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.
However, during normal development several vertebrae fuse together, leaving a total of 24, in most cases. The confusion about whether or not there are 32-34 vertebrae stems from the fact that the two lowest vertebrae, the sacrum and the coccyx, are single bones made up of several smaller bones which have fused together. This is how the vertebrae are counted: 24 separate vertebrae and the sacrum, formed from 5 fused vertebrae and the coccyx, formed from 3-5 fused vertebrae.