404 The Animal Kingdom 



The Classes of Platyhelminthes. — There are three recognized 

 classes in this rather diverse phykim of worms. They are : 



Class I. Turbellaria. These free-living worms have undivided bodies 

 covered with a soft ciliated epidermis. An intestine is usually present. Example : 

 Dugcsia. 



Class II. Trenialoda. Parasitic flatworms without cilia or epidermis, but 

 covered with a cuticle which is provided with suckers for attachment. The bodies 

 are undivided, and they have a digestive tract. Many have complex life cycles 

 involving more than one host. Example, Fasciola hcpatica, the sheep liver fluke. 



Class III. Cestoda. Parasitic flatworms which are without an epidermis 

 or cilia, lack a digestive tract, and have the body covered with a cuticle. The 

 body is divided into segments which usually remain together as a long chain. 

 One of these segments is modified for attachment to the host. Often these forms 

 have complicated life cycles involving more than one host. Nearly all are 

 parasitic only in vertebrates. Example: pork tapeworm of man, Taenia solium. 



THE CLASS TURBELLARIA 



The members of this class are usually small, free-living forms which 

 abound in both fresh and salt waters. A few forms are terrestrial but 

 have high moisture requirements, and are confined to tropical and sub- 

 tropical areas. 



The members of the common fresh- water genus, Ditgesia, are often 

 studied as representatives of this class. This is the form referred to as 

 planaria which formerly bore the generic name of Euplanaria. 



The Anatomy of Planaria.— The members of the genus Dugesia 

 are small, gray, flattened worms of about 15 mm. length. The head end 

 is easily distinguished as it is somewhat triangular in shape and has 

 small lateral projections. The eyes are placed close to the median 

 line on the dorsal surface of the "head." The mouth is ventral, near 

 the central portion of the body. The protrusible pharynx is extruded 

 through the mouth to obtain food. The genital pore is just posterior 

 to the mouth. On the dorsal surface are minute excretory pores which 

 are located near the lateral margin, but are very difficult to see. 



The body wall of planaria consists of the outer single-celled epi- 

 dermis formed of cuboidal cells with cilia on their ventral surfaces. 

 Possibly these cilia aid in locomotion. Opening between the cuboidal 

 cells of the epidermis are the ducts of many deep-lying gland cells. 

 Other cells contain rodlike bodies known as rhahdites, whose function 

 is unknown. The cells containing these rhahdites are usually deep- 

 lying, but the opening is on the surface of the body. Beneath the epi- 



