As in the genuine Isopoda, the eyes of Amphipoda are always sessile, and generally paired, and, in contrast to crabs and lobsters, these two groups have only four pairs of mouth-organs instead of six, but seven pairs of trunk-legs instead of five. ”

“ In so far as the cornea and its facets or lenses are to be regarded, there is the greatest analogy with the Isopoda. In vertical sections of Sphæroma we have the same sort of elongated, flattened biconvex lenses as in Sphærophthalmus and others. -33- Since Grenacher and Exner and others have published their excellent works on the eyes of the Arthropoda, there can be no foundation for speaking of the resemblance of the trilobitic eye with that of Limulus, as this genus stands completely isolated amongst all Arthropoda in that respect. ”

“ 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. ”

“ The whole organ can be rotated by special muscles. Organs of this type are best known in the Euphausiidae among the Schizopoda, but a modified form is found in some of the lower Decapods.
Reproductive System.—In the great majority of Crustacea the sexes are separate. Apart from certain doubtful and possibly abnormal instances among Phyllopoda and Amphipoda, the only exceptions are the sessile Cirripedia and some parasitic Isopoda (Cymothoidae) , where hermaphroditism is the rule. ”

“ The cephalic appendages are specialized, and the antennules double as in most of the Malacostraca. External epipodites are very numerous on the anterior limbs.
This group extends back as far as the Pennsylvanian and had then probably already become adapted to fresh-water life. It may be significant that the Palæozoic syncarids appear to have lacked epipodites. While differing very considerably from the Trilobita, the Syncarida could have been derived from them.
3. Isopoda. ”

“ Throughout the whole, the researches made since 1860 have not only added a great throng of new species, genera and families, but have thrown a flood of light upon questions of their phylogeny, systematic arrangement, horizontal and bathymetric distribution, organization, habits of life and economic importance. There are at least seven orders: the stalk-eyed Brachyura, Macrura, Schizopoda, Stomatopoda, and the sessile-eyed Sympoda, Isopoda, Amphipoda. ”

“ The choice was necessarily limited to those the chief forms of which were easily to be obtained alive in some abundance. The Crabs and Macrurous Crustacea, the Stomapoda, the Diastylidæ, the Amphipoda and Isopoda, the Ostracoda and Daphnidæ, the Copepoda and Parasita, the Cirripedes and Rhizocephala of our coast, representing the class of Crustacea with the deficiency only of the Phyllopoda and Xiphosura, furnished a long and varied, and at the same time intimately connected series, such as was at my command in no other class. ”

“ The uropods are generally very small, and do not form a tail-fan.
Unlike the Tanaidacea, the Isopoda, which form the fourth order of the Peracarida, are very numerous in species, and very varied in structure and habits. The most familiar are the Woodlice, or Slaters, which are commonly found in damp places, under stones and the like. Besides these, however, the order includes a vast number of forms living in the sea and a few that live in fresh water. ”

“ It is in this state that we have seen the males of a number of Crustacea taking on a temporary or partial hermaphroditism. We may, therefore, inquire, whether in these cases of normal hermaphroditism there is any evidence to show that here too the hermaphroditism has been acquired by the male sex as a response to the change in the metabolic conditions. In the parasitic Isopoda Epicarida (see pp. 129–136) the hermaphroditism is of a very simple kind; all the individuals are at first males, whose function it is to fix on and fertilise the adult parasites. ”

“ The parasitic habit is most common among the Copepoda and Isopoda, where it leads to complex modifications of structure and life-history. Perhaps the most complete degeneration is found in the Rhizocephala, which are parasitic on other Crustacea. In these the adult consists of a simple saccular body containing the reproductive organs and attached by root-like filaments which ramify throughout the body of the host and serve for the absorption of nourishment (fig. 1) . ”

“ With rare exceptions, branchial plates are developed either by modification of a ramus of the limbs or as processes on a ramus, or upon the sides of the body. No tracheate Crustacea are known, but some terrestrial Isopoda develop pulmonary in-sinkings of the integument. A characteristic, comparable in value to that presented by the pygidial shield of Arachnida, is the frequent development of a pair of long appendages by the penultimate somite, which with the telson form a trifid, or, when that is small, a bifid termination to the body. ”

“ The species of Epicaridea are very numerous, and they infest Crustacea belonging to nearly all the chief groups of the class, a few even being parasitic on other Epicaridea. Many of them differ greatly from the Bopyrus just described, and in some cases it would be impossible to guess from the structure of the adult animals that they were Isopoda, or even Crustacea at all. The life-history is not yet completely [223] known. When hatched from the egg, the free-swimming larvæ have a short and broad body, and, as in other Isopod larvæ, have only six instead of seven pairs of legs. ”

“ More than 30 per cent. of the Insecta exhibit this habit, which is also the dominant one in the Oligochæta, Symphyla, and in many of the soil Nematodes. Heterophagous species include all those of somewhat plastic habits; for the most part they are saprophagous, but, on the other hand, a considerable proportion of the species attack growing plants or exhibit both habits. Under this category are included a certain number of the Insecta, the Diplopoda, Isopoda, and some Acarina. ”

