THE NAUTILUS. 



hidia which have been described above as parasitic upon Nec- 

 turus I observed a similarity to the glochidia of Anodonta imbecil- 

 lis, Say. A minute comparison showed differences however and I 

 made an effort to determine if the differences might come within 

 the range of normal variation. To do this I secured as many 

 gravid examples of A. imbecillis as I could from the place where 

 the Necturus were captured. In a measurement of glochidia 

 from 16 individuals I found that only the very lowest point of 

 variation in A. imbecillis corresponded with the larger of the 

 unknown. It was quite obvious they could not be the same 

 species. While making this examination which yielded negative 

 results to my original search I noted a peculiarity which proved 

 quite a diversion. In measuring the glochidia I came upon in- 

 dividuals which had advanced far beyond the larval stage, hav- 

 ing in fact reached a point of development equal to that to be 

 seen in most Unionidae after metamorphosis parasitically upon 

 fishes. 



The young mussels, mostly of the same stage, lie crowded in 

 the marsupial gill of the parent without apparently any matrix 

 or conglutinate structure whatever. In this respect we have a 

 condition quite different from that reported by Lefevre and 

 Curtis ! for the non-parasitic development of Strophitus edentulus 

 (Say), in which there is a matrix sub-divided into the so-called 

 cords. 



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 difference. This 

 growth consists of a narrow rim only, around the edge of the 

 glochidial shell. The hooks of the glochidium are still much 

 in evidence, but are much weaker than in parasitic forms. A 

 noticeable feature is the large proportion 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 papillse, etc. 



1 Lefevre and Curtis. Bull, of the Bureau of Fisheries, vol. zzx, 1910 

 (issued 1912). 



