Thuridilla bayeri is a Sacoglossan with a smooth body. Parapodia of this species cover the body completely an meet in a straight line along the dorsal midline (Marshall 1999). T. bayeri is known for its distinct colouration (Cimino 1998). The parapodia extended from the neck to the anterior end. The body are generally with dark background with thin or broad, longitudinal, white-cream lines. Blue spots are very common, however, spots may be lacking in some occasion (Jensen 1992). Inner margen of the parapodia is brightly orange-red. The end of the posterior end, the front of the foot, and the tips of the rhinophores are also often brightly orange in colour. All integration is possible (Jensen 1992). The rhiniphores are always rolled and long.  They also usually have a rounded tail. The ciliated groove is present anteriorly to guide the egg mass during spawning period (Jensen 1992).

The integration of colour and pattern may lead to different colour morph form e.g. T. ratna and T. gracilis (Marshall 1999). Different morphology found in different location. For example, T. bayeri is common and can be seen throughout the year on Heron Island. However, no T.ratna were found during the sampling period. T. ratna, on the other hand, found more in South-East Asia e.g. Thailand, Philippines, and Indonesia. This might be the result of parallel evolution. However, the present evidences and information have not explain the parallel evolution of this species yet. Therefore, study should be done for better understanding of body colour and parallel evolution including factors that influences it.