354 



SCIENCE 



[N. 8. Vol. XL. No. 1027 



mussel escapes from its host. These young 

 mussels lie crowded in the marsupial gill of 

 the parent without apparently any matrix or 

 conglutinate structure whatever. The outer 

 giUs as in other Anodontas are marsupial and 

 these become well distended throughout their 

 whole length when gravid. 



In regard to the breeding of this species 

 Ortmann^ says it is gravid from September to 

 May. My observations, which are rather 

 limited on this point, I give below : 



In addition to these I have found numbers 

 of free juveniles not sexually mature ranging 

 in length from 5 to 30 millimeters. These 

 stages are remarkable for the thinness of their 

 shells and the flatness of the mussel as a 

 whole. The term " floater," of the mussel- 

 fishermen, for this type of mussel is well ap- 

 plied in its use for this immature stage. 



The presence of juveniles in the marsupia 

 during November in a majority of the specimens 

 examined seems to indicate that metamorphosis 

 is probably completed in the fall. The time 

 of discharge of the young mussels is yet to be 

 determined but the appearance of glochidia 

 again in early spring would seem to indicate 

 that the juveniles escape in the fall or early 

 winter. 



Among the six lots of marsupial juveniles 

 that I collected the degree of development 

 varied slightly as to amount of shell growth, 

 otherwise there seemed to be little diiierence. 

 This growth consists of a narrow rim only, 

 around the edge of the glochidial shell. The 

 hooks of the gloehidium are still much in evi- 

 dence but are much weaker than in parasitic 

 forms. A noticeable feature is the large pro- 



'' Op. ait. 



portion of gaping shells as compared with a 

 similar lot of glochidia. It would seem that 

 with the loss of the powerful single adductor 

 muscle the action of closing is less vigorous. 

 Between the gaping valves can be seen the 

 ciliated foot, two adductor muscles, the mantle, 

 on each side the gill papillae, etc., indicating a 

 development equal to that of other young 

 Naiades at the end of parasitism. 



I have tested the reaction of the glochidia 

 in the presence of fish and obtained strong 

 evidence that they do not respond as other 

 known parasitic forms. Mature glochidia 

 taken in March were employed; in an ex- 

 posure to fish for an hour they failed to give 

 the usual infection. A few glochidia lodged 

 in the mouths of the fish but no eneystment 

 could be detected. The fish showed no re- 

 sponse. Following this test the fish were ex- 

 posed for ten minutes to the glochidia of 

 Symphynoia complanata (Barnes). These 

 rapidly became attached and the fish showed 

 considerable uneasiness in marked contrast 

 to their indiflierence in the presence of the 

 other glochidia. 



From these observations I think we are war- 

 ranted in concluding that this mussel passes 

 through its metamorphosis in the entire ab- 

 sence of parasitism. The period immediately 

 succeeding this metamorphosis has not been 

 followed but there seems to be little reason for 

 suspecting any parasitism here. 



In Strophitus edentulus the mussel for 

 which Lefevre and Curtis found a non-para- 

 sitic metamorphosis the arrangement of the 

 glochidia in the gills is very unusual as has 

 been described by them and other authors. 

 The glochidia at first and later the juveniles 

 are imbedded in cords of a gelatinous semi- 

 transparent substance which lie like crayons in 

 a box packed in the water tubes of the mar- 

 supium. Under natural conditions these are 

 shed into the water from time to time. 

 Sterki^ called these cords placentae and Le- 

 fevre and Curtis" have concluded that they 



8 ' ' Some Observations on the Genital Organs of 

 UnionidaB," Nautilus, Vol. 12, pp. 18-21 and 

 28-32. 



9 Op. cit. 



