ISO WORMS. 



Like other Plathelminthes (flat-worms), they are bilaterally 

 symmetrical, un-segmented, with lateral nerve-cords, and 

 with primitive excretory tubes. 



Their more precise characteristics are : — 

 (i) The ectoderm is ciliated, as is natural enough in free- 

 living simple animals, and it contains peculiar rod-like 

 bodies (rhabdites), and, very rarely, stinging-cells. 



(2) The nervous system consists of two ganglia in the 



head-region (which is obviously the region most 

 stimulated), and of two nerve strands running back- 

 wards, but not united in a double ventral nerve-cord; 

 there are usually simple sense-organs. 



(3) The food canal has a muscular pharynx, is often 



branched, and is always blind. They are carni- 

 vorous. 



(4) There is no development of special respiratory or 



circulatory organs ; the body-cavity is represented at 

 most by small spaces ; the excretory system usually 

 consists of two branched canals, ending internally in 

 ciliated cells. 



(5) With the exception of two genera, they are herma- 



phrodite, and the reproductive organs usually show 

 some division of labour, e.g., in the occurrence of a 

 yolk gland, which seems to be an over-nourished por- 

 tion of the ovary. 



Classification of noteworthy forms of Turbeltarians. 



Order i. Polycladidea. Marine Planarians. Large flat leaf-like 

 forms, with numerous ovaries and testes, without yolk-glands, 

 mostly with two genital apertures. The food-canal is much 

 branched. 

 e.g. , Cycloponis. 

 Leptoplana. 



Order 2. Tricladidea. Fresh- water, marine, and terrestrial Plan- 

 arians. Elongated flat forms ; the mouth and tubular pharynx 

 he behind the middle of the body; two ovaries, numerous 

 yolk -glands and testes, and a common genital aperture. The 

 food-canal divides into two main lateral loops, which are 

 themselves branched. 

 e.g. , Planaria and Dcndroccelum (in fresh-water) ; the former 

 sometimes divides transversely. 



