The true nudibranchs are divided into two suborders – Suborder Cladobranchia and Suborder Doridoidea.
Without doubt the most noticeable external difference between the two is in the appearance of the gills. Here we must generalise of course with the dorids having a circle of gill branches around the anus, sometimes presenting as an arc or line of gills anterior to the anus, and located posteriorly on the dorsum.
The cladobranchs on the other hand have lost this “anal gill” arrangement. Instead, most possess secondary structures for respiration (not true gills) most usually down the length of the dorsum and the anus is relocated to the right side of the body wall. These anatomical differences are well illustrated in the accompanying images.
Top: The anal pore on the right side of the body wall of the dendronotid, Tritoniopsis elegans is indicated with an arrow. The animal is caught here in a scatological moment, about to pass a faecal pellet. It has also just laid its spiral of spawn on the substrate. The fine branching of the many cerata located down each side of the dorsum are acting in a respiratory capacity.
Bottom: The anal papilla of the dorid, Hypselodoris obscura can be seen here surrounded by the gill branches and located posteriorly on the dorsum. This arrangement in the dorids is often referred to as the anal gill. David A. Mullins September 2019