46 H. WHITEHEAD ON BRITISH FRESHWATER RHABDOCOELIDA 



an inch in length. They are generally found in ponds, lakes 

 and ditches, and less frequently in running water. Like many 

 other microscopic inhabitants of ponds, they appear in great 

 abundance at certain seasons of the year and then suddenly 

 disappear. 



The body is more or less transparent, slightly flattened, and is 

 provided with cilia. The Turbellaria are remarkable for peculiar 

 secretions given off from the epidermis. These secretions are of 

 two distinct kinds one a mucous fluid, and the other con- 

 sisting of very small solid bodies, or rhabdites, which, on coming 

 in contact with the water, produce mucus. Several forms of 

 rhabdites have been described (spindle-shaped, rod-shaped, egg- 

 shaped and spherical). They are formed in special glandular 

 cells which lie beneath the epidermis, and the rhabdites pass to 

 the surface by means of minute ducts. 



Another interesting feature is the presence, in certain species, 

 of nematocysts similar to those found in Hydra.* 



The Rhabdocoels are provided with a mouth, a pharynx and 

 an unbranched, sac-like gut. The position of the mouth varies 

 and affords a valuable generic character. It may lie at the 

 extreme anterior or in a median position anywhere along the 

 ventral surface as far down as two-thirds of the body length. 



The excretory system consists of renal organs which are, in 

 some cases, somewhat complicated in structure. 



The nervous system is simply, and comprises a two-lobed brain 

 and a pair of nerves running along the body close to the ventral 

 surface. In some species the pigmented eyes are clearly defined, 

 in others the eye pigment is scattered, and in some cases eyes 

 are absent. 



Some of the freshwater Rhabdocoels have at their anterior 

 end pit-like depressions which contain cilia (PI. 4, fig. 3, cp). 

 The ciliated pits rest upon a group of ganglion cells which are 

 connected with the brain. Similar structures are found in 

 Nemertine worms, and some zoologists consider that this 

 suggests affinity between the groups. Another interesting organ 

 is the statocyst, which is present in some species. This consists 

 of a cavity containing fluid, in which is suspended a highly 



* Mr. Scourfield has recently called my attention to a paper by C. H. 

 Martin (20) on this subject. The author shows conclusively that the 

 nematocysts are derived from the prey upon which the Turbellarian feeds. 



