There are a huge number of highly organized creatures on the planet that live not only on land, but also in the water column. One of these representatives are molluscs.
general characteristics
The class of cephalopods, or Cephalopoda, is also called gastropods. It has about 700 species that live in water bodies or at their bottom. The class is divided into two subclasses. The first includes the extinct ammonites and nautilus, which represent tetragill. The second includes cuttlefish, squid and octopus. These representatives represent a subclass of two-gill.
As a rule, the body of mollusks is characterized by bilateral symmetry. In this regard, the head and body are distinguished. The shell is present only in ancient forms, while in other representatives it is rudimentary. From above, the entire body of the mollusk is covered with a mantle, which consists of one layer of epithelium and connective tissue. Some species may have chromatophores, thanks to which the body can change its color.
Digestive system
The digestive system of cephalopods has a rather complex structure. But the pharynx deserves special attention, which is equipped with horny jaws in the form of a beak. In addition, salivary secretions that are secreted into the pharyngeal cavity are quite poisonous. Thanks to them, the mollusk immobilizes its prey.
The stomach is a sac that flows smoothly into the hind gut. There is also an ink bag in which ink is produced, thanks to which the mollusk has the ability to hide in case of danger. The digestive system ends with the anus.
Excretory and circulatory systems
The excretory system includes two or four kidneys. As a rule, their number depends on the representative. The circulatory system is represented by a heart with one ventricle and two atria. The two-gill representatives of the atria have two, and the four-gill representatives have four.
Sense organs
The sense organs are represented by osfadia and eye sacs. In two-gill representatives, instead of osfadia, there are olfactory pits, and as an organ of vision - compound eyes, which resemble the structure of eyes in mammals.
Genitals and skeleton
All cephalopods are dioecious organisms. Their fertilization is spermatophores, that is, spermatophores act as germ cells. The development of the offspring takes place in the egg, which is located in the mantle cavity of the female before hatching. The skeleton of most cephalopods is represented by a cartilaginous skull.
Nervous system
A feature of molluscs is their complex nervous system. It is represented by the brain. In this regard, animals lend themselves to training, have a very good memory and even distinguish geometric shapes.
