ing only with the observed facts of development.
Haeckel, applying all that was known of
embryology to the construction of the pedigree of the groups, made (1866) the
Articulata one of the five great trunks of the genealogical tree. The
Articulate phylum embraced the
Infusoria and
intestinal worms, as well as the
Annelids, along with the
Arthropoda as restricted above. The
Arthropoda further formed two groups: —
Carides, the branchiate Arthropods or
Crustaceans; and
Tracheata, the
Arachnids,
Myriapods, and
Insects, which breathe by
tracheæ. The term
Articulate disappears from
Haeckel's latest classification, in which a redistribution of the
phyla is set forth. From assumed ancestors destitute of body-cavity (
Acœlomi) descend those
Vermes with body-cavity, of whose plan
Echinodermata,
Arthropoda,
Mollusca, and
Vertebrata show special modifications. In 1870
Gegenbaur gave a general table, in which the
Vermes included
Tunicata and
Annulata. The former led towards the
Mollusca; the latter was the starting point of
Arthropoda,
Vertebrata, and
Echinodermata; of the Arthropods there are four
classes,—
Crustaceans,
Arachnids,
Myriapods, and
Insects. Amidst all the varying opinions as to the value of the group, the importance of the limb-structure has been recognised since
Latreille dwelt upon the articulations by which the parts of these appendages are connected.
The Arthropoda agree in the characteristic already mentioned, the articulations of their limbs, whence the class-name is derived. The body presents various degrees of complexity. In the caterpillar, the metamera, somites, somatomes, or annuli, owe their mobility to differences in thickness of the integument. In the Myriapods the numerous similar somites are flexed on each other by the overlap of the chitin-thickened portions of cuticle which protect the upper and lower surface of each division. The somites are more or less effaced in the abdomen of insects and spiders; head and thorax in crabs and spiders have their composite origin concealed. But the external signs of division of the body no longer correspond, as in Annelids, to the distribution of the internal organs, which, with a partial exception in the case of the nervous system, are now unities contributing to the well-being of the whole. Homonomy, the absence of segmentation, or the equivalence of the divisions of the body, among the Annelids, has been contrasted with the heteronomy, or segmentation of the arthropod body. The difference, however, is only one of degree, since both the cephalic and caudal extremities, at least of the higher Annelids, are true segments, i.e., fused somites which, in addition to fusion, have undergone some amount of specialisation. In the four classes of Arthropods the head is a constant segment. It consists of præ-oral and post-oral somites, the ganglia of which are represented by the supra- and sub-œsophageal masses. The number of somites, as represented by appendages, is not the same in the four classes, and as the variation affects the præ-oral appendages supplied from the supra-œsophageal ganglion the difference is of great importance. In the Crustaceans the somites of this segment are, according to Huxley,—
The
Podophthalmata present the
eyes as modifications of processes identical with those which become ambulatory limbs. In the rest the
eyes are sessile. In
Myriapods,
Arachnids, and
Insects, the
eyes are sessile, and the præ-
oral appendages are reduced to one pair of
antennæ, whose innervation is from the supra-
œsophageal ganglion. Apart from the value to be assigned to descent in the search for homologies among these
classes, it is a question of fact whether the
eyes are præ-
oral or lateral to the
oral aperture. The
cephalic lobes carry the
organs of sight probably in the earliest types of development at the angle of bifurcation, the position of the single
eye of
Ostracods. In more complex forms the
eyes appear more or less towards the outer margin of the lobes; and in
insects where the cephalic arch is high, these organs may appear to correspond not to the most anterior, but to a posterior part of the cephalic sterna, just as
sensory organs appear, the
gustatory at the base of the outer, the
auditory at the base of the inner
antennæ in the higher
Crustaceans. By shortening of the development process the change of position may be obscured, and the
eyes, primitively belonging to the extremity of the
embryonal body, may from the first appear connected with more posterior
somites. The identification of homologous parts of the præ-
oral region in the four
classes rests on the opinion held as to the origin of the
classes. If the
Crustaceans are regarded as the stock of the Arthropods, the homologies must be recognisable. If, on the other hand, all four are divergencies from a common stock, then the absolute identity of the parts must hold a second place in comparison with a general conformity to the common plan. The identification of the
eyes with a particular pair of appendages necessitates the assumption that these sense organs, when sessile, are so by non-development of their supports. The converse supposition is more admissible, that the
eyes are supported on stalks as the result of an adaptive modification. Further, among the
Crustaceans we find hints of the primitive composite character of the Arthropod. The
auditory sacs of
Mysis are at the caudal extremity of the body, the
respiratory organs of
Isopods are in the same position, and the
genital orifices vary in different
genera, and even in the
sexes of the same
species. Analogous (perhaps no more) is the distribution among
Molluscs of the
eye spots which fringe the mantle of
Pecten, are pedunculate in the
snail, and, with the otolithic sacs, are in close proximity to the nerve centres of
cuttlefishes. If this view is accepted, the close comparison of the limbs of Arthropods loses much of its importance, and it becomes more interesting to endeavour to trace the primitive form from which the divergences have occurred. Among
Crustaceans the
Nauplius is the earliest recognisable form,—“an unsegmented ovate body, a median frontal
eye, and three pairs of natatory
feet, of which the anterior are simple, and the other two biramose” (
F. Müller). The third pair of appendages is replaced by the
mandibles, the oval body is divided by a transverse fold, and the
Nauplius head and
tail thus marked off have the mid-body of the adult developed by intercalation between them. Appendages are developed before segmentation is indicated in the free living
Nauplius; but in some this stage is overpassed in the
egg, the evidence of its existence being the presence of a thin exuviated
membrane which is not
egg membrane, nor can it be termed amnion, without overstraining that term which is properly used in the higher
vertebrate embryology. In
Insects the vermiform stage is rapidly passed through, the priority of segmentation to the development of appendages being indicated in the
Trichoptera, according to
Zaddach, and in
Aphis, according to
Huxley. If we go to the
Rotifers, there are in that group types which are comparable with the
Nauplius of
Crustaceans, and with the vermiform
larvæ of
Insects, as
O. Schmidt and
Lubbock (
Origin and Metamorphoses of Insects) have shown.
Pedalion mira (
Hudson) has a very close resemblance to the
Nauplius,
Lindia to the vermiform
grub of Dipterous
insects. The resemblance is not impaired by the comparison suggested by
Ray Lankester between the
Molluscs and
Rotifers.
Huxley calls the
Molluscs “little more than oligomerous modifications of the polychætous
Annelids” (
Nature, December 10, 1874); and in this article it is attempted to show what are the simplest forms presenting common features with the Arthropods. The hexapod
Insect has been compared by
Haeckel,
F. Müller, and others to the
Zoëæ of Malacostracous
Crustaceans, a group in one
