LAGENIDIALES 



969 



diameter, irregularly crescent ; when old with branched rhizoids ; sporan- 

 gia formed from the swollen terminal cells, with a discharge tube 1 5- 1 42 fjt, 

 long and 5 fx in diameter; zoospores monoplanetic, more or less pear- 

 shaped in motion, at rest ovoid to fusiform to irregularly triangular, 

 biflagellate, 5 by 2 jj., swarming within the sporangium and swimming 

 out through the discharge tube ; resistant spores about 45-90 by 40-80 \i, 

 light golden-brown, with dense cytoplasm and relatively thick walls; 

 gemmae also formed. 



Parasitic in larvae of the clams Venus mercenaria and Venus mortoni 

 (and of hybrids between them) and of the oyster Crassotrea virginica, 

 United States. 



-"a a ft, * 



§£ D 



Fig. 78. Pontisma lagenidioides H. E. Petersen in Ceramium 



A. Typical segmented thallus. B. Curved unsegmented sporangium within 

 which zoospores have become quiescent. C. Discharge of zoospores from an 

 unsegmented sporangium. D. Zoospores. E. Group of empty sporangia, all 

 segmented portions of single thallus. 



(Sparrow, 1934c) 



