956 THE AMERICAN NATURALIST. [Vov. XXXII. 
has been modified and thickened by the addition to it of connective- 
tissue material from the deeper lying parenchyma. That the sub- 
muscular cells do not secrete this material is shown by the fact that 
the submuscular layer is absent from the caudal appendix of Apoblema, 
though the cuticular layer is as well developed there as over other 
parts of the body. It will be seen that the view which Pratt advo- 
cates is essentially the theory of Looss grafted upon the older view 
of Schneider and Minot. The evidence which he brings forward 
strongly antagonizes the views of Ziegler, Brandes, and Blochmann. 
Pratt’s account of the life-history of Apoblema differs from that 
given by previous investigators. He believes that the copepod is the 
primary host within which the young worm lives until it is nearly 
mature. It then escapes from the body of the copepod by forcing 
its way outward between two thoracic segments—a process which 
the author observed repeatedly. The young worm, while swimming 
about freely in the water, is probably swallowed by some fish, which 
thus becomes its final host. W S NICKERSON. 
Michigan Unionidæ.!— The distribution of the Unionidæ of 
Michigan has been worked out by Mr. Bryant Walker from. a 
census which the Conchological Section of the Michigan Academy 
of Science has taken of all the known public and private collections 
within that state. Michigan possesses the richest unione fauna of 
all the territory tributary to the Great Lakes, and, as the state is 
wholly within the St. Lawrence basin, the problem is not compli- 
cated by the political boundaries of the investigatión. Sixty-one 
species belonging to the genera Unio, Margaritina, and Anodonta 
are recognized. Of these only a small number have a general distri- ` 
bution; a few are peculiar to the northern part of the state; and 
several are confined to Lake Erie and the waters immediately tribu- 
tary to it. On the other hand, a great majority of the total number 
(75 per cent of the species of Unio and Margaritina) are confined to 
the Grand-Saginaw valley and the region to the south of it. These 
forms are members of the fauna of the Mississippi basin, while those 
of the southeastern part of the state show decided affinities to the 
Ohian fauna. But two species peculiar to the fauna of the Atlantic 
region are found, and these have a general distribution throughout 
the state. Their westward migration could have taken place readily 
along existing waterways. On the other hand, the explanation of 
1 Walker, Bryant. The Distribution of the Unionide in Michigan. Detroit, 
1898. Printed for the author. 20 pp., 3 pls. 
