Isopod

“ There are no living trilobites, but the creatures now nearest them are the isopods. The trilobite was more like an isopod in its larva state than like a full-grown one. The trilobite had a hard plate or shell over his head and the front part of his body. This piece of armor was shaped nearly like the large front portion of the shell of a horseshoe [Pg 119] or king-crab. The king-crab uses this shell as a mud-plough to plough its way through the slime, where it finds its food and deposits its eggs. ”

“ Among the Crustacea of the Carboniferous and Permian epochs, there are a number of forms of which the affinities were until recently quite obscure. They have two-branched antennules, a scale-like exopodite on the antenna, and the last pair of appendages (uropods) form, with the telson, [264] a tail-fan. In these points they resemble the shrimp-like forms, but there is no carapace, and all the somites of the thorax are distinct, so that the form of the body is rather that of an Amphipod or Isopod. ”

“ The Phreatoicidae are a curious family of Isopods confined to the fresh waters of Australia and New Zealand, which bear a remarkable resemblance to Amphipods, being laterally compressed and possessing a subchelate hand on the anterior thoracic leg. These Isopods are exceedingly common in small mountain pools and in streams in Tasmania, and in the Great Lake in that country I have recently found a number of species which, together with some species of Amphipods, make up the dominant feature in the Crustacean fauna. One of these species may grow to fully an inch in length. ”

“ This isopod crustacean, living at first in the same manner as the rest, looks out for a small cyprinoid fish, thrusts itself like a trocar behind the abdominal fins, through the scaly skin, and penetrates entirely into the abdominal cavity. The male always accompanies its female. It is remarkable that she, in contradistinction to many others, preserves all the attributes of her sex. She does not change her form more than the other free crustaceans of her order, and only differs from the male in size. It is well known that in all these animals the male is always smaller than the female. Mons. ”

“ 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. ”

“ Tanaidae, are interesting in that they afford a transition between the ordinary Isopods and the Mysidacea. The important features in which they resemble the Mysidacea are, first, the fusion of the first two thoracic segments with the head, with the coincident formation of a kind of carapace in which the respiratory functions are discharged by a pair of branchial lamellae attached to the maxillipedes; and, second, the presence of very small exopodites on the first two thoracic appendages of the Apseudidae. ”

“ 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 view [196] that some connexion may exist between the Isopoda and the Trilobita seems to have been based on the similar dorso-ventral flattening of the body, its division into three regions—head, thorax, and abdomen—and the presence of sessile eyes. Beyond this it is difficult to find any resemblance; whilst the differences, such as the variable number of thoracic segments and their biramous appendages in Trilobites, are important. ”

“ The members of this sub-order, although agreeing with the Isopoda in the essentials of their anatomy, resemble the Amphipoda in being rather laterally compressed, and in having the hand of the first free thoracic limb enlarged and subchelate. The abdomen is greatly produced laterally by expansions of the segments. In fact, the shape of the body and of the limbs is very Amphipodan.—Phreatoicus from New Zealand, Southern Australia, and Tasmania. ”

“ This same genus includes a species (Œga spongiophila) which lives in the magnificent sponge, the Euplectella. We know also a certain number of isopods which prefer the interior of their neighbour’s body, and instal themselves in the cavity of the mouth, either to fish at the same time as their host, or to seize the food on its passage; others are of such a cruel nature, that they make no scruple to establish themselves in the stomach of a peaceable white fish. ”

“ 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 female is very much larger than the male, and, as is usual with parasitic creatures, the greater part of the body-cavity is occupied by the well-developed organs of reproduction.
There are several other parasitic isopods, some of which live on the bodies of fishes, and are popularly known as fish-lice, but these are not so likely to come in the way of the sea-side naturalist as the more typical forms that crawl about on the rocks and among the weeds of the coast. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ Besides all these more or less temporary visitors, however, there are numerous species, even in the inshore waters, which are adapted to a floating life, and pass their whole existence as members of the plankton. Copepoda of many kinds, some Mysidæ, Amphipods like Hyperia—which is commonly found sheltering under large jellyfish—some species of actively swimming Isopods, and many other forms, are only to be captured by the tow-net; and now and then, in certain localities, winds and currents may drive into coastal waters shoals of species whose proper home is the open ocean. ”

“ 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. ”

“ 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. ”

“ Differences in size are frequent; in the higher Decapods the males and in the lower Crustacea the females frequently being larger, the disparity being extreme in some of the parasitic Copepods and Isopods where the males are minute and attached to the females, whilst in the Cirripedes, as Darwin himself discovered, very minute complementary males may live as parasites in the mantle cavity of large hermaphrodite or female forms. ”

“ 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 ”

“ The above and other isopods feed on various animal and vegetable substances, some species being quite omnivorous in habit. Most of them are eagerly devoured by birds and fishes.
The Amphipods, six species of which are shown in the above illustration, include the Sandhoppers or Beach Fleas, so numerous on our coasts that it is almost impossible to go any distance without making their acquaintance. They are invaluable as scavengers, as they rapidly devour decaying sea weeds, and will speedily reduce the body of any animal washed on the beach to a clean skeleton. ”

“ 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 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 Epicarida include an immense number of Isopods, parasitic upon other Crustacea. In the adult state they become greatly deformed, and offer very few characters of classificatory value, but they all pass through certain highly characteristic larval stages which are essentially similar in the different families. All the species are protandric hermaphrodites, each individual being male while in a larval state, and then losing its male organisation and becoming female as the parasitic habit is assumed. ”

“ 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 ”

“ The abdomen or pleon carries the remaining six pairs, of which from three to five are called pleopods and the remainder uropods. Underlying the diversity of names and functions and countless varieties of shape, there is a common standard to which the appendages in general can be referred. In the maxillipeds and the trunk-legs it is common to find or otherwise easy to trace a seven-jointed stem, the endopod, from which may spring two branches, the epipod from the first joint, the exopod from the second.1 The first antennae are exceptional in branching, if at all, at the third joint. ”

“ They have become successfully adapted to a wide variety of environments and live on land, in water, and in the air. The typical arthropod has a segmented body which is usually covered by a chitinous exoskeleton which, in some forms, contains additions of calcium carbonate. They are highly specialized and well-developed animals in which locomotion is by means of paired jointed appendages.
Although the arthropods are of great importance in nature today, only a few groups are of importance to the paleontologist. Only two of these, the trilobites and the ostracodes, are discussed herein. ”