OCCURRENCE AND HABITS OF THE RHABDOCOELIDA.—The _Acoela_ are usually minute, active Turbellaria abounding amongst weeds throughout the lower half of the Littoral, and the whole of the Laminarian zone, but are most plentiful in the pools exposed during spring-tides on our coasts, especially on the shores of Devonshire. The species of _Haplodiscus_, however, and _Convoluta henseni_ are modified pelagic forms found in the Atlantic Ocean.[61] _Convoluta paradoxa_ (Fig. 19, B) is the commonest British species. It is from 1 to 9 mm. in length, and of a brown colour, marked above by one or more transverse white bars. The brown colour is due to a symbiotic alga, the nature of which has not been thoroughly investigated. In an allied species, however (_C. roscoffensis_), from the coast of Brittany, the alga, which is here green, has been carefully examined by Professor Haberlandt,[62] and it appears from his researches that the algae form a special assimilating tissue, enabling the _Convoluta_ to live after the fashion of a green plant. At Roscoff, these elongated green _Convoluta_ live gregariously in the sandy tide-pools, fully exposed to the sun's rays, and have the appearance of a mass of weed floating at the surface of the water. Access to the atmosphere and to sunlight are necessary in order to enable the assimilating tissue to form the carbohydrates, upon which this form lives exclusively. Not only has the alga itself undergone such profound changes (loss of membrane, inability to live independently after the death of the host) as to disguise its true nature (a tissue-cell derived from algal ancestors), but the _Convoluta_ has also undergone {44}concomitant changes, in form, in the loss of a carnivorous habit, and in the development of marked heliotropic movements, thus adapting itself to an holophytic or plant-like mode of nutrition. Nevertheless the Acoela, as a group, are carnivorous, feeding upon Diatoms, Copepoda, and small Rhabdocoela, the absence of a digestive tract indeed being probably more apparent than real.[63]