Metameriya can be full and incomplete. Metameriya is full in case repetition covers all organization of an animal, and incomplete if it covers one devices, one segments of bodies and does not cover others.
Jellyfishes and their relatives from type coelenterates, and also the lowest worms are deprived of the real metamer (. However it either polymeric bodies, or a psevdometameriya, that is, existence of false segments are peculiar to much. At such
Further division of a body of animals was possible thanks to complication of nervous system and, mainly, emergence of blood system. Isolation of branchings of nerves (with formation and blood vessels (with formation became possible.
Except the usual metameriya caused by existence of a simple axis of transfers in the organic world two more more rare meet like a metameriya: one with existence of a screw axis of transfer, another – a combination of forward symmetry, existence of the plane of the sliding reflection.
Metameriya can arise in the organization of animals in several ways: metameasured streamlining of originally chaotically located gomotipny parts, (the parts belonging to the same type); metamenrny differentiation lengthways originally uniform whole; connection of originally independent gomotipny parts. The first and second way of emergence of a metameriya have among themselves that general that here the metameasured whole arises from same whole or at all deprived of a metameriya, or possessing uncertain forward symmetry. In the third way the metameasured whole arises from set of not metameasured units of the lowest order independent among themselves.
The external structure of Crustacea can be very various. The body at them is dismembered on three departments: head (cephalon), breast (thorax), and paunch (abdomen). The structure of the head can have the features. On segments of the head various appendages which can be steam rooms (antennas and modified extremities) and unpaired (top lips can settle down. Segments of the head bear extremities which are homologous to parapodiya polikht. Other three segments of the head also bear extremities, but they usually change in oral parts and carries out functions of primary processing of food. Extremities of the second segment will be transformed to oral jaws. Extremities of chest segments also specialize. At many
The second big group of arthropods is represented by CHELICERATA. For a subtype of the helitserovy merge of head and chest segments therefore it is formed uniform, consisting of seven merged segments is characteristic. bears six couples extremities, first of which changed in helitser, the second pair of extremities changed in pedipalpa. Other four couples are hodilny feet.
During evolutionary development there were species of flat worms which increased length of the body, letting grow a segment behind a segment. These beings simply added the next muscular ringlet. But then changes captured all internal structure of worms. Their bodies also began to grow parts. As a result there were animals with external and internal partitioning of a body in which almost each new segment in general copies the previous. Such repetition of uniform segments of a body received the name of a metameriya in science.
Metameriya can be simple and difficult. Speak about a difficult metameriya when the metameasures making a body of an animal settle down the repeating groups constructed according to one plan so these groups are as if metameasures of the 2nd order.
Metameriya can be gomonomny and geteronomny. Metameriya true is only the metameriya. The Geteronomny metameriya arises in case metameasured educations are differentiated, and thus stop being quite similar among themselves, though keep the general plan of a structure.
The screw axis is widespread as an element of symmetry of a stalk at vascular plants. Thus the angle of rotation on one step of transfer (on one metameasure) determines by itself an order of screw symmetry. Among METAZOA screw symmetry in an arrangement of parts is found, for example, by heads of a gidroid of STAUROCORUNE FILIFORMIS. The extended head of a hydrant bears 7-9 metamerno of the located rings of golovchaty feelers. In each ring up to 4 feelers. Rings alternate so that everyone the overlying is turned on 45 degrees in comparison with the next.