RHABDOCOELIDA 43 



(2) Rhabdocoela, which possess a complete alimentary tract 

 separated from the body-wall (except for a few suspensory 

 strands) by a space or body-cavity, filled with fluid. This space 

 is sometimes ( Vortex viridis) lined by an endothelium of flattened 

 parenchymatous cells. There are two compact testes, which are 

 enclosed (as are the ovaries and yolk -glands) in a distinct 

 membrane. An otolith is present in some genera and species. 

 Terrestrial, fresh-water, marine. 



(3) Alloeocoela, in which the body-cavity is greatly reduced. 

 Except in the Bothrioplanidae, the gonads have no distinct mem- 

 brane. Testes numerous ; yolk-glands present. Marine with a 

 few exceptions. 



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. 1 

 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, 2 and 

 it appears from his researches that the algae form a special assimi- 

 lating tissue, enabling the Convoluta to live after the fashion of 

 a green plant. At Eoscoff, 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 sun- 

 light 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 con- 



1 Bohmig, Ergcbnissc d. Plankton Expedition, Bd. ii. H. g. 1895. 

 2 von Graff, Die Acoela, Leipzig, 1892. Appendix. 



