The body consists of a number of exactly similar or closely similar segments, which are never fused and metamorphosed, as in the Arthropoda, to form specialized regions of the body.
Where locomotive appendages (the parapodia of the Polychaeta) exist, they are never jointed, as always in the Arthropoda; nor are they modified anteriorly to form jaws, as in that group.
Thus in the Chaetopoda the perivisceral cavity is coelomic; in this respect the group contrasts with the Arthropoda and Molluscs, where the perivisceral cavity is, mainly at least, part of the vascular or haemal system, and agrees with the Vertebrata.
The cerebral ganglia constitute an archicerebrum for the most part, there being no evidence that, as in the Arthropoda, a movement forward of post-oral ganglia has taken place.
HEXAPODA (Gr., six, and g obs, foot), a term used in systematic zoology for that class of the Arthropoda, popularly known as insects.
And the wings, though not always present, are highly characteristic of the Hexapoda, since no other group of the Arthropoda has acquired the power of flight.
Lecaillon (1898) on various leaf beetles, tend to show that the organ " in the embryos of the lower Arthropoda corresponds with whole of the " mid-gut " arises from the proliferation of cells at the the region invaginated to form the serosa of the hexapod embryo.
His order of wingless insects (Aptera) included Crustacea, spiders, centipedes and other creatures that now form classes of the Arthropoda distinct from the Hexapoda; it also included Hexapoda of parasitic and evidently degraded structure, that are now regarded as allied more or less closely to various winged insects.
Relationships And Phylogeny The Hexapoda form a very clearly defined class of the Arthropoda, and many recent writers have suggested that they must have arisen independently of other Arthropods from annelid worms, and that the Arthropoda must, therefore, be regarded as an " unnatural," polyphyletic assemblage.
In 1735 appeared the first edition of the Systema naturae of Linnaeus, in which the "Insecta" form a group equivalent to the Arthropoda of modern zoologists, and are divided into seven orders, whose names - Coleoptera, Diptera, Lepidoptera, &c., founded on the nature of the wings - have become firmly established.
The Insecta of Linnaeus was a group exactly equivalent to the Arthropoda founded a hundred years later by Siebold and Stannius.
The position of the Arachnida in the great sub-phylum Arthropoda, according to recent anatomical and embryological researches, is explained in the article Arthropoda.
- Modern views as to the classification and affinities of the Arachnida have been determined by the demonstration that Limulus and the extinct Eurypterines (Pterygotus, &c.) are Arachnida; that is to say, are identical in the structure and relation of so many important parts with Scorpio, whilst differing in those respects from other Arthropoda, that it is impossible to suppose that the identity is due to homoplasy or convergence, and the conclusion must be accepted that the resemblances arise from close genetic relationship. The view that Limulus, the king-crab, is an Arachnid was maintained as long ago as 182 9 by Strauss-Diirckheim (1), on the ground of its possession of an internal cartilaginous sternum - also possessed by the Arachnida (see figs.
But it is necessary to remember, in the light of recent discoveries, that the sixth prosomatic pair of appendages is carried on the seventh somite of the whole series, there being two prosthomeres or somites in front of the mouth, the first carrying the eyes, the second the chelicerae; also that the first mesosomatic or genital somite is not the seventh or even the eighth of the whole series of somites which have been historically present, 1 See the article Arthropoda for the use of the term " prosthomere."
- It appears (see Arthropoda) that there is embryological evidence of the existence of two somites in Arachnida which were originally post-oral, but have become prae-oral by adaptational shifting of the oral aperture.
The genital ducts of Arthropoda are, like the green glands, shell glands and coxal glands, to be regarded as coelomoducts (gonocoels).
We know that such a lateral position of the nerve cords preceded the median position in both Arthropoda and Chaetopoda.
It seems that there is a primitive tendency in the Arthropoda for the arteries to accompany the nerve cords, and a " supra-spinal " artery - that is to say, an artery in close relation to the ventral nerve cords--has been described in several cases.
It is very probable that in Scorpio they do not serve merely to secrete a digestive fluid (shown in other Arthropoda to resemble the pancreatic fluid), but that they also become distended by the juices of the prey sucked in by the scorpion - as certainly must occur in the case of the simple unbranched gastric caeca of the spiders.
In Limulus Lankester found (15) the spermatozoa to possess active flagelliform " tails," and to resemble very closely those of Scorpio which, as are those of most terrestrial Arthropoda, are actively motile.
(From Lankester, loc. cit., after Packard.) with all air-breathing Arthropoda except Peripatus, possesses these tubules, which are often called Malpighian tubes.
They have appeared independently in connexion with a change in the excretion of nitrogenous waste in Arachnids, Crustacea, and the other classes of Arthropoda when aerial, as opposed to aquatic, respiration has been established - and they have been formed in some cases from the mesenteron, in other cases from the proctodaeum.
