158 THE MASTIGOPHORA 



fission (Uroglena). On the other hand, the units often retain a 

 facultative independence, and the coenobium may then undergo 

 dissociation (Synura). 



The principal forms of association of individuals are the follow- 

 ing : 1. Linear aggregates, e.g. Hirmidium, Chlorodesmus, Ceratium; 



2. Rosettes, e.g. Bicosoeca socialis, Cydonexis annularis, Gonium pedorale ; 



3. Plates, e.g. Proterospongia, Platydorina ; 4. Spherical aggregates, e.g. 

 Sphaeroeca, Uroglena, Volvox ; 5. Dendroid associations, e.g. Dinobryon, 

 Hyalobryon, Poteriodendron, Anthophysa, Ehipidodendron, Dendromonas, 

 Phalansterium. 



Of the above colonial assemblages it is to be remarked that the 

 dendroid form is the most polymorphic in actual appearance. As 

 for transitional forms, it is not difficult to construct a series, while 

 analogies are stupefying in their abundance. Thus a biserial linear 

 aggregate like Chlorodesmus in comparison with a rosette like Cydo- 

 nexis is absolutely paralleled by species of the pelagic Ascidian, Salpa. 

 A transition from a rosette to a plate is afforded by Gonium, and 

 from a rosette to a spherical aggregate by the volvocine genus 

 Stephanosphaera, in which the units are arranged in a rosette 

 though surrounded by a common gelatinous envelope. 



The Mastigophora as a class may be defined broadly as uni- 

 nucleated Protista which perform their movements by means of one, 

 two, or several flagella, usually arising at or near the anterior end, 

 i.e. the end which is directed forwards during locomotion. The typical 

 motion of the flagellum has been described as one of circumduction 

 (Delage), by which the cell is drawn along at the same time that 

 it rotates about its axis. The flagellum of a typical Flagellate 

 Infusorian is therefore a tractellum, as opposed to the tail of a 

 spermatozoan, which is a pulsellum. 1 It acts, however, as a pulsellum 

 in exceptional cases among the Monadidea, and with the Choano- 

 flagellata when they quit their attachment in order to effect change 

 of position. 



According to the number, position, and proportions of the 

 flagella we recognise monomastigote forms, with a single porrect 

 flagellum ; paramastigote, with one or two small accessory flagella at 

 the base of the main one ; isomastigote, with from two to four equal 

 flagella ; heteromasligote, with divergent flagella, one directed forwards 

 or transversely, the other directed backwards ; polymastigote, with 

 more than four flagella ; to which may be added holomastigote forms, 

 with numerous flagella distributed over the entire surface of the 

 cell. The disposition of the flagella has a distinct systematic 

 importance, but of much more limited application than was formerly 



1 The terms " tractellum " and " pulsellum " were suggested by Prof. Lankester. 

 In some elongate metabolic species (Astasiina) the tractellum is directed straight 

 forwards, and only the apical portion of it executes rapid vibrations, drawing the body 

 .along without rotation. 



