A GROUP OF TURBELLARIA. 47 



refractive particle of calcium carbonate the otolith (or statolith). 

 The statocysts serve as organs of equilibration. 



Reproduction is, in most cases, sexual. The animals are 

 hermaphrodite, but the male organs ripen first. The sexual 

 organs are very complicated, and the details of their structure 

 are of great value in classification. On this account it is often 

 impossible to determine the species of immature individuals, and 

 sometimes it is necessary to have specimens in both the male and 

 the female stages before identification can be certain. Fresh- 

 water Turbellaria undergo no metamorphosis, and newly hatched 

 individuals are similar to their parents in general appearance. 



Asexual reproduction occurs only in the section Hysterophora. 

 A chain of individuals is formed by the development of mouths, 

 eyes, etc., at intervals along the body. Constriction of the body 

 and gut then follow, and fresh individuals are produced by 

 fission. The process is illustrated in PI. 4, fig. 3. Some species 

 which reproduce asexually throughout the year develop sexual 

 organs in the autumn. These produce eggs which lie dormant 

 through the winter. 



Considerable interest has recently been aroused in certain 

 green or yellow cells which are found in the bodies of some 

 species of Turbellaria. The green cells contain chlorophyll and 

 are able to decompose carbon dioxide in the presence of sunlight. 

 Two marine species, Convoluta roscoffiensis and G. jmradoxa, 

 found on the coast of Brittany, have been the subjects of detailed 

 study, and the results have been summarised by Prof. Keeble in 

 a little book entitled Plant- Animals. The genus Convoluta 

 belongs to a group of Turbellaria, the members of which have 

 not, up to the present, been found in fresh water. The green cells 

 or zoochlorellae, as they are termed, are now regarded as algae 

 similar to Chlamydomonas. In the case of Convoluta it is 

 certain that the presence of zoochlorellae is of benefit to the 

 Turbellarian, and that the relationship is a true symbiosis. 



Von Graff (17) mentions twenty-five species of freshwater 

 Rhabdocoels in which green cells have been found. The fresh- 

 water species containing zoochlorellae have not been well 

 studied, and some zoologists doubt whether there is mutual 

 benefit in the association. This aspect of the subject will, 

 however, be dealt with later. 



The Rhabdocoelida live under various conditions, but generally 