“ Trilobites have been most often compared with Branchiopoda, Isopoda, and Merostomata, the present concensus of opinion inclining toward the notostracan branchiopods (Apodidæ in particular) as the most closely allied forms. It seems hardly worth while to burden these pages with a history of opinion on this subject, since it was not until the appendages were fully made out, from 1881 to 1895, that zoologists and palæontologists were in a position to give an intelligent judgment. The present status is due chiefly to Bernard (1894) , Beecher (1897, 1900, et seq.) , and Walcott (1912, et seq.) . ”

“ In the primitive Malacostraca the gills were probably, as in the Phyllopoda and in Nebalia, the modified epipodites of the thoracic limbs, and this is the condition found in some Schizopoda. In the Cumacea and Tanaidacea only the first thoracic limb has a branchial epipodite. In the Amphipoda, the gills though arising from the inner side of the bases of the thoracic legs are probably also epipodial in nature. In the Isopoda the respiratory function has been taken over by the abdominal appendages, both rami or only the inner becoming thin or flattened. ”

“ In the foregoing account of the Cirripedia we have met with certain peculiar sexual relations in which closely allied species exhibit marked differences in regard to the distribution of the qualities of sex among their individuals; we have seen that the majority of species are hermaphrodite, unlike most Crustacea which, with the other exception of the parasitic Isopoda, are normally dioecious; and that in some species complemental males exist side by side with the hermaphrodites, while, in yet others, the individuals are either females or dwarf males. ”

“ The arrangement of the processes of the carapace, and, still more, the similar number of the caudal setæ in the most different Zoëæ of Crabs (Figs. 19–23) prove this. Such an accordance in the number of organs apparently so unimportant is only explicable by common inheritance. We may predict with certainty that amongst the Brachyura no species will occur which, like Penëus, still produces Nauplius-brood. [3]
As we have already seen, Mysis and the Isopoda depart from all other Crustacea very remarkably by the fact that their embryos are curved upwards, instead of, as elsewhere, downwards. ”

“ The simple median eye appears earlier, and would therefore be more important than the compound paired eyes; the scale of the antennæ in the Prawns would be more important than the flagellum; the maxillipedes of the Decapoda would be more important than the chelæ and ambulatory feet, and the anterior six pairs of feet in the Isopoda, than the precisely similarly formed seventh pair; in the Amphipoda the most important of all organs would be the “micropylar apparatus,” which disappears without leaving a trace soon after hatching ”

“ Not unfrequently we might indeed directly reverse the proposition and assert that the difference becomes the greater, the further we go back in the development, and this not only in those cases in which one of two nearly allied species is directly developed, and the other passes through several larval stages, such as the common Crayfish and the Prawns which are produced from Nauplius-brood. The same may be said, for example, of the Isopoda and Amphipoda. In the adult animals the number of limbs is the same ”

“ A very remarkable feature of the subterranean fauna is that a number of the animals appear to be more closely allied to marine species than to any known from fresh waters above-ground. This is especially the case with some of the Isopoda belonging to typically marine families like the Cirolanidæ and Anthuridæ, and it has been suggested that these have been derived from marine species which have entered the underground waters directly from the sea by way of submarine fissures in the crust of the earth. ”

“ In the cheliferous Isopods (Tanais) the heart resembles that of the Amphipoda in its elongated tubular form, as well as in the number and position of the fissures, but with this difference, that the two fissures of each pair do not lie directly opposite each other.
Fig. 14. Heart of a young Cassidina. Fig. 15. Heart of a young Anilocra.
Fig. 16. Abdomen of the male of Entoniscus Cancrorum. h. Heart. l. Liver.
In all other Isopoda the heart is removed towards the abdomen. ”

“ The intestine terminates at the posterior end of the body, and the gills and circulatory and reproductive organs are paired. It includes the chitons (Polyplacophora) , together with Neomenia and Chætoderma, which are wormlike forms without a shell.
I"so*pod (?) , a. (Zoöl.) Having the legs similar in structure; belonging to the Isopoda. -- n. One of the Isopoda.
||I*sop"o*da (?) , n. pl. [NL. See Iso-, and -poda.] (Zoöl.) An order of sessile-eyed Crustacea, usually having seven pairs of legs, which are all similar in structure. ”

“ The examination of a common Woodlouse, such as Oniscus or Porcellio (Fig. 20) , will give a general idea of the form and structure of a typical Isopod, although many curious modifications are found, some of which will be mentioned in later chapters.
There is no distinct carapace, but the last vestige of one may be indicated by the fact that the first [52] thoracic somite is completely fused with the head. All the other somites of the body are distinct (in some Isopods, however, the abdominal somites are coalesced) , but the telson is not separate from the last somite. ”

“ To avoid having the exopodites and endopodites intermingled in this way, the animal would have to bring all the endopodites together along the axial line in a plane approximately perpendicular to the dorsal test, in which case the exopodites would be free to act as swimming organs. The fact that the setæ of an exopodite stay together like the barbs on a feather would of course tend to strengthen the idea that the exopodites could be used in swimming, but that is not the only possible explanation of this condition. ”

“ In most species, both exopodite and endopodite suffer reduction. The exopodite springs from the basipodite and that segment is closely joined to the coxopodite, producing a protopodite. In some cases the original segments of the endopodites fuse to form a stiff rod. While highly diversified, these appendages are very trilobite-like, and some Ostracoda even have biramous antennæ.
The non-parasitic Copepoda have limbs exceedingly like those of trilobites. Many of them are biramous, the endopodites sometimes retaining the primitive six segments. ”

“ 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. ”