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.
At the end of the spinal column is the coccyx or tailbone. Most people have 33 vertebrae in total, although there may be 32 or 34. Variations are usually found in the lumbar or sacral regions.