CHAPTER X. ANNULOSA. THE members of this sub-kingdom have received the name Annulosa in consequence of their annulated or ringed appearance; they have also been called Homogangliata (¿μoû, together; and yάyyλa, ganglia-Owen), and Articulata (articulatus, jointed-Cuvier). The body is composed of a series of segments, (somites), arranged in a longitudinal manner. The limbs, when present, are turned towards the neural aspect of the body. The alimentary canal is distinct, and does not communicate with the general body cavity. Respiration is performed either by the general surface of the body, or by "trachea," "pulmonary sacs," or "branchiæ." Circulation is effected either by a systemic heart, consisting of one ventricle, or a single segmented dorsal vessel, or a pseudo-hæmal system; or the blood circulating system may be absent. The nervous system is situated on the ventral aspect of the body, and consists, in its typical form, of a pair of symmetrical ganglia in each segment (anterior cephalic segment excepted), which are connected by two nervous cords that extend along the whole length of the animal. The most anterior ganglion is single, and called the supra-æsophageal or cerebral. The sense of sight exhibits some extremely interesting gradations, and the following different kinds of eye are met with:-(1.) The eye spot. In the lowest members of this sub-kingdom, the annelida (anellus, a little ring; and eldos, form), we find the organ of vision existing in its simplest form, and termed an "ocellus," or "eye spot.” This ocellus is formed by a single optic nerve passing from the supra-æsophageal ganglion, and expanding into a retina behind a delicate transparent integument that serves the purpose of a cornea; situated immediately posterior to the retina there is a single pigmentary or choroid coat-such is the structure of an ocellus wherever met with in the animal kingdom. (2.) The simple eye. This form of eye, found in the insecta, crustacea, and arachnida, consists of an external coat formed by the choroid, which, bending down in front, constitutes an iris with a central opening or pupil; inside this there is a retina, vitreous humour, and globular lens, in close contact with a minute, smooth, convex cornea. (3) The conglomerate eye.—It is composed of a number of simple eyes massed together, and is found in the myriapoda (μupio, innumerable; and Tódes, feet). (4.) The compound eye.Its structure is extremely intricate, and conflicting accounts of it have been given by different accurate observers. The following is the most generally received opinion concerning its formation :—The optic nerve, immediately after its origin from the supra-æsophageal ganglion, swells into a secondary ganglion; from this secondary ganglion arise a great number of short nerves, which terminate by forming a nervous expansion called the "general retina," situated behind a pigmental layer called the common choroid; a multitude of minute filaments arising from the general retina pierce the common choroid, and terminate in a number of separate corneal facettes, each filament ending in a single cornea; the corneal facettes are described as hexagonal lenses united together. This form of eye is found among the insecta and crustacea, and it is stated that the common house-fly possesses about 4,000 of those distinct corneal facettes in each compound eye. The other senses, though present, become more rudimen tary and imperfect as we descend through the different members of this sub-kingdom. The remaining peculiarities, especially those of reproduction, will be mentioned in describing the different classes. The Annulosa are divided into I. Arthropoda (áp@pov, a joint; and woús, the foot). I. ARTHROPODA. The members of this division are more correctly denominated articulate animals, because they have jointed appendages articulated to the body. The covering of the body is either a "chitinous" integument or calcareous shell, from the inner surface of which the muscles arise. In the embryo the nervous system always presents the true homogangliate type, although in the adult it may be variously modified. Ciliæ are never present. The Arthropoda are sub-divided into 1. INSECTA (inseco, I cut into).This class is extremely numerous, and is said to contain more species than all the rest of the animal kingdom, no less than 150,000 having been already described. Insects can easily be recognised by the following characters:-The body is covered by an integumentary envelope, more or less hardened with "chitine," and is divided into three distinct parts, viz., the head, thorax, and abdomen. The head, formed of several smaller segments amalgamated together, supports a pair of eyes, usually compound, a pair of antennæ (feelers), of mandibles, and maxillæ. The thorax, composed of three segments, prothorax, mesothorax, metathorax, carries two or four wings (the wings are often absent, sometimes deciduous), and three pairs of legs. The wings are composed of a double membranous expansion of the integument, supported by “nervures " (nervus, a sinew), and are attached to the dorsal surface of the meso-thorax and meta-thorax. The legs are always six in number, a pair being connected to each segment of the thorax. Each leg is composed of a series of pieces named in order, coxa, trochanter, femur, tibia, and tarsus; the tarsus contains two to six, generally five joints, terminated by claws. The abdomen, composed of nine segments amalgamated together, contains the viscera and organs of reproduction-never carries legs, and sometimes has peculiar appendages, such as the forceps of the earwig, the stings of bees and wasps, the ovipositors of the ichneumon. The muscles are striped, and arranged in small bundles or fasciculi without sarcolemna; they are extremely numerous, Lyonnet having described nearly 1,800 in the larva of the goat moth. The digestive apparatus consists of (1) mouth, (2) pharynx, (3) œsophagus, (4) crop (in masticatory insects), (5) proventriculus or gizzard, (6) ventriculus or stomach, (7) intestine, (8) cloaca. There are two distinct types of mouth found in insects, viz., the masticatory, and suctorial or haustellate; these are sometimes modified, and occasionally combined. The masticatory mouth (e.g., beetles) consists of (1) labrum or upper lip, (2) a pair of mandibles or biting jaws, (3) a pair of maxillæ or chewing jaws, with one or more pairs of maxillary palpi, (4) a labrum or lower lip, with labial palpi. In the suctorial mouth (butterflies) the labrum and mandibles are rudimentary; the maxillæ are elongated, and form a spiral tube (proboscis), while the labial palpi form two hairy cushions, between which the proboscis is received when not in use. The gizzard is muscular, and often has ridges on its internal surface, furnished with plates of "chitine" for crushing the food. The glands present are (1) salivary, (2) malpighian, or biliary tubes, (3) uriniferous tubes opening into the cloaca. Respiration is effected by "trachea," or air tubes, that open externally by apertures called "spiracles," and as they pass inwards through the tissues, they branch and anastomose ; inside these tubes there is a filament of "chitine," coiled in a spiral manner. Many insects undergo an interesting metamorphose before arriving at maturity. Take, for an example, the common butterfly; on leaving the egg the insect is termed a larva, but is better known by the name of caterpillar. This caterpillar has an elongated, worm-like body, with numerous legs; it eats ravenously, increases rapidly in size, and frequently changes its skin; after changing its skin for the last time the animal wraps itself up in a case called the cocom, and becomes perfectly quiescent and torpid; it is now called a chrysalis, nympha, or pupa; in this state it remains for some time, often for many months, and then bursts through its covering, and becomes the imago, or perfect insect, that alone has the power of propagating the species; having accomplished this duty it dies. A peculiar method of reproduction has been particularly described by Professor Owen under the name of Parthenogenesis (rapoévos, a virgin; and yéveσis, production), or the power of propagation, "sine concubitu ;" it is well exemplified in the aphides, or plant lice, sic. :—In the autumn only do perfect male and female insects make their appearance; at this time the female becomes impregnated, and produces true ova, which, lying dormant throughout the winter, are hatched in spring, and become developed into female aphides, which in turn produce other females, without any coitus, to the ninth, tenth, or perhaps eleventh generation; when the final generation again appears in autumn perfect males and females once more make their appearance, and so on. A variety of this form of propagation occurs in the bee. Every bee-hive contains from fifteen to twenty thousand |