This order is dominated by the Aplysiidae family – the “sea hares”. Less well known and significantly different in appearance are the members of the Akeridae family.
The Aplysiidae are usually large and plump in appearance with a distinct head carrying a pair of enrolled and erect rhinophores and large enrolled oral tentacles. If the shell is not lost it is reduced to a plate that may be somewhat exposed covering the mantle cavity or wholly internal protecting the gill and heart.
The Akeridae however, are considered primitive with an external shell and headshield not unlike the cephalaspideans and in fact their external appearance and lifestyle is suggestive of those “head shield slugs”. The akerids possess an elongate body without rhinophores or oral tentacles and burrow through the substrate.
The attributes that both of these families have in common include the possession of two glands that separately secrete a dark ink and an unpleasant smelling clear fluid for defensive purposes, and a somewhat similar digestive system anatomy. All feed upon algae.