there is still a lot of unexplored. Another thing is interesting. This jellyfish - it was called Blackfordia (Blackfordia) after 15 years was a floodplain in the Black Sea. Neither in the Mediterranean Sea, whose fauna is very well known, nor on the European coast of the Atlantic Ocean does this species live. How did the American Blackfordia end up in the Black Sea? The second incident happened to everything recently. One of the species of hydroids that live in the Kiel Canal is bougain villia. Again, unexpectedly, B was discovered in the Black Sea. Both blackfordia and the mentioned Baltic hydroid (Bougainvillia megas) are brackish water species; in order to get from one basin with low salinity to another, they must, like the Cordylophora, overcome the obstacle of the sea with its high
salinity. Before the construction of the canal between the Volga and the Don in the Caspian Sea, there were only two types of coelenterates - the Caspian Merizia and Cordylophora. When the canal was ready and navigation began, three more species moved from the Azov-Black Sea basin to the Caspian Sea. A year after the canal was put into operation, blackfordia moved to the Caspian Sea, a year later the Black Sea merizia, and after it the Baltic hydroid (Bougainvillia megas), which had recently entered the Black Sea from Kiel's bay. Of course, not only coelenterates travel this way, but also mollusks, and crustaceans, and worms, and other brackish-water organisms.
"SAILING FLEET" OF INTESTINAL
The class of hydroids is divided into two subclasses - hydroids and siphonophores. We now turn to the description of these amazing pelagic colonial coelenterates.
A whole world of living beings lives on the verge of the two elements of water and air. On floating algae, fragments of blue wood, pieces of pumice and other objects, one can find a variety of
grown or firmly clinging animals. It should not be thought that they got here by chance "in distress." On the contrary, many of them are closely connected with both the water and the air environment, and in other conditions they cannot exist. In addition to such "passive passengers", here you can also see animals actively swimming near the surface, equipped with floats of variously arranged organs, or animals that are held using a film of surface tension of water. This whole complex of organisms (pleuston) is especially rich in the subtropics and tropics, where the destructive effect of low temperatures is not felt.
Above, when the action of stinging cells was discussed, the "Portuguese warship" large siphonophore physalia was already mentioned.
Like all siphonophores, physalia is a colony, which includes both polypoid and honeycomb individuals. An air bubble, or pneumatophore, rises above the surface of the Water, a species-modified medusoid individual of the colony. In large specimens, the pneumatophore reaches 30 cm, it usually has a bright blue or reddish color. An air bubble floats on the surface of the sea like a tightly inflated rubber balloon. The gas filling it is similar in composition to air, but differs in an increased content of nitrogen and carbon dioxide and a reduced amount of oxygen. This gas is produced by special gas glands located inside the bubble. The walls of the pneumatophore can withstand rather strong gas pressure, since they are formed by two layers of ectoderm, two layers of endoderm, and two layers of mesoglea. In addition, the ectoderm secretes a thin chitinoid membrane, due to which the strength of the pneumatophore also significantly increases, although its walls remain very thin.
