TURBELLARIA. 171 



tion, and occasionally with stinging cells. A pair of ganglia 

 in the anterior region give off lateral nerve-cords, and there 

 arc usually simple sense organs. The food canal has a pro- 

 trusible muscular pharynx, is often branched, and is always 

 blind. There are no special respiratory or circulatory organs ; 

 the body cavity is not represented, unless it be by intercellular 

 lacuncz in the parenchyma ; the excretory system usually con- 

 sists of two longitudinal canals, whose branches end in- 

 ternally inflame cells. The Turbellarians are almost always 

 hermaphrodite ; and the reproductive organs usually shoiv some 

 division of labour, e.g. in the development of a yolk gland, 

 which may have arisen as an over-nourished (Jiypertrophied] 

 part of the ovary. The eggs are usually enclosed in shells or 

 cocoons, and the development may include a metamorphosis. 

 Some forms multiply by fission. There seem to be undeniable 

 affinities between Turbellaria and Ca'lentera, especially the 

 Ctenophora. 



The Turbellarian worms form an exceedingly interesting group ; they 

 are often beautiful, and the ciliated ectoderm enables them to move with 

 singular grace. Although the bilateral symmetry and the distinction of 

 anterior and posterior ends is quite marked, the "mouth" or single 

 opening of the food canal is often near the middle of the ventral surface. 

 The anterior region is usually furnished with tactile processes. The 

 shape of the body in the aquatic forms is flattened and leaf-like, as in 

 the delicate Leptoplana, the "living film" found on the shore-rocks. 

 Fresh-water forms are usually small and often minute, but those living 

 in the sea may attain a length of six inches. Land Planarians are elon- 

 gated and more worm -like in shape ; they may measure a foot or more 

 in length, and are most abundant in tropical countries. Some, like 

 rianaria, have so much regenerative capacity that half a dozen or more 

 may be produced by cutting one into pieces. 

 Classification. - 



Order I. Rhabdoccelida small fresh-water and marine forms. 

 The food canal is very slightly branched, or quite straight or 

 blocked. 



Rhabdocrela. With straight intestine, e.g. Microstoma, a fresh- 

 water genus with stinging cells. It is first male and then 

 female (protandrous hermaphrodite) ; it forms temporarily 

 united asexual chains, sometimes of sixteen individuals, sug- 

 gesting the origin of a segmented type ; Graffilla and Anoplo- 

 diiun, parasitic on Gastropods. 



Alloiocrela. With irregular creca on the gut, e.g. Allostoma. 

 All marine except one from Swiss lakes (Plagiostoma 



Accela. Without intestine, e.g. Convoluta, which contains green 

 cells, regarded by some as symbiotic Alga.-. 



