The fronds of the order Collemacei are gelatinous; the medullary stratum seems to occupy the whole, though in two species of the genus there is a distinct epidermic cellular coat. There are moniliform strings of minute gonidia in the gelatinous matter, and others that are single, arising from the division of each gonidium and its supporting thread into two: both kinds differ from the gonidia in other lichens. The fruit-bearing discs on the surface are open. Members of this order occur in Europe, Africa, and Australia. The order Coccocarpei is mainly distinguished by having orbicular discs entirely deprived of the cortical envelope called an excipulum, or, if it does exist, it is confounded with the thin membranaceous thallus. The discs spring at once from the medullary stratum, and contain asci and sporidia, similar to those of minute fungi (Sphæriæ). Some species of the genus Coccocarpia only differ from Lecidea in the total absence of an excidium. The order is chiefly parasitic, and in some cases the whole plant is little more than a mass of fructification, parasitic upon and continuous with the substance of other lichens, at whose expense they live, thus forming an exception to the general habit of lichens, which are fed by the atmosphere alone. M. Tulasne has discovered that the genera Abrothallus and Scutula, though consisting almost entirely of fruit, produce secondary spores—they are the only lichens in which they occur; while Phacopsis and Celidium bear spermatogonia, which, analogous to antheridia, contain minute fertilizing particles. The parasitic genera occur in most parts of Europe and North America.