864 



SCIENCE 



LN. S. Vol. XXXIII. No. 857 



call " placentulse," is not through the lamells 

 of the gills, as Simpson has maintained, but 

 that it occurs in the usual manner through 

 the suprabranchial chambers. 



Strophitus edentulus is a rare species in all 

 of the localities in which we have collected 

 mussels, and we have obtained, until recently, 

 only the following records of its breeding from 

 individuals taken in the Mississippi River 

 near La Crosse, Wis., during the summer of 

 1908: 



Number of 

 Date Individuals Stage of Graviditj* 



June 10 1 glochidia fully formed. 



July 6 1 glocliidia fully formed. 



July 9 3 glochidia fully formed. 



July 10 1 glochidia fully formed. 



July 9 2 not gravid. 



July 17 1 not gravid. 



July 18 2 not gravid. 



July 29 1 not gravid. 



July 29 4 young embryos. 



August 11 1 young embryos. 



August 11 3 late embryos. 



Since these records include the interim be- 

 tween the breeding seasons, they confirm the 

 statement of Ortmann and others that Stro- 

 phitus edentulus is one of the so-called 

 " winter breeders," or those species which have 

 the long period of gravidity. The interval be- 

 tween the seasons, however, as indicated in the 

 above records, is seen to be a much shorter one 

 than that observed by Ortmann. 



After verifying the main observations of 

 Lea and Sterki, so far as was possible at that 

 season of the year, we examined the glochidia 

 carefully with a view to determining whether 

 their subsequent life-history would exhibit any 

 peculiarities, as might be suspected from their 

 relation to the cords. At that time we did not 

 observe the normal discharge of the cords by 

 the female; but we removed them from the 

 marsupium, placed them in water, and, after 

 the glochidia had emerged, employed various 

 means to bring about their attachment to fish. 

 None of these attempts, however, was success- 

 ful, although the fish were left in small dishes 

 containing many cords for as long a time as 

 twelve hours. In the light of these results, 

 which indicated the inability of this glochid- 



ium to attach itself to fish, and in view of the 

 fact that the cords so evidently seemed to be 

 a nutritive device, we felt it to be highly prob- 

 able that in this species the metamorphosis 

 would be found to occur in the absence of 

 parasitism — a prediction which has been re- 

 cently verified. 



On February 6, 1911, a single female of 

 Strophitus edentulus, which had been under 

 observation in the laboratory since last No- 

 vember, was seen discharging its cords from 

 the exhalent siphon. The discharge has con- 

 tinued to the present date, March 25, and dur- 

 ing this time the cords have been thrown out 

 in varying numbers from day to day. They 

 measure from 2 to 10 mm. in length and about 

 1 mm. in diameter, although they become 

 more or less swollen after lying in the water 

 for a time. Each cord contains from 10 to 24 

 glochidia arranged in an irregular row. In 

 many eases the glochidia emerge from the 

 cords in a few minutes after the latter are 

 discharged, and then usually remain attached 

 by the thread in essentially the same manner 

 as has been described by Lea and Sterki. The 

 thread, which is apparently a modified larval 

 thread, is continuous at its distal end with the 

 egg-membrane, which generally remains em- 

 bedded in the cord ; so intimate, in fact, is the 

 union between the two, that at times the 

 membrane, adhering to the thread, is dragged 

 out of the cord when the glochidium is ex- 

 truded — in which case, of course, the glochid- 

 ium becomes entirely detached from the cord. 



All attempts to infect fish with these fully 

 formed glochidia have again been unsuccess- 

 ful, even when the exposure has been of long 

 duration. Within a few days, the extruded 

 glochidia die in spite of every effort to provide 

 the most favorable conditions for their main- 

 tenance. 



When the cords first began to be discharged, 

 one of our students, Miss Daisy Young, hap- 

 pened to notice that not all of the larvae were 

 extruded, and that among those which re- 

 mained in the cords some had lost the larval 

 adductor muscle, possessed a protrusible foot, 

 and showed other signs of having undergone 

 the metamorphosis. Upon careful examina- 



