46 



INVERTEBRATE ANIMALS. 



mouth. The Infusoria may be defined as Protozoa which 

 are provided with a mouth, and generally a rudimentary 

 digestive canal. They do not possess the power of emitting 

 pseudopodia, but are furnished with vibratile cilia or con- 

 tractile filaments. They are mostly microscopic in size, and 

 their bodies usually consist of three distinct layers. They 

 are mostly simple free-swimming organisms, but they some-' 

 times form colonies by budding, and are fixed to some solid 

 object in their adult condition. As types of these two sec- 

 tions of the Infusoria, we may take respectively Paramoecium 

 and Upistylis. . 



Paramoecium (Fig. 10) is a beautiful slipper-shaped little 



Fig. 10. — Ciliated Infusoria. A. Param(scium, sbowin*? the nucleus («) i^nd two con- 

 tractile vesicles (y); B. Piw(tm(Bcium bursaria (after Stein), dividing transversely, 

 n Nucleus ; »' nucleolus ; t Contractile vesicle ; 0. Paramoecium aureHa (after Ehren- 

 berg), dividing longitudinally. 



creature, which may be found commonly in stagnant waters 

 or in artificially-prepared infusions. The body is nearly quite 

 transparent, and consists of three layers — 1. A structureless, 

 transparent, external film or pellicle ; 2. A central mass of soft 

 semi-fluid sarcode ; and 3. An intermediate layer of firm and 

 consistent sarcode. The external membrane or cuticle is 

 richly covered with minute vibrating hairs or cilia, which ap- 

 pear, however, to be really derived from the middle layer. 

 The cuticle is also perforated by the aperture of the mouth, 

 which is continued into a short, funnel-shaped gullet. The 

 gullet, however, is not continued into any distinct stomach, 

 but opens directly into the soft, semi-fluid sarcode which con- 

 stitutes the central abdominal cavity. The particles of food 



