24 ELECTRON-MICROSCOPIC STRUCTURE OF PROTOZOA 



while in Euglena gracilis (Wolken, 1956, 1959, 1960; Gibbs, 1960) 

 it is a separate body (Fig. 8, PL II). In all of these instances, it 

 consists of a number of dense granules, apparently membrane- 

 limited, arranged in one to three flat or curved plates. The 

 granules are up to 400 m/x in diameter and show in some aspects 

 a rather regular hexagonal packing. 



The stigma is considered in most organisms to be a primary 

 light receptor, but in Euglena it has been thought to function as a 

 light-absorbing shield that permits directed phototropic response 

 by intermittently shading the true photoreceptor, a swelling on 

 the adjacent flagellum (Fig. 8, PI. II). This swelling or para- 

 flagellar body consists of a dense, homogeneous mass bordered 

 by a light space and surrounded by the flagellar membrane (Roth, 

 1958a; de Haller, 1959, 1960; Gibbs, 1960). It is not known to 

 contain any photosensitive pigment; hence, as Gibbs points out, 

 its function as a direct light receptor is extremely dubious. 



Whether or not the paraflagellar body is directly sensitive to 

 light, it is highly significant that a stigma-flagellum association has 

 been detected in other flagellated algal cells by recent electron- 

 microscope studies. Thus in the spermatozoid of Fucus, Manton 

 and Clarke (1956) found that the recurrent flagellum is swollen 

 and adherent to the cell surface over the length of the peripherally 

 located eyespot, becoming free and cylindrical thereafter. In 

 Ochromonas danica (Fig. 9, PL II) a paraflagellar swelling similar to 

 that oiEuglena is seen adjacent to the stigma (Gibbs, unpublished). 

 The most intriguing example yet is that of Chromnlina psammobia, 

 previously supposed to be uniflagellate, where Rouiller and 

 Faure-Fremiet (1958a) found a short, swollen, but structurally 

 impeccable, flagellum entirely enclosed within a cylindrical 

 invagination of the cell surface and occupying the groove formed 

 by the curved plate of the stigma. Here is a flagellum, clearly not 

 active in locomotion, whose only obvious association is with a 

 light-sensitive organelle. Many examples of cilium-associated 

 sensory structures in metazoa come to mind, notably the retinal 

 receptor cells of vertebrates. In these, the inflated outer segment 

 of the rod or cone is packed with pigment-bearing lamellae and 

 is connected to the inner segment, and hence to the sensory nerve 

 fiber, only by a short, slender, ciliary stalk (p.45). 



