182 



THE MASTIGOPHORA 



of 1500 to a maximum of 22,000. In /'. aureus, Ehrb., the number 

 of cells varies from 200 to 4400. In a third species. V. tertius, 

 Meyer, intercellular protoplasmic threads are only present in young 

 unhatched colonies, not in the adult condition. 



The form of the coenobium varies in the different genera. 

 Gonium, Miiller (Fig. 5 (14)) ; cells 4-16, arranged in a squarish plate 

 with flagella upon one face only ; envelope closely adherent. Stephano- 

 fphaera, Cohn ; cells 4-8, arranged in a rounded plate with flagella upon 

 one face only ; envelope swollen ; oval or spherical. Eudorina, Ehrb. ; 

 coenobium ellipsoidal or spherical ; cells 16-64, similar, not crowded nor 

 reaching towards centre. Pandorina, Bory ; coenobium ellipsoidal or 



spherical ; cells 16-32, simi- 

 lar, crowded, reaching to- 

 wards centre ; outer mem- 

 brane or sheath of coero- 

 biuni showing characteris- 

 tic concentric stratification. 

 Platydorina, Kofoid (Fig. 

 8) ; coenobium horseshoe- 

 shaped, flat, one cell deep, 

 with 3-5 prolongations of the 

 gelatinous matrix at the pos- 

 FIG. 8. terior end ; cells 16 or 32 ; 



Platydoniw. w.mlata, Kofoid. A plate-like Volvocine flagella upon both sides of 



colony. The two surfaces of the colony are alike. .-, , . ., ,, nlfpr. 



The aspect of the adjacent cells alternates, so that l 



the pole bearing the flagella and stignia of one 'cell natin". In side view the 



is turned in the opposite direction to that of its im- , . , , 



mediate neighbours, B, x. A, front view of the colony ; plate IS seen to be twisted 



B, side view; C, a single cell showing, /, the flagella; cli a htl v in a Ipft sniral r> at 



T, the vacnoles \ *t, the stignia ; N, the nucleus ; and sll g ntl y !1 



P, the pyrenoid. (After Kofoid.) to describe a figure of 8. 



The asexual reproduction of 



Platydorina has been observed by its discoverer (Kofoid, 1900) repeatedly 

 during five years, but sexual reproduction has not been seen in this genus. 

 All the cells are gonidial, each capable of dividing to form a daughter 

 coenobium. The daughter colonies acquire the adult form and torsion 

 before escaping from the maternal matrix, which then undergoes dis- 

 integration. Pleodorina, Shaw (Fig. 9) ; coenobium ellipsoidal ; cells 32, 

 arranged in 5 circles, 4 in each polar circle, 8 at the equator, and 8 

 in each intervening tract. Vegetative cells always 4 at the anterior 

 pole. Gonidial cells twice as large as the vegetative. 



SUB-CLASS IV. DINOFLAGELLATA. 



The Dinoflagellata or Peridiniales, formerly called Cilioflagellata 

 under an erroneous impression concerning the nature of the trans- 

 verse flagellum, are heteromastigote forms usually possessing a 

 complete cellulose membrane or cuirass which is never silicified. 

 The chromatophores are predominantly brownish coloured with a 



