John Young

“ 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. ”

“ In any one of these stages, even in the adult, multiplication may be provided for by a process of budding, the bud from which the new form emerges being in essence undistinguishable from the ovum for whose further development impregnation is necessary. ”

“ 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. ”

“ Further, they are developed from the upper surface of the body. Now, in the Annelids the limbs are typically double pairs projected from the sides of the body; the parapodia consist of two branches, notopodium and neuropodium, and the gills when present are modifications of the notopodium. In the Crustaceans this bipartite condition is indicated by the exopodial and endopodial divisions of the limbs. In the adult insect this duplicity has disappeared, unless we recognise in the position of the gill tracheæ the equivalent of the branchiferous notopodium. ”

“ 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. ”

“ 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. ”

“ Hence the same limbs are tactile in Crustaceans, prehensile in Arachnids, ambulatory in some Crustaceans, accessories of mastication in others, locomotive in some, respiratory in others. In Insects the abdomen of the adult is destitute of appendages, but many larvæ are provided with tracheal gills, that is, external processes in which air-canals ramify, and in which a large quantity of blood is received within the cavity of the thin-walled dilatable process. ”

“ Their relations may be “falsified by the struggle for existence,” the details of the developmental history of the family (phylogenesis) may be crowded into a short space in the development of the individual (Ontogenesis) . The description of these variations belongs to the particular treatment of the Crustaceans, Myriapods, Arachnids, and Insects. ”

“ All the Arthropods agree in having the terminal portion of the intestinal canal derived from the outer, the middle portion from the inner embryonic layer. Lastly, the Arthropods, in common with the Molluscs and Annelids, have their body-cavity, or perivisceral space, formed by the splitting of the mesoblast, or derivative layer formed between the outer and inner layer (epiblast and hypoblast, and ectoderm and endoderm) . In Insects, in the higher Arachnids, and Crustaceans, yelk segmentation is partial; in Myriapods, and the lower Arachnids, and Crustaceans, it is total ”