1920] Allen: Plankton of the San Joaquin River 87 



Rhizota 



station I Station II Station III Daily Hourly 



Number of forms reeorded 2 3 3 .... 3 



Indivicluals per cu. meter.. 1,509 37,181 30,185 16,145 



Practically all of the Rhizota found at any station were single 

 individuals without tubes. Only two or three times were colonial 

 forms recorded and then only part of the colony was present. These 

 facts serve to emphasize the adventitious character of the Rhizota 

 and to indicate that their presence in the plankton was due to broken 

 anchorage. The large numbers at Stations II and III as compared 

 with Station I suggest a decidedly deterrent influence of sewage. 



Discussion of Species 

 Collothrca pclagica Rous. 



Station I Station II Station III Daily Hourly 



Average 1,325 21,747 7,629 1,477 



Identification uncertain. May include two or three species, one 

 of whieli is possibly C. mutabilis. The preserved condition does not 

 permit of very accurate judgment and the animals were usually with- 

 out tubes. So many of the tubes which were seen were of a slender 

 type that it was thought that they indicated the species designated. 

 This form was only recorded at Station I six times (in small num- 

 bers), all in August, September and October with the largest number 

 in October. Occurrence at vStation II was from August 15 to Novem- 

 ber 22. Most of the catches were fairly large and there were only two 

 failures to appear in that period. The largest number recorded was 

 in August, due probably to some unusual disturbance of the water by 

 barges or dredges. 



Attached eggs of this genus and almost entirely of this form were 

 recorded with averages as follows : Station I, 185 ; Station II, 14,288 ; 

 Station III, 15,332. Althougli certainly adventitious, the combined 

 numbers of eggs and adults make this form of some importance in the 

 local river plankton for a brief period. It is not adapted to life in 

 sewage, however. 



The following forms were recorded only once or twice or were 



thought to be present in fresh material : 



CoIIotheca ornata Ehrbg. Once, I and III. 



Conoehiloides dossuarius Hudson. 



Conochiloides natans Seligo. 



Conochilus hippocrepis Sohrauk. 



Conoehilus unicornis Rous. Once, II and III. 



Cupelopagis vorax Leidy. Once, III. 



Ptygura brevis Rous. 



