Vertebrates

VERTEBRATES (Vertebrata), a term introduced by Lamarck (1801) to denote all animals in whose skeleton there is a vertebral column or spinal column consisting of vertebrae. Vertebrates are contrasted with all other animals as invertebrates (Avertebrata). However, the significance of these two terms is by no means equivalent. "Vertebrates" denotes one type or subtype of animals, whereas invertebrates include all other types of animals (arthropods, mollusks, worms, echinoderms, coelenterates, etc.). Consequently, only the term "vertebrates" or Vertebrata has taxonomic significance, while "invertebrates" is a collective, non-systematic term. V. in the proper and exact sense of the concept include animals with a vertebral column, whether it is constructed of cartilaginous or bony elements. In this sense, the classes belonging to V. are: cyclostomes (Marsipobranchii), fishes (Pisces), amphibians, or naked reptiles (Amphibia), reptiles, or scaly reptiles (Reptilia, s. Sauria), birds, or feathered animals (Aves), and mammals (Mammalia). The vertebral column is, in origin, a secondary internal mesodermal skeleton, which embryologically develops in the connective tissue-mesenchymal sheath of the dorsal string or notochord (chorda) (see Spine, comparative anatomy). V. compared to other types are highly developed animals, and man belongs to V. Phylogenetically they appeared later than other types of animals. The study of the embryonic development of some animals lacking a vertebral column showed that they have a dorsal string in early stages of development, which either completely reduces or remains throughout life as a supporting spindle-shaped formation, but without parallel development of elements of the mesodermal internal skeleton. Great theoretical importance is attached to the laying down of the notochord, and on this basis the type Chordata is distinguished. The type Chordata includes animals constructed on bilateral symmetry, having permanently or temporarily a cellular entodermal skeletal axis arising from the dorsal part of the wall of the primary intestine (notochord). Above the notochord is the central nervous system, laid down in the form of an invagination of the outer germ layer on the dorsal side of the embryo. A circulatory system is present; the heart or equivalent segment of the system lies ventrally. In the entodermal part of the anterior intestine there are paired gill slits (organs of aquatic respiration). The type Chordata is divided into three subtypes: I subtype-tunicates (Tunicata, or Urochorda). The notochord rarely remains throughout life, as it reduces in adults. Similarly, the circulatory system reduces, from which only the heart and blood lacunae remain. Many tunicates lead a sedentary lifestyle. Due to strong reduction in organization, tunicates take on a very distinctive appearance. They were even distinguished as a separate type, which was placed after worms (Haeckel), others referred to them as an additional class to worms (R. Hertwig), still others distinguish them as a separate type (Ziegler). Tunicates live in the seas. II subtype-vertebrates without skulls (Agnatha, or Cephalochorda) in the form of very few representatives (Amphioxus-lancelet) have a large, permanently remaining notochord, but without rudiments of a vertebral column; these animals also lack a skull; the general appearance is fish-like, but without true limbs. The heart as a sac-like organ is reduced; it is considered that the ventral gill artery corresponds to it, and on this basis the lancelet is characterized as a tubular-heart animal (Leptocardii). The gill system is strongly developed. Remarkable is the presence of excretory organs of the nephridial type (as in many annelid worms). The lancelet lives in the seas. Due to important features of similarity in the architecture of the body, many authors unite the skullless with the skull-bearing into one subtype or type Vertebrata. III subtype-V. in the proper sense (Vertebrata, s. Craniota, or skull-bearing). There is always an internal skeleton of mesodermal origin, constructed from metameric arranged, formed vertebrae or their parts. At the anterior end of the body, the vertebral column is transformed into a skull, which is a container for the brain; in connection with this, in V. the process of cephalization (i.e., isolation of the head) reaches the greatest expression. Similarly, paired limbs are characteristic of V.; due to these features, the body of V. is divided into parts-head, trunk, and tail; limbs belong to the trunk. According to the nature of their architecture, the limbs of V. are divided into two groups: 1) undivided along their length, paddle-like organs of aquatic locomotion (fish fins) and 2) lever-like supporting limbs for movement on a solid substrate. Typically, there are four paired fins and terrestrial limbs. Terrestrial limbs, called legs, typically end in five fingers. Fingers and limbs can reduce to complete disappearance. Having phylogenetically developed in the sea as fish-like creatures, V. in the form of amphibians moved to land, maintaining a greater or lesser connection with the aquatic environment, but already having the characteristics of typical terrestrial animals. The next stage in the evolution of V.-reptiles-typically broke the connection with the aquatic environment, becoming characteristic terrestrial animals. However, some of them secondarily returned, partially or with changed organization, to the aquatic environment (e.g. ichthyosaurs). Reptiles gave rise to the most highly organized V.-namely birds, specialized in conquering the aerial environment for locomotion (transformation of the anterior pair of limbs into wings), and mammals-with the most highly developed central nervous system, to which man also belongs.-If within the subtype of vertebrates the phylogenetic relationships of these animal classes are clear, then the relationships of V. and chordates to other types of animals are interpreted differently. Of the existing theories, the recognition of the origin of chordates from oligomeric worms (enteropneusts-Balanoglossus) has greater weight. The study of comparative anatomy, embryology, and paleontology of V. provides important material for understanding the origin of man.

E. Pavlovsky.