■92 Protozoa. 



:flexible to a certain extent, and may alter in form. The ectosarc 

 is absent from two spots on the surface ; one of these serves 

 for the ingestion of food, and is termed the mouth, whilst 

 through the other, the anus, undigested material is ejected. 

 The two openings are usually situated at opposite extremities of the 

 body; the end where the mouth lies is termed anterior, the other 

 posterior. The mouth usually opens at the base of a funnel, which 

 is often fairly deep, whilst the anus is only visible as a slit, when 

 excreta are ejected. The Infusoria are all provided with cilia, 

 which constitute the chief locomotor apparatus. In some forms they 

 are distributed over the whole surface, and are then often arranged 

 in longitudinal rows ; in others, some of the cilia are specially 

 developed as spines or hooks, or there may be one or several rows 

 «f these between the other cilia; specially common is the presence 

 of a spiral row of powerful cilia, at the anterior end of the body, 

 by means of which the food is driven into the mouth. In others, 

 again, the whole covering, with the exception of this row, or this 

 row and one other, is completely lost.* Hard skeletal structures 

 like those of the Gymnomyxa do not occur here ; but some of the 

 Infusoria secrete round the body a gelatinous or membranous shell, 

 beaker-shaped or tubular, into which their protoplasm may be with- 

 drawn : the relation between the animal and its shell is the same as 

 that between a tubicolous worm and its tube. The shell is generally 

 attached to a foreign object, but it is carried about by some marine 

 Infusoria. 



In the protoplasm there is a large macronucleus, which 

 may be round, sausage-shaped, ribbon-like, or moniliform (Fig. 52), 

 and one or more smaller micronuclei; rarely there are several 

 macronuclei. Near the surface are the contractile vacuoles, 

 which excrete the water they contain through one or more fine 

 pores, and then take up a fresh supply from the protoplasm. In 

 the outer portion of the protoplasm there are frequently delicate 

 threads of a contractile substance, muscle fibrillse: fat 

 globules and pigment granules also occur in the endosarc. 



The Infusoria reproduce by fission (Fig. 52), which usually 

 occurs at right angles to the long axis, and is thus transverse ; it 

 is preceded by a division 'of both macronucleus and micronucleus. 



Whilst permanent conjugation is rarely met with here, a tem- 

 porary fusion of two individuals, which separate again later, occurs 

 very frequently. They are adherent over a limited area, and during 



* In some Infusoria there are the so-called membranellse, vibrating, laminate 

 structures, each of which is regarded as a short row of fused cilia. The " undulating 

 membranes," long, vibratile ribbons attached by one edge, are also to be regarded 

 as membranellse, which have arisen from long rows of cilia. 



