THE PH^OPHTCEAN ZOID 3 



( = Hymenomonadace£e), Green Algae, as marine Codium and freshwater 

 Volvox, etc. ; aa also as flagellated phases of many Heliozoa, Radiolaria, 

 Chara, the Bryophyta, and even Selaginella. 



The movement of the anterior tractor is always that of a sharp 

 rhythmic contractility in three-dimensional space, working out a 

 movement which Avould be observed as a spiral vortex if the zoid 

 were still ; but being freely suspended in the medium the body is 

 pulled along and at the same time rotated on its axis, while the 

 movements of the tractor appear as a mere undulatory lashing like 

 tha shaking of a rope. Such a mechanism is clearly the expression 

 of a limiting term of economy and precision in swimming, and it is 

 to this that it owes its constancy. It can be only improved by 

 increasing the effective power of the tractor, as by increasing the 

 mass of the contractile plasma, or by adding to the length of the 

 stroke. 



The feeblest monokont flagella are usually about one body-length (flagel- 

 lulae of Mycetozoa) ; yet many Cryptomonads are intensely active, with 

 darting action, with flagella relatively no longer (Cliilomonas). 



In megazoids of the Phseophyceae efficiency obviously falls off as the 

 flagella are left less than one body-length. 



The most efficient are 3-4 body-lengths, though ranging to 4-6 body- 

 lengths with more rounded zo'ids {Dictyota). Among the Phfeophycese the 

 finest expressions of the type are found in the Cutleriaceae (Yamanouchi). 

 Thus :— 



Cutleria megazoid 26/j long, ovoid, anterior flagellum 40ju.. Aglaozonia 

 zoid 22"5/it long, ant. flagellum 65^. Zanardinia, zoogonidium 22'5 long, 

 ant. flag. 45/i. 



Where the zoid is enlarged in correlation with phenomena of heterogamy 

 the relative dimensions are diminished ; and the large oosphere of C^itle7-ia 

 with 30 chloroplasts presents an anterior flagellum of 40/i only, or 1*5 body- 

 lengths. It may be noted that 3 body-lengths bears a suggestive relation 

 to the circumference of the zoid, as one complete turn of the body in 

 ontogeny. 



In many more specialized and powerful holozoic flagellates, the tractor- 

 flagellum is distinctly broadened to a band-form. [Cf. also Euglena, 

 Cyathomonas, and animal spermatozoa (Retzius, 1906, Doflein, 1916, 

 p. 38).] 



Again, regarded as complex kinetic organs of primary signifi- 

 cance involving problems of life and death to the organism, such 

 flagella are the result of long and complex natural selection on 

 pre-existing factors of growth and contractility. Their structure, as 

 presenting a thin him of intensely katabolic contractile plasma 

 apparently investing an axial core of more resistant endoplasm, 

 possibly affords in its minute dimensions ('5 /j, diam.) as vivid an 

 idea of the complex nature of living plasma as any other part of the 

 cell ; such a structure as the first evolved " member " or " organ " of 

 the cell-soma, projecting far beyond the limit of the main body, must 

 involve a special system of conduction, nutrition, and control, of 

 which little is yet known, but is to be considered from the standpoint 

 of "basal granule," *' mitochondria," and *' rhizoplast." 



The primary function is undoubtedly that of a means of vertical 

 ascent for autotrophic photosynthetic pelagic ph3'toplankton ; lateral 

 progression in such a medium is meaningless : but it is obvious that 



