THE "BELL" ANIMALCULES. 



369 



Fig. 26. A, B, UROCENTRUM 

 TURHO ; C, TRANSVERSE 

 FISSION. 



which form colonies, and the commonest of which have their stalks contracting, often in a corkscrew 

 shape, the end of the bell being provided with a circle of long active cilia. Occasionally they may 

 be seen freely swimming, and then there is a second circlet of cilia at the tail end ; but they soon 

 settle down, become attached, and grow a stalk, the lower circlet of cilia disappearing. Very often 

 the o-roup of these stalked Vorticellidse are so large that they are visible to the naked eye, and hence 

 they were amongst the first animalcule described. There are numerous genera, ai-ranged in sub- 

 families, and some have no stalk and others have it, and they may be 

 solitary or social, arranged in branching groups on a common stem or im- 

 mersed in mucus. The animalcules are highly contractile, and vary in 

 shape from that of a long egg to sub-cylindrical, or a long or broad bell 

 shape (Fig. 11). The free end of the bell consists of an outer raised border, 

 sometimes but not always ciliated, and this closes the opening like a 

 sphincter when the animalcule shuts up. As it reopens this peristome is 

 seen to environ a spiral membrane with a circle of cilia on its free surface, 

 and this projects beyond the peristome and the cilia produce very forcible 

 currents in the water. On one side the circle is incomplete, and leads to 

 a furrow which is often prolonged backwards on the body to a canal-like 

 opening to the mouth. The movements of the cilia cause the particles of 

 food to take the direction of this furrow, which has often a long solitary 



cilium at its free end. The spiral part, or disc, can be protruded or retracted. The endoplast is 

 band -like and lai-ge, and the contractile vesicle is single, spherical, and is placed close to the anal 

 aperture, which is distinct near the furrow. The stalk, when it exists in its highest degree of 

 perfection, has an outer cuticle continuous with that cf the body, and an inner spiral tissue more 

 or less longitudinally fibrous, which is continuous with the myophan layer of the hinder part of 

 the bell. Contraction 'produces spiral winding of the stem in some species, and a slow unwinding- 

 happens subsequently. 



The animalcules rarely divide by transverse and usually by longitudinal fission, which takes 

 place through the endoplast and contractile vesicle. The offshoot grows a circle of cilia close to the 



stalk, which does not divide, and after a while it escapes 

 as a free swimmei'. In some species there is a free- 

 swimming and small animalcule, which finally settles 

 on the side of one of the larger fixed individuals, and 

 either penetration occurs or the contents of the smaller 

 pass into the larger. The endoplast subsequently 

 develops a host of germs, which escape and become 

 like the parents with growth. 



In the sub-family Vorticellina the animalcules are 

 naked, long, without a stem, and are sessile on sub- 

 stances ; some have a distinct sucker, by which they 

 cling on, mostly to moving invertebrata and sometimes 

 to weeds in fresh water. One of the genera (Spirochona) 

 has solitary individuals, and the peristome is developed 

 into a spiral funnel, and in Stylochona there is a rigid 

 pedicle or stem instead of a sucker at the tail end. Then 

 there is a genus with all the characters of the genus 

 Vorticella, but the stem is rigid and uncon tractile, and 

 the animals are solitary ; and in the genus Pyxiclium the 

 solitary animalcules have a rigid stem and a ciliary 

 disc projecting beyond the peristome. These forms 

 lead up to Vorticella as a genus, which is the type 



of the family. Vorticella nebulifera (Fig. 27) is common in ponds attached to duckweed or 

 other water plants, and is a very beautiful object under the microscope. The bell-shaped 

 body of each individual is about -gtTo tu of an inch in length, and is attached to a long 



Fig. 27. VORTICELLA NEBULIFERA. 



cv, Contractile vesicle. 



85 



