With the exception of _A. mediterranea_ Lamx. (fig. 34) the few living species of this genus are confined to tropical seas. The habit of _Acetabularia_ is well illustrated by the photograph of a cluster of plants of _A. mediterranea_ Lamx.[294] reproduced in fig. 34. The thallus consists of a delicate stalk attached to the substratum by a tuft of basal holdfasts, and expanded distally into a small circular disc 10–12 mm. in diameter and more or less concave above. This terminal cap is made up of a number of laterally fused appendages given off from the upper part of the stalk in the form of a crowded whorl. The whole thallus resembles a small and long-stalked calcareous fungus. In each radially elongated compartment of the fertile cap (fig. 33, I) there are several sporangia (_gametangia_) developed; these eventually open and produce numerous ciliated gametes which give rise to zygospores by conjugation. Fig. 33, I, represents the cap of an _Acetabularia_ in radial section and surface-view; the two radial compartments seen in section contain the elliptical gametangia; the circular markings at the base of the figure are scars of sterile deciduous branches. The whole plant is unicellular, each chamber in the disc being in open communication with the stem of the plant. _Acicularia._ Fig. 33, C–H. In a recent monograph on the Acetabularieae, Solms-Laubach[295] has described a new type of these algae which is of special importance from the point of view of the past history of the family. Möbius described an example of _Acetabularia_ in 1889 under the name _A. Schencki_; this species has since been placed in D’Archiac’s genus _Acicularia_[296]. _Acicularia Schencki_[297] bears a close resemblance as regards external form to _Acetabularia mediterranea_. In the latter species the walls of the terminal disc compartments are calcified, and the cavity of each of the laterally fused members contains numerous free spores; in _Acicularia_, the cavity of each disc-ray is occupied by a calcareous substance in the form of a spicule containing numerous cavities in each of which is a single sporangium. A single spicule is seen in fig. 33, H, showing the spherical pockets in which the sporangia were originally situated. This species, _Acicularia Schencki_, has been recorded from Martinique, Guadeloupe, Brazil, and a few other places.