The study of the cephalic capsule of Caecosphaeroma burgundum, a subterranean waters Isopod, demonstrates improved adaptations to volvation; these concern in a similar manner the other regions of the body, particularly the pleotelson. From a primitive aquatic Isopod structure, the head of this blind Spheromid has been completely fashioned by many mechanical factors: posterior margin of pleotelson providing support on the head, relation of anterior angles of the second pereionit and, above all, the mandibular palps and antennae which retract into two deep grooves of the face. The comparative study of the head of other volvational Isopods shows the importance of that “antennary factor”, e.g. in Oniscoids, epigean Spheromids and some other subterranean waters Isopods (two Spheromids and one Cirolanid). This comparison shows that Caecosphaeroma burgundum is certainly the most specialized of all; it approaches perfection in volvation for it is the only one which rolls up into a hermetic sphere without outwards projections. Volvation seems to play a two-fold role. It is a mean of defence against predators used by single specimens and by copulating pairs, males and females being then associated in two concentric spheres. Furthermore, it is a very important way for passive dissemination allowing settlement of these Crustacea in distant subterranean waters.