436 zooLOGf. 



the common characters. They are elongated, bilaterally 

 symmetrical animals, not nietamerically segmented ; the 

 alimentary canal has no anus and may be absent entirely. 

 Coelom represented by the cavities of the generative 

 organs, which form a complicated system of hollow organs 

 and ducts, usually hermaphrodite; usually a number of 

 yolk glands distinct from the ovaries. Nephridia, a single 

 pair of branched tubular organs, the branches ending 

 internally in flame-cells. Blood-vessels absent. 



The Nemarteans were formerly included in the Platyhel- 

 mia, and there are reasons for thinking that they are allied 

 to this phylum. But they differ in the remarkable eversible 

 proboscis, which lies above the alimentary canal, in the 

 possession of an anus, and of blood-vessels, and in the 

 simple character and rnatameric arrangement of the genera- 

 tive organs. Many of them are extremely long and 

 flexible ; they have no external signs of segmentation. 



Apart from the Nemertinea the Platyhelmia comprise 

 the Turbellaria, the Trematoda, and the Cestoda or tape- 

 worms. The last group are all internal parasites, the 

 second are either internal or external parasites, while the 

 Turbellaria are free-living and inhabit the sea or fresh 

 waters ; a few, however, live in moist soil in the tropics. 



CLASS : TURBELLARIA. 



Mostly small animals flattened dorso-ventrally and not 

 much elongated, ciliated over the whole of the external 

 surface ; by means of these cilia they glide along at the 

 bottom of the water or on the lower side of the surface 

 film; they can also swim by undulations of the body. 

 The mouth is usually on the lower surface of the body 

 and leads into a muscular pharynx connected with a blind 

 intestinal cavity which is either simple (Rhabdocoelida), 

 or consists of a median anterior division and two paired 

 posterior divisions (Tricladida), or is complexly branched 

 (Polycladida). In a few species the digestive cavity is 

 absent and the endoderm is a solid mass of cells (Acoela). 

 In the Polycladida there are numerous testes and ovaries 

 but no separate yolk-glands, and male and female genital 

 aDertures are usually separate. In Tricladida there are 



