16 General Morphology of the Protozoa 



monad flagellates. The function of the costa is uncertain, although it may 

 add firmness to the cytoplasm underlying the undulating membrane. 



The cresta (Fig. 1, 9, E, F), possibly a homologue of the costa, is 

 present in Macrotrichomonas and related genera. This organelle is a 

 somewhat triangular membrane, often visible in the living organism and 

 apparently capable of independent movement (125). The broad anterior 

 end is usually joined to a blepharoplast, while the rest of the cresta ex- 

 tends posteriorly with its outer margin near the periplast. The length, in 

 different species, ranges from about 1.5[x to almost that of the body. A 

 trailing flagellum, sometimes loosely adherent to the periplast anteriorly, 

 may parallel the cresta (Fig. 1. 9, F) and thus simulate the relationship 

 between the undulating membrane and the costa. 



The pelta (Fig. 1. 9, A, C), demonstrable by the Bodian silver tech- 

 nique, is a crescentic membrane lying anterior to and separate irom the 

 blepharoplasts in certain flagellates. The pelta may be homologous with 

 a membranous extension of the axostylar capitulum in certain devesco- 

 vinid flagellates (128). 



The acicuhim (Fig. 1. 9, D) of Cryptobia helicis, a needle-like structure 

 lying opposite the kinetoplast and extending approximately to the origin 

 of the anterior flagellum, is detectable in living material but is best 

 demonstrated by the Bodian silver technique (141). 



The parabasal apparatus 



In many parasitic and a few free-living flagellates a parabasal 

 apparatus, an organelle of unknown function, forms part of the mastigont 

 (Fig. 1. 10). The simplest type is a small compact body, often attached 

 by a rhizoplast to a blepharoplast. At the other extreme, the apparatus 

 may be a large branched structure or may be composed of separate 

 elements. 



The index of refraction of the parabasal body is approximately that 

 of the cytoplasm and vital staining is rather slow (71); consequently, 

 the organelle is not readily seen in the living flagellate. The apparent 

 internal structure may vary with the species as well as with methods of 

 fixation and staining (47, 123). 



The parabasal apparatus of free-living flagellates shows little variety 

 (Fig. 1. 10, H, L, N-P). One or two small parabasal bodies have been 

 described in several species; one or more long slender bodies, in certain 

 others. In the tetranucleate Polykrikos schwartzi (27), each band-like 

 parabasal body is attached to a ring encircling the intracytoplasmic por- 

 tion of an axial flagellum. The parabasal body of Codosiga elegans (196) 

 is of special interest because it closely resembles a structure (Fig. 1. 10, 

 M) described in choanocytes of calcareous sponges (218). 



Among parasitic flagellates, the complexity of the parabasal apparatus 

 varies widely. The small kinetoplast of Trypanosoma hnicei (Fig. 1. 10, 



