366 



THE INFUSORIA 



in the stalk of Vorticella, and the remarkable myoneme bands for 

 the retraction of the peristome in Cycloposthium. 



It is usually the alveolar sheath which bears the pigment of 

 those forms that are specially coloured, such as the blue Stentorin 

 of Stentor coeruleus, but the colour may also be due to pigments in 



Fio. 6. 



Section through the outer layers of 

 Holophrya discolor, a, the alveolar 

 sheath covered by a very thin pellicle ; 

 R, the surface ridges, between which 

 are the rows of cilia ; M, the vertical 

 canals bearing the myoneme fibres; E, 

 the medulla. (After Btitschli.) 



the pellicle or in the medulla. It is in the same layer that the 

 trichocysts occur. These are described more fully below. 



The inner layer of the cortex does not present any features of 

 very special interest. It is frequently quite inconspicuous. 



The MEDULLA. This part of the body is, in almost all cases, 

 the larger in bulk. It is usually semifluid in consistency, and 

 exhibits a constant rotatory movement. In Trachelius (Fig. 7) 

 and some others it exhibits a reticulate char- 

 acter, with irregular branching pillars stretching 

 from the centre to the cortex. In Dendro- 

 cometes among the Acinetaria very fine lines may 

 sometimes be seen stretching from the arms to 

 the region of the meganucleus, but in most 

 cases the particles composing the medulla seem 

 to be freely interchangeable in position. 



The bodies held in suspension by the 

 medulla are very diverse and variable. Apart 

 from the food vacuoles, contractile vacuoles, 

 and nuclei, which are described in some detail 

 a frlSTr HoZtric^an,' in an <>ther place, there are often to be found 

 showing the reticulate ar- pigmented granules, colourless spherules, crystal- 



rangement of the medul- ? v j- j -M _, i i -i 



lary protoplasm. 6, 6 the line bodies, and smaller particles, which vary 

 the SSStCdSW in size and number according to the state of 

 x so. nourishment and sexual condition of the in- 



dividual. 



The Mouth (Cytostom) is present in nearly all the Ciliata, the 

 parasitic Opalina, TrichonympJut, and one or two other genera forming 

 interesting exceptions to the general rule. In the Acinetaria there 

 is usually no mouth. 



In its simplest form the mouth is represented by a small slit- 

 shaped break in the continuity of the cortex at the anterior end of 

 the body. This can be opened for the reception of food, but in 

 the intervals of ingestion is kept closed (Enchelina, etc.). In other 



Fio. 7. 



