50 INVERTEBRATE ZOOLOGY 



Both free-living and parasitic forms are common. Though the 

 cell is commonly covered by a pellicle or cell wall, there are 

 definite openings for the ingestion of food and discharge of waste. 

 A cytostome leads through a cytopharynx down into the endo- 

 plasm and a cytopyge for the elimination of solid waste is present 

 though usually not observable except at the time of elimination. 



The Cytoplasm. — The cytoplasm is divided into an ectoplasm 

 and an endoplasm of which the former comprises a number of 

 rather sharply differentiated regions not uniformly encountered 

 in the various groups. The alveolar layer is the outermost and 

 is marked by a striated appearance due to the arrangement of 

 the alveoli of the cytoplasm. Beneath this lies the trichocyst 

 layer in which the spindle-shaped trichocysts are embedded with 

 their tips at the surface ready for discharge as long, stiff organs 

 of defense. A contractile layer underlies this one. It consists 

 of myonemes which ordinarily run parallel to the rows of cilia. 

 Between or external to the myonemes are found the basal granules 

 from which the cilia take their origin as they pass between the 

 alveoli of the alveolar layer to the outer surface. A spongy zone 

 of ectoplasm traversed by fluid-filled spaces and channels overlies 

 the endoplasm and with the contractile vacuoles and their 

 associated radial canals represents an excretory layer. 



The endoplasm is less highly organized. It comprises a fluid 

 cytoplasm within which are contained the nuclei, mitochondria, 

 Golgi apparatus, and various inclusions such as food and water 

 vacuoles, excretory granules, and sometimes symbiotic algae. 



Trichocysts, commonly associated with the ectoplasm, have 

 their origin in the endoplasm and later migrate to their peripheral 

 location. Trichites are elongated rods usually surrounding the 

 mouth, apparently giving support and protection to the body. 

 Similar supporting rods are frequently combined to form a tube 

 known as a pharyngeal basket leading from the mouth into the 

 endoplasm. 



The Nuclei. — The macronucleus is extremely variable in 

 shape, while the micronucleus is usually a single rounded mass. 

 In the Peritricha, the macronucleus is frequently a long ribbon- 

 shaped structure and in the Heterotricha it usually assumes a 

 distinctly beaded or moniliform condition. An exceptional dis- 

 tribution is found in the genus Trachelius the members of which 

 have a single large macronucleus and thirteen micronuclei. The 

 number of micronuclei is not constant even for the species 



