THE INFUSORIA 185 



body, and leading by a mere passage into the endoplasm below. 

 Mouthless forms are known, but these have degenerated through 

 parasitism and are not primitive (Opalinidae). In the majority of 

 forms the mouth is displaced from the original terminal position and 

 has become ventral and central. Biitschli maintains that this change 

 in the position of the mouth is brought about by the gradual shifting 

 from the anterior end, as shown by the meeting point of the lines 

 of ciliary markings. As previously indicated, the original course of 

 these lines is from the anterior to the posterior end, but in numerous 

 transitional forms in which the mouth has a more or less ventral 

 position, the markings become curved to agree with the changed 

 position, while the course which the mouth is supposed to have taken 

 is shown by the converging lines (Fig. 95). 



In almost all cases the mouth is not in direct communication with 

 the endoplasm, but is separated from the latter by a longer or shorter 

 pharynx, oesophagus, or gullet, which frequently bears cilia, mem- 

 branes, or membranelles. The oesophagus is likewise an ectoplasmic 

 invagination, as is also a second oesophageal apparatus, found in 

 some forms ( Vorticellidae), where the mouth leads into a comparatively 

 large ciliated or membraned space, known as the vestibitle ( Fig. 101, C, 

 D), and this leads into the oesophagus or gullet proper, which, in turn, 

 communicates with the endoplasm. This space begins as a wide 

 tube and gradually narrows down to a more or less narrow aperture 

 or constriction at the oesophagus. The anus and the contractile 

 vacuole, in some forms, open to the exterior through the vestibule. 

 In some of the Holotrichida, the region about the pharynx is 

 strengthened by accessory apparatus developed in the cortical layer, 

 which in this region is greatly thickened and which in some cases 

 contains secretions in the form of bars arranged in a peculiar basket 

 structure (A, B). 



The membranelles which surround the mouth are usually in motion, 

 as are the membranes and cilia which extend into the vestibule 

 or oesophagus. Even while the animal is lying quiet, the membra- 

 nelles continue their, active vibrations, keeping a constant current of 

 water toward the mouth. This current brings a supply of bacteria, 

 diatoms, algae of various kinds, rotifers of small size, or parts of 

 animals undergoing disintegration, flagellates, etc. A distinction 

 can be made here between herbivorous and carnivorous forms, 

 although the differences can hardly extend to structural adaptations, 

 unless it be, perhaps, in some carnivorous forms, where special 

 weapons of offence (the trichocysts) are found. Probably all forms 

 are more or less omnivorous and make little or no selection of food. 

 The food particles are thrown by the current of the membranelles 

 into the peristomial depression and thence into the vestibule or 



