REFERENCE NOTES FOR LECTURES IN ZOOLOGY

Module 3. ARTHROPODS, TARDIGRADES, PENTASTOMIDS, ONYCHOPHORANS

Content Module 3.1. Arthropods

Topic 3.1.1. Phylum Arthropoda. Subphylum Branchiata

General characteristics of the phylum. Origin, evolution, paleontological data, modern diversity, geographical distribution, and their role in nature and human life. Overview of major taxonomic groups important for human life and the functioning of natural ecosystems. Class Cephalocarida. Class Branchiopoda. Class Remipedia. Class Maxillopoda. Class Ostracoda. Class Malacostraca.

The richest phylum of the animal kingdom in terms of species diversity. Comprising over 1.5 million species, with the actual number of living species estimated at 3 to 5 million. They have successfully colonized all major environments. Their life forms, feeding habits, and environmental adaptations are exceptionally diverse. Many species are animal parasites. Based on their feeding strategies, they can be herbivores, carnivores, saprophages, or parasites. They share numerous common features with Annelids, and were previously grouped together under the phylum Articulata. However, Arthropods have achieved a higher degree of Tissue and organ differentiation, as well as more advanced nervous systems and Sensory Organs; some exhibit complex behaviors (such as brood care, sophisticated construction activities, social structures, and even a primitive 'language' found in higher Hymenoptera).

Metameric animals characterized by markedly heteronomous metamerism. Groups of similar segments are fused into distinct body regions known tagmata. There are three primary tagmata: the HEAD, Thorax, and abdomen. In arachnids, the head is fused with the thorax to form a single tagma—the cephalothorax; in trilobites and myriapods, There are two tagmata—the head and trunk. Cephalization is the evolutionary process leading to The formation of the head, a tagma where sensory organs (antennae, eyes) and structures for the preliminary Processing of food (mouthparts) are concentrated. The head consists of the prostomial lobe (acron) and several anterior segments. The acron is homologous to the prostomium of annelids. The head typically comprises 4 to 8 or more segments, most commonly considered to be 6. The number of segments forming the thorax and abdomen (which terminates in the anal lobe, or telson, homologous to the pygidium) varies; however, the overall evolutionary trend within the phylum is toward stabilization and a reduction in segment number. Tagmatization also results in the internal integration of segments, with certain organs expanding at the expense of equivalent ones. There is a marked concentration of Nervous System ganglia into a Brain. A defining feature of arthropods is The Structure of their appendages. These consist of individual articles flexibly joined by articulating membranes to form multi-jointed levers capable of precise and complex movements. Appendage specialization is pronounced: on the head, appendages perform sensory Functions (one or two pairs of antennae), while several other pairs are modified into mouthparts that capture, hold, and crush food (mandibles and maxillae, or chelicerae and partially pedipalps). Appendages of the thoracic tagma serve a locomotor function (swimming, crawling, walking, running, jumping), occasionally acquiring additional functions such as Respiration or sensory perception. Abdominal appendages are frequently reduced. Only in trilobites and certain crustaceans (class Malacostraca) are they adapted for swimming; one or two pairs of abdominal appendages are often specialized as reproductive organs (for Fertilization and mating).

Subphylum Branchiata, or Crustacea.

Sizes range from fractions of a millimeter to 80 cm. While external body shapes and appendages vary widely, the internal Organization remains remarkably uniform. The body is segmented, with the number of segments varying across a broad range. The body is divided into three tagmata: head, thorax, and abdomen. The segments of each tagma may be distinct or fused. Sometimes the thoracic segments are clearly demarcated ventrally, while dorsally they are covered by a common shield, or carapace, which shelters PARTS OF THE body like a flat or pitched roof. The head comprises the acron and several segments, typically numbering between four and eight. The thorax and abdomen contain varying numbers of segments, except in species of the class Malacostraca (where the thoracic tagma

consists of eight, and the abdominal tagma of six to seven segments). The abdomen terminates in the anal lobe (telson), which often bears a pair of appendages (the furca, or caudal fork). Frequently, the division into three tagmata is modified. In some cases, only the part of the head bearing the antennules and antennae—the protocephalon—is incorporated into the head. The jaw-bearing segments may remain free or fuse with the thoracic segments to form the gnathothorax. When The process of cephalization encompasses not only the head segments but also one or two anterior thoracic segments, a synocephalon is formed. Occasionally, the head and all thoracic segments fuse into a cephalothorax. In other instances, several posterior abdominal segments fuse with the telson to form a pleotelson.

Appendages of different tagmata and body segments vary in Structure and function. An appendage consists of a basal part, the protopodite (comprising 1–3 articles), which bears a biramous branch known as the telopodite. The outer branch (exopodite) and the inner branch (endopodite) consist of a varying number of articles. On certain body segments, one or two respiratory outgrowths—epipodites (acting as cutaneous gills)—also arise from the protopodite.