MORPHOLOGY All Arthropoda differ from the Annelida in having some of the anterior somites coalesced to form the "head." This is doubt less associated with the modification of some of the anterior ap pendages to serve as jaws. Further, there has taken place, as Ray Lankester first showed, a backward shifting of the mouth opening, from its primitive position on the first somite (peristom ium of Annelida), so that one or more somites which were origi nally postoral come to be situated, with their appendages, in front of the mouth. At the same time the nerve-ganglia belonging to these somites have been moved forward to become incorporated in the "brain." The evidence for the forward shifting is par ticularly clear in the case of the antennae of the Crustacea and their nerve-ganglia (see CRUSTACEA).
The number of somites constituting the head differs in the classes of the Arthropoda but considerable difficulties are en countered in attempting to determine the actual numbers, and to compare the various classes with one another in respect of this character. Embryological study has shown that clearly mark ed somites may be present in the early stages which disappear in the course of development and that the number of appendages present in the adult does not always correspond to the number of somites originally present. Even where no such embryonic ves tiges can be discovered it cannot safely be assumed that the cephalic appendages are serially homologous in widely different types of Arthropoda. The following comparison of the consti tution of the head in the different classes must be understood as subject to this reservation.
The structure of the head is simplest in the Onychophora (Peripatus) which in this, as in many other respects, occupy a very primitive and isolated position among existing Arthropoda. In the early embryo the first pair of mesoblastic somite-rudi ments lie, at their first appearance, well behind the mouth, but move forwards in the course of development until they are in front of the mouth. From these arise a pair of limb-buds which become the antennae of the adult. From the similar limb-buds on the second pair of somites arise the single pair of jaws and from those on the third somites the "oral papillae" of the adult. In all other Arthropoda there is evidence of more than one pre oral segment. Even in the Arachnida, in which only one pair of appendages, the chelicerae, are in front of the mouth, a pair of pre-cheliceral somite-rudiments have been found in the embryo in spiders. In Crustacea there are three preoral somites, a pair of preantennular somite-rudiments being present in the embryo in front of those that carry the antennulae and antennae of the adult. In the Chilopoda (Myriopoda) there are also three preoral somites, transitory embryonic rudiments being found in front of and behind the antennae, which are the only preoral appendages of the adult. In insects also a preantennal somite has been demon strated and there is stated to be some evidence of the existence of another somite in front of it ; a transitory somite between antennae and mandibles completes the resemblance to Chilopods and Crustacea.
Subject to the reserves indicated above, the probable homologies of the anterior somites and their appendages in the chief classes of Arthropoda may be tabulated as follows (embryonic somites are indicated by italics) :— The constitution of the head in the Diplopod Myriopoda (sometimes regarded as a class distinct from the Chilopoda) is not clearly ascertained, but for further details reference must be made to the articles dealing with the different groups.
The sensory organs of the Arthropoda present numerous and diverse complexities of structure many of which are clearly as sociated with the presence of a hard and often opaque cuticle. With the body and limbs so encased it may be supposed that the sense of touch would be very dull unless it were localized in special organs. These organs are provided by the hairs or "setae" which are scattered over the surface in all Arthropoda. They are hollow projections of the cuticle containing prolongations of the underlying cells of the hypodermis into which nerve-fibrils can be traced. At the base, the seta arises from a cup-shaped de pression where the cuticle is thinned away to form a more or less movable joint, and movements caused by touching solid bodies will cause afferent impulses to be transmitted to the central nervous system. The finer setae are often furnished with secondary hairs set along their length like the barbs of a feather, and such setae, which are especially common in aquatic Arth ropoda, can be seen to respond to movements in the water and even to the delicate vibrations caused by sound waves. Another modification of setae gives rise to organs of taste and smell or, more generally, of chemical sense. In these the cuticle of the setae is exceedingly thin, permitting diffusion of substances from the surrounding medium which thus reach the nerve-endings within.
The eyes of Arthropoda are very characteristic of the group, and here again the structure seems to be conditioned by the presence of a thick cuticle. Only in Peripatus (Onychophora) are the eyes of a type found in Annelida, consisting of a closed sack or vesicle, folded off from the skin and enclosing a lens that seems to be cuticular in origin. In all other Arthropoda the eyes are developed as pits in the ectoderm, the cavity of the pit being filled by a transparent thickening of the cuticle which acts as a lens, concentrating light on the receptive cells beneath. These receptive or retinal cells contain rod-like bodies (rhab domes) perhaps also cuticular. The simpler types of eye consist each of a single pit with single lens overlying a group of retinular cells, or a number may be grouped together, leading to the re markably complex "facetted" or "compound" eyes which are especially characteristic of crustaceans and insects. In these the transparent cuticle forming the cornea which covers the eye is divided into a number (often a very large number) of lenticular facets each of which corresponds to a little group of retinular cells and usually an additional refractive body, the "crystalline cone." While there is considerable diversity in minute details of structure it should be emphasized that the agreement in general plan between the compound eyes of Crustacea and those of insects is remarkably close.
