156 UNSEGMENTED "WORMS." 



well-defined mesoderm, and a ccelom or body cavity lined 

 with mesoderm and distinct from the gut. It must be noted, 

 however, that the appearance of a well-developed ccelom 

 and mesoderm is very gradual; thus there is practically 

 no ccelom in the Platyhelminthes, and the mesoderm is 

 only represented by " mesenchymatous " cells, comparable 

 to those of Ctenophora. 



Class Turbellaria. Planarians, etc. 



Turbellarians are unsegmented "worms," living in fresh, 

 brackish, or salt water, or in moist earth. Almost all are 

 carnivorous, a few are parasitic. They represent the 

 beginning of definite bilateral symmetry. 



The ectoderm is ciliated, and contains peculiar rod-like 

 bodies (rhabdites), and occasionally stinging cells. A pair of 

 ganglia in the anterior region give off lateral nerve cords, and 

 there are usually simple sense organs. The food canal has a 

 protrusible muscular pharynx, is often branched, and is 

 always blind. There are no special respiratory or circulatory 

 organs; the body cavity is represented at most by small 

 spaces ; the excretory system usually consists of two longitud- 

 inal canals, whose branches end internally in ciliated (flame) 

 cells. Excepting two genera, the Turbellarians are her- 

 maphrodite ; and the reproductive organs usually show some 

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

 which may have arisen as an over-nourished (hypertrophied) 

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

 cocoons, and the development may include a metamorphosis. 



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 lateral 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 elongated and more worm-like in shape ; they may 

 measure a foot or more in length, and are most abundant in tropical 

 countries. 



There seems little doubt that the next two classes (Trematoda and 



