2 MORPHOLOGY OF INVERTEBRATE TYPES 



body consists of a single cell which has therefore to perform 

 within itself the various functions of life, such as nutrition, 

 sensation and reproduction. Nature has assigned these func- 

 tions to various parts of the cell, which may be compared with 

 organs of multicellular animals. One must bear in mind, how- 

 ever, that no homology exists between cell-parts and organs, 

 and that even the analogy is quite superficial. It is advisable, 

 therefore, to use the term organellum in treating the structures 

 of unicellular organisms and to speak of sensory, nutritive, 

 protective, reproductive and other organella. 



The body of Paramaecium is asymmetrical, i. e., it cannot be 

 divided into two like parts. The anterior end is wider and 

 more blunt than the posterior one. Paramaecium appears al- 

 most round in a cross-section, yet there is a distinct difference 

 between two surfaces one of which may be regarded as dorsal 

 and the other as ventral. The latter is characterized by the 

 presence of a large depression or peristome which extends from 

 the anterior end to somewhat beyond the middle of the body 

 and is as wide as half the width of the animal. At the bottom 

 of the peristome is the mouth or cytostome which leads into a 

 short, curved cytopharynx with an undulating membrane. Half- 

 way between the lower end of the latter and the posterior end 

 of the body is a small opening functioning as an anus and called 

 cytopyge. There are two pulsating vacuoles, one about one- 

 quarter of the entire length from the anterior end, the other 

 about the same distance from the posterior end of the animal. 

 In about the middle of the body is a large macronucleus and a 

 small micronucleus. 



Protoplasmic structures. The protoplasm of Paramaecium 

 shows the characteristic differentiation into ectoplasm and 

 endoplasm. The latter is devoid of particular structures. It 

 has a finely granulated appearance and contains food-vacuoles 

 with food in various stages of digestion, and the two nuclei. 

 The ectoplasm, on the other hand, presents many structures 

 and is subdivided into three layers: pellicula, alveolar layer and 