Respiration in aquatic Arthropoda is carried on by gills or, in the more minute forms, by the general surface of the body. The gills are plate-like or branching outgrowths from some of the limbs or from the surface of the body. The organs of respiration in air-breathing Arthropoda are more varied. The characteristic "lung books" of many Arachnida are plainly de rivable from the very similar "gill books" of the aquatic Xiphosura and their allies (see ARACHNIDA). The most common method of aerial respiration, however, is by tracheae. These are delicate tufted or branching tubules, opening to the exterior and filled with air. They are lined by a delicate continuation of the external cuticle, of ten strengthened by a spiral thickening. In the simpler forms the tracheae penetrate only a little way into While the number and the importance of the characters which are common to all the classes composing the Arthropoda are such as to remove all reasonable doubt that the group is a "natural" or monophyletic one, it is not possible to say that the phyletic relations of the classes themselves have been clearly ascertained. No phylogenetic scheme has been suggested that does not involve the independent origin of similar features in different lineages. Thus, the possession of tracheae was formerly considered as clear evidence of common descent in the Onychophora, Myriopoda, Insecta, and Arachnida. The demonstration by Ray Lankester, however, of the close affinity between the air-breathing Arachnida and the gill-bearing Xiphosura and Eurypterida involved the con clusion that the tracheae of Arachnida must have arisen independ the body and the blood no doubt acts as intermediary in the respiratory exchange with the tissues. In more highly developed types, especially in insects, the tracheal tubules ramify through out the whole body, carrying air into direct contact with the cells of the tissues and the respiratory function of the blood is almost or quite superseded. Tracheae are possessed by Onycho phora, Myriopoda (Chilopoda and Diplopoda), Insecta, most terrestrial Arachnida, and even by the terrestrial Isopoda among Crustacea. Similar structures are said to occur in certain Coel enterata (Siphonophora) and Mollusca, but with these exceptions they are peculiar to the Arthropoda.
The structure of the heart has been already alluded to. In the more primitive forms it is a long dorsal tube running throughout the greater part of the body and lying in a chamber, the peri cardium, which contains blood. From the pericardium the blood enters the heart by paired openings or ostia with valvular lips which permit blood to enter but prevent its egress. Typically there is a pair of ostia in each somite. The heart is rhythmically contractile and propels the contained blood towards the head where it may be discharged directly into the lacunae of the haemo coel or may pass through one or several arterial vessels before entering the lacunar system. In more specialized forms the heart tube is shortened and the number of ostia is reduced and in some of the smaller Crustacea it forms a globular sack with a single pair of ostia. In many minute forms of Crustacea and Arachnida the heart is absent and the blood is simply driven hither and thither by movements of the body, limbs and alimentary canal.
ently of those of the insects. The assumption of a simple phyletic series leading from the Onychophora through the Myriopoda to the Insecta became untenable when it was shown that the Chilopod Myriopods and the insects agreed closely with Crustacea in the segmentation of the head-region and that the similarity in struc ture of the compound eyes of insects and Crustacea was too close to be imagined as arising independently in the two groups. Again, the resemblance of the tracheae of terrestrial Isopod Crustacea to those of the other tracheate groups is not disposed of by call ing them "pseudotracheae" and it is not seriously suggested that so specialized a group of Crustacea can have given rise to insects.
The suggestion that all the existing Arthropoda can have been de rived from tracheate air-breathing ancestors, implying, as it does, the existence of a land-fauna in Pre-Cambrian times, cannot be considered seriously in the present state of our knowledge. The only conclusion that can be drawn, therefore, is that tracheae have been acquired at least four times in the evolution of the Arthropoda.
The fact that convergent evolution must be admitted in the case of a system of organs so characteristic as the tracheae de mands a careful scrutiny of the other structural features on which classification may be based. Some uncertainties with regard to the constitution of the head have already been alluded to. Where there is a clear correspondence of somite for somite and appendage for appendage, as in the comparison of Xiphosura and Eurypterida with scorpions, a natural or phyletic relationship may be taken as proved. On the other hand, while the dismembering of the old group Myriopoda may be justified by the wide differences separating the Diplopoda and Chilopoda, it is not clear that the association of the Symphyla with the former can be sustained solely because of the anterior position on the genital openings.
The palaeontological record of Arthropoda gives some help to wards understanding their phylogeny. Particularly interesting are the Trilobites, some of which are among the earliest known fossils and of which the structure is now known in considerable detail in several genera. It is clear that they are not far removed from a primitive stock which may well have given rise to the Crustacea and it is probable that the Eurypterida, and with them the Arachnida, may have come from the same source by way of the imperfectly known Cambrian Limulava. Of the origin of insects and of the Myriopod groups palaeontology has, as yet, nothing to tell us although Diplopod-like forms and probably insects occur in the Devonian.
The position of the Onychophora with respect to the other Arthropoda is quite obscure except that they must have diverged very early from the primitive stock and have acquired independ ently the features which they have in common with the other terrestrial groups such as tracheae and simple clawed limbs.
The Pycnogonida are another group which it is impossible, in the present state of knowledge, to link up with any of the larger groups of Arthropoda. Their traditional association with Arach nida encounters difficulties in the interpretation of the appendages and these difficulties have not been diminished by the discovery of genera which have five, instead of the usual four, pairs of legs.
The Tardigrada and Pentastomida (Linguatulida, qq.v.), are two small groups whose title to be regarded as Arthropoda is doubt ful. If they are, the simplification of structure and the disap pearance of connecting links has entirely obscured their relation ships with the more normal members of the group.
It may be mentioned that, in point of size, the Arthropoda have a wider range than any of the other main groups of the animal kingdom. The same minimum of i/ioo inch (.25 mm.) is reached both among Crustacea (Cladocera) and among insects (Coleop tera). At the other extreme stands the Giant Japanese Crab (Macrocheira or Kaempferia) which can span I 1 ft. with its claws. Some of the extinct Eurypterida, reaching 5ft. in length of body, were probably still bulkier.
See ARACHNIDA, CRUSTACEA, INSECTS, MYRIAPODA, ONYCHOPHORA, PYCNOGONIDA, TRILOBITES. (W. T. C.)