PLECTUS CIRRATUS 207 



the body, and about two to two and one-half times as long as wide. They appear 

 in the uterus one at a time and seem to be deposited before segmentation begins. 



Habitat: Sand, Washington filter beds, early in January 1916, at the end 

 of about five months' use. Not common. No males seen. 



PLECTUS Bastian 1865 



10. Plectus cirratus, Bastian. ~iT? ?!,4 - 47? TT "iV 1 - "" Taking the de- 

 scriptions and figures of de Man as the basis of his identification, Maupas 

 made numerous and very careful observations upon this species; he never 

 found males, and concluded from this and his other observations that the species 

 is purely parthenogenetic. As my own observations do not agree with those of 

 Maupas, it is well to consider carefully whether the species observed by us are the 

 same. My specimens came from the Potomac River near Washington, D. C., U.S.A., 

 and in size and proportions agree in all respects with the figures and descriptions 

 of de Man. The anatomy of my specimens also appears to agree in all essential 

 respects with that set forth by de Man. There is a little uncertainty about the 

 number of cephalic setae as reported by de Man; in one instance he says four, 

 in another six. Bastian's original description, made from specimens ^ in. 

 long, says four. In the description in which de Man placed the number at six 

 his corresponding illustrations are possibly open to the interpretation that four 

 only were present. The Potomac specimens always present four cephalic setae. 

 The caudal setae of the Potomac specimens are not so prominent as indicated in 

 de Man's figures. My specimens present the amphids at precisely the point 

 indicated by de Man, and of very nearly the same size. De Man does not give 

 a very clear indication of the shape of the amphids, but I see no reason to consider 

 the two forms specifically or varietally different on the basis of the shape and 

 position of the amphids. There exists, therefore, only the uncertainty with 

 regard to the cephalic setae, and this may not in fact be a discrepancy. Maupas 

 shows five eggs in each uterus. It is uncommon for the Potomac specimens to 

 present as many eggs as this in the uterus; the common number is one or two, but 

 it may rise to four. De Man describes the egg-shells as smooth ; Maupas, however, 

 says they are covered with minute points. The Potomac specimens agree 

 with Maupas' description. I am therefore most strongly inclined to believe that 

 the forms examined by Maupas and myself are the same. 



Maupas, in his description of the development of the egg, notes the following 

 points: In one case only among a large number of developing ova examined 

 did he see more than a single nuclear figure. He considers this an important 

 point in his demonstration of the parthenogenetic character of the develop- 

 ment. He observed that when the egg entered the uterus it lost its definite 

 nucleus for an hour or more, and concluded that during this period the polar 

 bodies were formed, although, as he says, he never saw any polar bodies, but 

 observed amoeboid movements in the ovum during the absence of the nucleus. 

 Maupas says that it was with much difficulty that he convinced himself that no 

 spermatozoa were present in the sexual organs of this Plectus, but that in spite 

 of careful examination he never succeeded in seeing any. Nor did he see the 

 slightest trace of spermatozoa on removing the sexual organs and treating them 

 with acetic acid. He therefore took the species to be parthenogenetic. Never- 

 theless, he noted occasionally in the distal end of the uterus refractive bodies 

 of exceedingly small size. As will appear later, it is not at all improbable that 

 these minute bodies were really spermatozoa which he failed to recognize as such. 



