It is complexly represented by skeletal needles consisting of silica or strontium sulfate. Skeletal needles are riveted to the outer layer of the cytoplasm with the help of muscle fibers capable of contracting or lengthening.
The external skeleton is produced by skin cells and consists of a chitin polysaccharide, which is why it is called a chitinous cover.
Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.