2 THE PH^OPnTCEAN ZOID 



of granules between the insertion of the flagella and the nucleus, in 

 the manner of a rhizoplast-strand, but decided against any connection 

 between such basal granule and a blepharoplast as a " cell-organ." 

 lletzius (190G), for Fucus Areschoiigii and F. seDYrfits, showed the 

 presence of a group of 4 (rarely 5) " Nebenkern "-granules (Mito- 

 chondria, Plastochondria) probably of nutritive function, and confused 

 by previous observers with the nucleus (Guignard). Ketzius also 

 demonstrated the " end-piece " of the flagella, as a short delicate 

 terminal portion (5/i long), regarded either as a projecting core 

 (Minchin, p. 52), or as a prolongation of the extreme plasmatic 

 him. Meves (1908) confirmed the presence of plastochondria which 

 might become confluent, and also demonstrated distinct "basal 

 granules " in Fucus serratus associated with each flagellum, to be 

 described as "centrioles" ( = blepharoplasts), and in direct contact 

 with the nuclear body of the protoplast. According to Meves also 

 the two flagella of Fucus ser)Y(tus grow round the zoid in the same 

 sense (cf. (-hara), while according to Retzius they are so exactly in 

 the same line that they appear as pi-actically continuous. Older 

 hypotheses that flagella are formed from a peripheral zone of cyto- 

 plasm (Guignard, Yamanouchi, 1913) require to be replaced b}^ the 

 view that they grow outwards from the point indicated by the basal 

 granule, in the manner generall}^ characteristic of flagellates (Minchin, 

 P-^2). 



The special feature of the mobile zoid is its asymmetrical organi- 

 zation as expressed in the " lateral insertion " of the two unequal 

 flagella ; and this arrangement, so constant and characteristic, is in 

 marked contrast with the isokont habit of the zoids of the Chloro- 

 phyceai-lsokonttfi and Chrysomonadina-Isokontse. It ma}^ be termed 

 the '' anisoko7if^' condition. 



Of the two flagella one projects forward beyond the pointed anterior end 

 as a tractor mechanism ; the posterior merely trails behind as a long 

 '' steering-oar " : to what extent it acts as a definite propeller is still obsciire. 

 The names " tractor " and "trailer" maybe adopted as suflSciently distinctive 

 and concise (Minchin, p. 53). hohont is preferable to isomastigote, and 

 ft ni>!o]iont to heteromastigote : "trailer" is preferable to guhernaculum (cf. 

 Willey & Hickson in Mastigophora, Lankester's ' Zoology,' i. (1909) p. 158, 

 Minchin, p. 259) ; Lankester (Enc. Brit., Protozoa) introduced the terms 

 tractellum and puUellurn respectively, the latter indicating the propeller; 

 and the?e terms have been retained in works on Protozoa, as Saville Kent 

 (1880), Infu oia; Minchin, Pio'.ozDa (1912), p. 52. 



On general principles it may be concluded that the asymmetrical habit is 

 secondary, as the transformation of a symmetrical mode of construction ; 

 and that the differentiation of two flagella with different fimctions is 

 secondtiry to that of the type of the isokont green algffi ; the latter may be 

 so far regarded as a more primitive phase of zoid construction ; as in turn 

 the condition of the single anterior tractor-flagellum may be considered to 

 represent the first step in the evolution of such a kinetic mechanism. 



Zoids with a monokont organization survive in the case of Botrtjdium 

 (Chlorophyceae), and among several groups of Brown Flagellates (Chryso- 

 monadina-Monokontae, Silicoflagellatae, Coccolithophoridae, Hydrnnis ; 

 among Fungus phyla in Chytridiaceae and Monohlepharis ; in vestigfial 

 flagellated Eadiolarians, flagellulffi of Mycetozoa, and as reproductive phases 

 in Foraminifera as Peneroplis. 



The Isokont condition is characteristic of Chrysomonadina-Isokontee 



