246 CLASSIFICATION OF ANIMALS 



developed, of both sexes. A "ripe" proglottis contains a large number of 

 fertilized eggs. It is cut off from the main chain and passes from the host 

 with the faeces. The embryonic stages develop in a second host as is 

 described in the case of a typical example in Part III. Page 367. 



538. Class n. Aschelminthes. — The animals comprised in 

 this class have usually a cylindrical body and a simple tubular 

 digestive tract opening posteriorly by a vent. A false body 

 cavity originating from the blastula cavity is often of consider- 

 able size. The sexes are usually distinct. 



539. Order i. — The Rotatoria are small, mostly microscopic, free-living 

 animals. They are found chiefly in fresh waters. The anterior end of 

 the body is provided with a contractile crown of cilia by which locomotion 

 is effected. At the posterior end there is usually a stalk-like "foot" 

 which is provided with adhesive glands. By means of this foot the 

 animal attaches itself temporarily. The name Rotifer has reference to 

 the apparent revolution of the crown when the cilia are in motion. The 

 currents produced by the cilia carry food particles to the mouth which 

 lies in the centre of the crown. The oesophagus opens into a stomach 

 which is provided with a set of cuticular teeth. A pair of excretory 

 tubules opens into the posterior end of the intestine. 



540. Order 4. — The Nematoda are in part free living, in part parasitic. 

 They are often called thread worms or round worms. The mouth is at 

 the anterior end. There is then a sucking oesophagus and a simple 

 tubular intestine which opens on the ventral side near the posterior end. 

 Special sense organs are practically wanting. There is a nerve ring around 

 the mouth and from this a pair of nerves, one dorsal, one ventral, extend 

 the length of the body. There is a pair of excretory tubules, one on either 

 side, which extend from end to end of the body. The life histories of 

 several parasitic forms are described in Part III. 



541. Class rV. Nemertini. — This group is composed chiefly 

 of free-living marine forms. The body is much elongated and 

 muscular. The epidermis is ciliated. There is a long eversible 

 proboscis and the intestine opens posteriorly by a vent. Eyes 

 are often present and in large number and there is a pair of 

 sensory grooves on the head. The nervous system consists of 

 a supra-oesophageal ganglion and a sub-oesophageal ganglion. 



