st.c, Statocyst containing the minute tentaculocyst. nr1, Ex-umbral nerve-ring. nr2, Sub-umbral nerve-ring. ex, Ex-umbral ectoderm. sub, Sub-umbral ectoderm. c.c, Circular canal. v, Velum.] Besides the ordinary type of ocellus just described, there is found in one genus (_Tiaropsis_) a type of ocellus in which the visual elements are inverted, and have their cones turned away from the light, as in the human retina (fig. 30). In this case the pigment-cells are endodermal, forming a cup of pigment in which the visual cones are embedded. A similar ocellus is formed in _Aurelia_ among the Scyphomedusae (q.v.). Other sense organs of Hydromedusae are the so-called _sense-clubs_ or _cordyli_ found in a few Leptomedusae, especially in those genera in which otocysts are inconspicuous or absent (fig. 39). Each cordylus is a tentacle-like structure with an endodermal axis containing an axial cavity which may be continuous with the ring-canal, or may be partially occluded. Externally the cordylus is covered, by very flattened ectoderm, and bears no otoliths or sense-cells, but the base of the club rests upon the ex-umbral nerve-ring. Brooks regards these organs as sensory, serving for the sense of balance, and representing a primitive stage of the tentaculocysts of Trachylinae; Linko, on the other hand, finding no nerve-elements connected with them, regards them as digestive (?) in function. The sense-organs of the two fresh-water medusae _Limnocodium_ and _Limnocnida_ are peculiar and of rather doubtful nature (see E. T. Browne [10]). FIG. 38.--Ocellus of _Lizzia koellikeri_. oc, Pigmented ectodermal cells; l, lens. (After Hertwig.)