Members with remarkable lifestyle and features, colours and shapes, which rival thescience fiction beings of outer space (Coleman 2001). These molluscs may have reduced external shells, or no shells at all in their adult form. They may have exposed gills, hidden gills, or no gills at all (depending on the respiratory functions of the skin, or other parts of the body - such aspustules or cerata). Most have a well-defined head, one pair of rhinophores and at least one pair of oral tentacles. 

Shell
Cephalaspideans have a variety of shells: some families have shells large enough to hold the animal inside; others have reduced shells and some that only have a small, vestigial shell.

Hydatina physis has a reduced shell, making in incapable to retract inside. H. physis is sometimes referred to as a "paper bubble" due to its lightly calcified shells (Figure 11). 

Figure 11. The a) internal and b) external striations, c) width and d) cracked textured shell of Hydatina physis

Eyes on H. physis are reduced (Figure 12), and act only as light receptors.

Figure 12. Close up on H. physis eyespots as indicated by the black arrows. Photos taken using a dissection microscope at 10X magnification. 

Head Shield
Wedge-shaped, allowing the animal to burrow just below the sand surface to look for prey. 

Figure 14. Prominent anatomical features of H. physis 

Sensory and Nervous System

​Sight
Opisthobranchs have some form of eyes, however they are not used for sight, but instead act as light receptors (Coleman 2001). 

Smell
Opisthobranchs utilise their nose structure, the rhinophore, to smell. Rhinophores are able to detect chemical molecules in the water and are sensitive enough to distinguish between different types (Coleman 2001). In some species, rhinophores are also used as a method of tracking down food. Rhinophores are connected to the brain by a pair of large nerves. The nerve endings contain two types of cell structures: branched (dendritic) cells, which are involved in chemical reception, and ciliated cells that function as mechanoreceptors to sense vibrations or changes in water pressure. 

Rhinophores exist in different forms, distinguished by an array of shapes and colours unique to certain species.

Taste
H. physis lack taste buds, but employ their rhinophores to sense chemical compounds to distinguish between favourable and unfavourable foods, triggering a feeding reaction. 


Hearing 
"Hearing", or the ability to have a better sense of its surroundings through detecting vibrations and changes in pressure is achieved by ciliated cells in the rhinophores of H. physis. Opisthobranchs do not detect sound like humans do, though they do have otoconia or otoliths, the evolutionary precursor to human ear bones. These small, spherical calcareous structures are housed inside an organ called the statocyst. The "ear bones" of opisthobranchs are thought to function as gravitational or hydrostatic sensors, allowing the interpretation of spatial orientation rather than sound. 


Touch 
H. physis have tentacles on its head and oral region, which provide the species with a sense of touch. Similar to almost all opisthobranchs, H. physis reacts to touch, suggestion a system of nerves, sensitive to external stimuli that would extend over their bodies. 

Rhinophores (Figure 15) and gills exhibit a similar sensitivity to touch. 

Figure 15. The eye spots and rhinophore of Hydatina physis 

An additional tool that the order Cephalaspidea use to sense is through what is called Hancock's Organ. It is a chemosensory organ in primitive Cephalaspidean sea slugs, including H. physis. It is usually a pad or groove on each side of the head, but in the Hydatina species, it is made up of rows of folds or flaps (Figure 16). 

Figure 16. Hancock's organ in H. physis as indicated by the white arrow (Rudman, W.B., 2008).