Foale & Willan (1987) and Gosliner (1994) described in detail the fine structure of the caryophyllidia. The caryophyllidia are structures formed by the stretching of the epidermis over a framework of spicules. At the apex, there is a spherical knob (or ciliated tubercle), which bears several tufts of cilia. The concentration of these tufts of cilia varies between different species. The size and shape of the knob is also highly variable. Each caryophyllidium is supported by a variable number of spicules that emerge near the apex and radiate in circular crown. Even after they emerge at the apex, the spicules remain covered with a thin layer of epidermis. In several species, the spicules are longer than the ciliated tubercle, whereas in other species they are shorter. Also, the spicules may be free apically or remain attached to the ciliated tubercle for all of their length. In some cases, the spicules are almost surrounded by the cilia of the ciliated tubercle. The epidermis of the caryophyllidia is entirely covered with a dense mat of microvilli. In addition, the caryophyllidia have muscles and nerves associated with them. These muscles could confer some mobility and retractibility to the caryophyllidia (Labbé, 1929; Foale & Willan, 1987).
There are other species of cryptobranch dorids that have tubercles with protruding spicules. However, in these species the spicules are not organized in a circular pattern and there is no a central ciliated tubercle. On the other hand, Kress (1981) showed caryophyllidia-looking spiculose tubercles in two species of Onchidoris Blainville, 1816 (phanerobranch dorids). However, according to Foale & Willan (1987), because of the phylogenetic distance between the genus Onchidoris and the caryophyllidia-bearing dorids, any similarities in mantle structures should be attributed to evolutionary convergence. The fine structure of these caryophyllidia-looking tubercles of some phanerobranch dorids has not been investigated in order to determine possible homologies with the caryophyllidia.
Very often, the caryophyllidia are difficult to recognize in preserved specimens. The spicules can be easily dissolved by a number of fixation methods, and they are often damaged as the consequence of harmful collecting methods. Many of the deep-water specimens examined in this paper have the caryophyllidia partially destroyed. The presence of a ciliated tubercle is normally the best indication of the presence of caryophyllidia in old or poorly preserved material.