VARIATIONS IN SHAFT STRUCTURE — CILIA 33 



Sensory cilia often lack the central fibrils. The rods and cones 

 of the vertebrate eye are composed of two segments connected by 

 a narrov^ neck about '5 to 1 '5 jit long and about 0-2 jjl in diameter 

 (PL Via, b and c) (Sjostrand, 1953; De Robertis, 1956a; De 

 Robertis and Lasansky, 1958; Tokuyasu and Yamada, 1959; 

 Eakin and Westfall, 1959, 1960). This neck contains a ring of 

 nine longitudinal fibril doublets which have their origin at a 

 typical basal body in the inner segment and terminate in the outer 

 segment, where they may make contact with the edges of the 

 flattened sacs which form the photoreceptive part of both rods and 

 cones. There is no well-defined fibrillar material at the centre of 

 this fibril bundle. The " connecting cilium " forms the only 

 contact between the receptive part of the cell and the dendritic 

 nerve ending, so that some part of the cilium must conduct the 

 impulse generated in the outer segment. Striated roots from the 

 basal body run deep into the inner segment. 



Faure-Fremiet (1958) has pointed out the parallel structural 

 developments found in the rods and cones of the vertebrate eye 

 and the photoreceptive region of the phytoflagellate Chromulina 

 psammohia. A short internal flagellum runs from the base of the 

 emergent flagellum to lie alongside the eye-spot chambers ; this may 

 conduct information from the eye-spot to the locomotor flagellum. 



The distal sense cells of the eye of Pecten carry curious oval 

 appendages about 1/x in diameter. Transverse sections show 

 concentric banding of alternate light and dark zones which diverge 

 from the appendage at the outside and end in the cytoplasm. 

 These terminations have the structure characteristic of ciliary 

 basal bodies; and from them ciliary shafts with nine dense loci 

 run to the appendage, apparently forming its whole struture. The 

 presence or absence of central fibrils in the cilia does not seem to 

 have been established for certain by Miller (1958) who has 

 described this structure. It is suggested that these appendages 

 form the receptive part of the sense cell. 



Until recently it was believed that cilia were not present in 

 arthropods, then insect sperm (Bradfield, 1955), the scolopale 

 organ of the locust (Gray and Pumphrey, 1958; Gray, 1960), and 

 the plate organs on the antenna of the honey bee (Slifer and 

 Sekhon, 1960), were found to contain the typical ciliary structure. 

 The scolopale sense organ detects vibrations of the tympanic 



