THE ANIMAL KINGDOM 399 



Examples: Pleurobrachia pileus, sea comb; Cestus veneris, a 

 flattened, ribbonlike form; Mnemiopsis leidyi, the sea walnut 

 (Fig. 226). 

 CLASS II. NUDA. Tentacles and oral lobes absent. 

 Example: Beroe ovata. 



PHYLUM 5— PLATYHELMINTHES 



Platyhelminthes (flatworms) are the first forms in which a 

 definite "head end" is present. The process of cephalization 

 as pointed out in a preceding chapter (p. 170) involves the con- 

 centration of sense organs at the anterior end of the animal and 

 a corresponding concentration of nervous control mediated 

 through cephalic nerve ganglia. Usually the body in cross 

 section is an oval, flattened dorsoventrally. Flatworms occur 

 in water, moist earth, and as parasites in animals and plants. 

 The body is triploblastic, unsegmented and covered by a ciliated 

 epithelium or by an unciliated cuticle. An alimentary canal 

 may be present but not a body cavity, the space between the 

 alimentary canal and the body wall being filled with a loose 

 connective tissue known as parenchyma, of mesodermal origin. 

 The alimentary canal like that of coelenterates is usually a blind 

 tube lacking an anal opening. The excretory system consists 

 of numerous flame cells connected with tubes opening by single 

 or paired excretory pores to the outside of the body (Fig. 91). 

 There is no blood circulatory system, nor a respiratory system. 

 The nervous system consists of a pair of cephalic ganglia from 

 which nerves pass to various parts of the body. Reproduction 

 is sexual, but fission and budding are also common. Her- 

 maphroditism is the rule. The ovaries and testes are well- 

 developed internal organs provided with ducts leading to the 

 outside. 



CLASS I. TURBELLARIA. Ciliated, free-living forms, living 

 in fresh or salt water, or in moist soil. In most cases the cili- 

 ated glandular epidermis contains rhabdites, very small rodlike 

 bodies, produced by the epidermal gland cells or by the 

 parenchyma. They are discharged in the slimy secretion of 

 the ectoderm. Functional nettle cells, nematoblasts, are 

 sometimes found, but these have been shown to have been 

 acquired from coelenterates taken as food. Adhesive suckers 

 are present in some. The mouth is an opening at the end of a 



