400 ZOOLOGY. 



surfaces there is a shallow groove (answering to the hollow 

 of the slipper). The rounded end always moves first, and 

 is therefore anterior ; the pointed end being posterior. Here 

 we see the same adaptation of shape to the swimming habit 

 that we found in the dogfish. The grooved surface, being 

 always towards the surface over which the animal glides, 

 is ventral. These terms " anterior," " posterior," " dorsal " 

 and " ventral," are merely used for convenience of descrip- 

 tion. As applied to an animal consisting of one cell only, 

 they cannot imply any morphological comparison with the 

 regions similarly named in a Vertebrate. 



2. Protoplasm and Cuticle. While having a definite 

 and constant shape, Paramecium is flexible and elastic 

 being able to squeeze itself through narrow openings and 

 bend round corners. These characters are due to the 

 enclosure of the protoplasm in a thin but firm membrane 

 secreted by its outer surface ; a membrane of this kind 

 secreted by the outer surface either of a unicellular or multi- 

 cellular animal, is called a cuticle. Since its function is to 

 keep the animal in shape, and aftbrd a firm support for all 

 internal motions of the protoplasm, it may be regarded as 

 the skeleton of the Paramecium. A skeleton forming the 

 external surface, as this does, is called an exoskeleton. 



The protoplasm within is differentiated into two layers, 

 which are far more unlike and permanently distinct than 

 the ecto- and endoplasm of Amoeba. The terms cortex and 

 medulla are therefore given to the inner and outer proto- 

 plasm respectively. 



From the cortex there project through apertures in the 

 cuticle into the surrounding water a great number of 

 delicate cilia, just like those of the ciliated epithelium of a 

 Vertebrate. It is by the vibration of these cilia that the 

 locomotion of the animal is effected. 



Embedded in the cortex at right angles to the surface are 

 numerous little bodies called trichocysts. When pressure, or 

 some irritant liquid, is applied to the surface of the animal, 

 a fine long thread is ejected from each trichocyst. Owing 

 to their small size it is difficult to say how this takes place, 

 but it seems probable that these structures are similar to 