Further, it is pointed out by Korschelt and Heider that the hinder portion of the gut frequently acts in Arthropoda as an organ of nitrogenous excretion in the absence of any special excretory tubules, and that the production of such caeca from its surface in separate lines of descent does not involve any elaborate or unlikely process of growth.
When we consider the relationships of the various classes of Arthropoda, having accepted and established the fact of the close genetic affinity of Limulus and Scorpio, we are led to important conclusions.
It has been insisted, by those who accepted Lankester's original doctrine of the direct or genetic affinity of the Chaetopoda and Arthropoda, that Apus and Branchipus really come very near to the ancestral forms which connected those two great branches of Appendiculate (Parapodiate) animals.
When it is admitted - as seems to be reasonable - that the primitive Arachnida would, like the primitive Crustacea, be anomomeristic and anomotagmic, we shall not demand of claimants for the rank of primitive Arachnids agreement with Limulus and Scorpio in respect of the exact number of their somites and the exact grouping of those somites; and when we see how diverse are the modifications of the branches of the appendages both in Arachnida and in other classes of Arthropoda, we shall not over-estimate a difference in the form of this or that appendage exhibited by the claimant as compared with the higher Arachnids.
(From Korschelt and Heider.) quently take place (as in all Arthropoda) at the posterior extremity of the body, whilst excalation of somites may occur at the constricted areas which often separate adjacent " regions," though there are very few instances in which it has been recognized.
Claus, " Degeneration of the Acari and Classification of Arthropoda," Anzeiger d.
"Evropa, insected Anaema or Insects (= Arthropoda, exclusive of Crustacea).
Annelids are segmented worms, and differ from the Arthropoda, which they closely resemble in many respects, by the possession of a portion of the coelom traversed by the alimentary canal.
TRILOBITES, extinct Arthropoda, formerly classified with the Crustacea, but of late years relegated to the Arachnida, which occurred abundantly in seas of the Cambrian and Silurian periods, but disappeared entirely at the close of the Palaeozoic epoch.
21), so that its fibres join the anterior faces of the nerve-end cells as in Vertebrates, instead of their posterior faces as in the cephalic eyes of Mollusca and Arthropoda; moreover, the lens is not a cuticular product but a cellular structure, which, again, is a feature of agreement with the Vertebrate FIG.
The perioesophageal nerve-ring of Chaetopoda and Arthropoda is represented, not by the collar first mentioned in the above description, but by the commissures connecting the cerebral and pedal ganglia.
(1875); " Note on the Coelom and Vascular System of Mollusca and Arthropoda," Quart.
Experiments to test distastefulness have also been made with various kinds of insectivorous Arthropoda, like spiders and mantises.
Among the remaining divisions of Invertebrata special mention may be made of the air-breathing Arthropoda - on the whole the most important and interesting group. About one-third of the animals belonging thereto that occur in the higher regions are exclusively alpine (or alpine and northern); these characteristically alpine forms being furnished chiefly by the spiders, beetles and butterflies.
The animals thus associated, the Rotifera, Chaetopoda and Arthropoda, are composed of a larger or smaller number of hollow rings, each ring possessing typically a pair of hollow lateral appendages, moved by intrinsic muscles and penetrated by blood-spaces.
Arthropoda are very abundant and as yet little known.
Collectively, they are ranked as one of the classes forming the sub-phylum Arthropoda, and their distinguishing characters are discussed under that heading.
It will be sufficient here to define them as Arthropoda for the most part of aquatic habits, having typically two pairs of antenniform appendages in front of the mouth and at least three pairs of post-oral limbs acting as jaws.
- As in all Arthropoda the body consists of a series of segments or somites which may be free or more or less coalesced together.
This is especially the case in the anterior part of the body, where, in correlation with the " adaptational shifting of the oral aperture " (see Arthropoda), a varying number of somites unite to form the "cephalon " or head.
As in all Arthropoda, it is composed of three divisions, a fore-gut or stomodaeum, ectodermal in origin and lined by an inturning of the chitinous cuticle, a mid-gut formed by endoderm and without a cuticular lining, and a hind-gut or proctodaeum, which, like the fore-gut, is ectodermal and is lined by cuticle.
As in the other Arthropoda, the circulatory system in Crustacea is largely 1 6/ lacunar, the blood flowing in spaces or channels without definite walls.
There is every reason to believe 2, Gastric teeth of common that, in the primitive Arthropoda, lobster, Homarus vulgaris.
A very remarkable condition of the blood-system, unique, as far as is yet known among the Arthropoda, is found in a few genera of parasitic Copepoda (Lernanthropus, Mytilicola).
In addition to these two pairs of glands, which are in all probability the survivors of a series of segmentally arranged coelomoducts present in the primitive Arthropoda, other excretory organs have been described in various Crustacea.
The central nervous system is constructed on the same general plan as in the other Arthropoda, consisting of a supra-oesophageal ganglionic mass or brain, united by circumoesophageal connectives with a double ventral chain of segmentally arranged ganglia.
The compound eyes are very similar in the details of their structure (see Arthropoda) to those of insects (Hexapoda).