ZOOLOGY STUDY GUIDE - Ye. O. Nevedomska - 2013

LECTURE 12. PHYLUM ARTHROPODS (Arthropoda)

1. General characteristics of the phylum Arthropoda

The most numerous phylum of animals, with over 1.8 million species known, distributed worldwide, including about 25,000 species in Ukraine. Arthropods are integral PARTS OF THE most diverse biocenoses, inhabiting seas and oceans, freshwater bodies, soils, and terrestrial surfaces across all climatic zones from the tropics to polar regions. Even in the high latitudes of the Arctic and along the coast of Antarctica, individual species of mites and springtails have been discovered. Arthropods are also found in deserts. Many species are parasitic. The remains of extinct primitive arthropods—trilobites—have been preserved in deposits from the beginning of the Paleozoic Era.

The Classification of the phylum is presented in Table 8.

Class="center">Table 8

CLASSIFICATION OF THE PHYLUM Arthropoda

Typical Representatives of the phylum Arthropoda include insects, ticks and mites, myriapods, spiders, crayfish, and shrimp.

The Characteristic Features of arthropods include:

✵ the appearance of jointed appendages that elevated the animals' bodies above the substrate, reducing friction against the surface and enabling the Progressive development of locomotion Methods;

✵ a segmented body; the segments are unequal—heteronomous (from Greek heteros meaning other, nomos meaning part);

✵ the fusion and stabilization of the number of segments forming tagmata (regions): the HEAD, Thorax (or cephalothorax), and abdomen.

✵ the body of arthropods is covered with a cuticle consisting of Chitin (a complex polysaccharide) and Proteins; the cuticle Functions as an external Skeleton (exoskeleton) to which Muscles attach, and also protects the Organism from adverse environmental influences; arthropods lack a Skin-muscular sac;

✵ due to the presence of a cuticle, the growth of arthropods is accompanied by molting (ecdysis);

✵ The Emergence of striated musculature, which, compared to smooth musculature, possesses a significantly higher contraction capacity and ensures the diversity and high precision of movements;

✵ the body cavity in arthropods is mixed (mixocoel); it is formed As a result of the fusion of the coelom with the remains of the primary body cavity;

✵ The Structure of the digestive Organs in arthropods depends on their feeding type; what they have in common is that they consist of three regions: the foregut, midgut, and hindgut; the Mouth is surrounded by modified appendages—mouthparts that assist in capturing, grinding, and swallowing food; the foregut and hindgut are lined with cuticle, so Digestion and nutrient absorption occur primarily in the midgut; a set of glands is present;

✵ the Circulatory system is open, which is associated with the presence of a mixed body cavity in these animals, and is characterized by the appearance of a central pulsating organ—The Heart; an aorta and Arteries are present, from which Blood—hemolymph—pours into the body cavity and bathes the Internal Organs; blood enters the heart from the body cavity through openings in its walls called ostia; in some arthropods, the circulatory system is absent or represented solely by the heart;

✵ the Respiratory system: in aquatic forms, gills; in terrestrial forms, Lungs (modified appendages) and a tracheal system;

✵ the excretory system is represented by modified metanephridia or Malpighian tubules—thin tubes that are outgrowths of the posterior part of the midgut; they absorb dissimilation products from the blood and excrete them into the hindgut;

✵ The Nervous system consists of a paired supraesophageal ganglion, a circumesophageal nerve ring, and a ventral nerve cord;

the level of development of this system is characterized by the complication of the supraesophageal ganglion forming a Brain and the concentration of the ganglia of the ventral nerve cord;

✵ arthropod Sense Organs are diverse and sophisticated; most of these animals possess simple or compound (facet) eyes, with some species having both; compound eyes consist of numerous ommatidia, each of which sees only the part of the object directly in front of it; overall, the compound eye forms a direct mosaic image of the object; simple eyes are designed to distinguish light intensity; arthropods have well-developed organs of Touch, chemical sense, statocysts, etc.;

✵ in arthropod behavior, alongside simple unconditioned Reflexes of the taxis type, complex forms of instinctive behavior are observed, associated with food procurement, reproduction, offspring care, and defense against enemies; these animals possess The ability to form conditioned reflexes;

✵ arthropods are predominantly dioecious, and occasionally hermaphroditic; Sexual Dimorphism is frequently observed;

✵ Fertilization is mostly internal, although some species exhibit parthenogenesis (The Development of an unfertilized egg);

✵ development can be either direct or involve metamorphosis.