Instances are recorded of algal colonies adhering to, and even penetrating, the thallus of lichens, but as they never enter into relationship with the lichen hyphae, they are antagonistic rather than symbiotic and have no relation to cephalodia. D. DEVELOPMENT OF CEPHALODIA _a._ ECTOTROPHIC. Among the most familiar examples of external cephalodia are the small rather dark-coloured warts or swellings that are scattered irregularly over the surface of _Peltigera_ (_Peltidea_) _aphthosa_. This lichen has a grey foliose thallus of rather large sparingly divided lobes; it spreads about a hand-breadth or more over the surface of the ground in moist upland localities. The specific name “aphthosa” was given by Linnaeus to the plant on account of the supposed resemblance of the dotted thallus to the infantile ailment of “thrush.” Babikoff[467] has published an account of the formation and development of these _Peltidea_ cephalodia. He determined the algae contained in them to be _Nostoc_ by isolating and growing them on moist sterilized soil. He observed that the smaller, and presumably younger, excrescences were near the edges of the lobes. The cortical cells in that position grow out into fine septate hairs that are really the ends of growing hyphae. Among the hairs were scattered minute colonies of _Nostoc_ cells lying loose or so closely adhering to the hairs as to be undetachable (Fig. 78 A). In older stages the hairs, evidently stimulated by contact with the _Nostoc_, had increased in size and sent out branches, some of which penetrated the gelatinous algal colony; others, spreading over its surface, gradually formed a cortex continuous with that of the thallus. The alga also increased, and the structure assumed a rounded or lentiform shape. The thalline cortex immediately below broke down, and the underlying gonidial zone almost wholly died off and became absorbed. The hyphae of the cephalodium had meanwhile penetrated downwards as root-like filaments, those of the thallus growing upwards into the new overlying tissue (Fig. 78 B). The foreign alga has been described as parasitic, as it draws from the lichen hyphae the necessary inorganic food material; but it might equally well be considered as a captive pressed into the service of the lichen to aid in the work of assimilation or as a willing associate giving and receiving mutual benefit. [Illustration: Fig. 78 A. Hairs of _Peltigera aphthosa_ Willd. associated with _Nostoc_ colony much magnified (after Babikoff).] [Illustration: Fig. 78 B. _Peltigera aphthosa_ Willd. Vertical section of thallus and cephalodium × 480 (after Babikoff).] Th. M. Fries[468] had previously described the development of the cephalodia in _Stereocaulon_ but failed to find the earliest stages. He concluded from his observations that parasitic algae were common in the cortical layer of the lichens, but only rarely formed the “monstrous growths” called cephalodia.