ABSTRACT
In most brooding echinoids, the young are housed in a shallow depression on the outside of the test. Urechinus mortenseni, a holasteroid of the family Urechinidae, broods its juveniles inside a deeply invaginated extension of the body wall suspended from the interior edges of the female’s apical plates. Although technically not inside the test, the juveniles are completely invisible from the exterior, and develop in a brooding system consisting of an apical aperture, birth canal, and brood pouches. Only a few (generally < 5), very large, long-spined juveniles are present in the system at one time. The salient characteristics of the brooding system found in U. mortenseni are contrasted with those of brooding strategies in other echinoids because they suggest a particular reproductive behavior. Regardless of the precise site of fertilization, there can be no doubt that U. mortenseni undergoes a process analogous to internal fertilization. This leads to an essentially internal development which results in juveniles much larger than the apparent size of the apical aperture in the center of the apical system. To allow passage of the juveniles to the exterior of the female, it is necessary for the apical plates to move, thereby enlarging the apical aperture. Topologically, this is analogous to “birth”.
