What are the main features of the similarities and differences in the external and internal structure of mollusks and annelids?
Similarities in structure:
two-sided symmetry of the body;
secondary body cavity;
digestive system: mouth, pharynx, esophagus, stomach, intestines and anus;
sense organs: balance, touch, sight and chemical sense;
gender: hermaphrodite or dioecious;
fertilization: internal or external;
development: indirect or direct.
Differences in structure:
The body of mollusks is not segmented, the head, trunk and legs are distinguished. In annelids, it is divided by segments (rings), it is made up of the end of the head, trunk, anus.
Body integuments. In mollusks, the body is surrounded on the sides by a mantle (fold of skin), inside which is a mantle cavity; with the help of glandular cells of the mantle, substances are released, from which a protective turtle is formed. In annelids – a bag of skin and muscles, which is formed from a thin cuticle, two layers of muscles – longitudinal and annular and unilamellar epithelium; there are many glands in the integument that secrete mucus.
The body cavity in mollusks is small, between the internal organs it is filled with parenchyma (connective tissue). In annelids, it is covered with a layer of epithelial cells that form partitions between the segments.
Respiratory system. Molluscs have gills or lungs. Ringed worms breathe over the entire surface of their body.
The circulatory system in mollusks is not closed, the movement of blood occurs between organs and through the vessels; the heart is represented by a ventricle and one or two atria. In annelids, it is closed, it is represented by the vessels – dorsal and abdominal, united by an annular in each segment.
Allocation system. In molluscs, it is represented by kidneys, which with one opening open into the cavity surrounding the heart, and the other into the cavity of the mantle. In annelids, excretory tubes (metanephridia) are located in pairs in segments.
Nervous system. In mollusks, there are three (head, trunk and lower) paired nodes of nerves, united by nerve trunks. In annelids, the abdominal chain of nerves, the suboesophageal, and supraopharyngeal nerve nodes.
