Characteristics: In all sea slugs (formerly known as Opisthobranchia) you will find the gills behind the heart. The sea slugs originate from the sea snails which have the gills in front of the heart. The shell is usually internal or completely lost at some point during evolution.The name Heterobranchia means different-gilled snails.he sea slugs originate from the sea snails which have the gills in front of the heart. The shell is usually internal or completely lost at some point during evolution. The name Heterobranchia means different-gilled snails.
The twelve thoracic vertebrae are numbered T1 to T12. The range of motion in the thoracic spine is limited. Lumbar (low back) - the main function of the lumbar spine is to bear the weight of the body. The five lumbar vertebrae are numbered L1 to L5.
In other animals the number of thoracic vertebrae can vary greatly; for example, most marsupials have 13, but koalas have only 11. 12 to 15 is common among mammals, with 18 to 20 in horses, tapirs, rhinoceroses, and elephants, and extremes in mammals are marked by certain sloths with 25 and cetaceans with 9.y convention, the human thoracic vertebrae are numbered T1-T12, with the first one (T1) located closest to the skull and the others going down the spine towards the lumbar region.