THE NATURAL HISTORY SURVEY. 49 
containing aberrant forms which are peculiar to North Amer- 
lea. * 
The embryo of Unio is called a glochidium (Fig. 10) and 
as the latter is frequently found in the gills of Anodonta and 
Unio, it may be well to briefly outline the characters of the larva, 
that it may be identified when seen. It is quite important to 
record the date upon which specimens are seen with embryos 
in the branchial uterus, and also whether the outer or all four 
gills are used as a marsupium. 
The young of the Unionide develop to a certain stage 
called, as above, the glochidium. After fertilization the mantle 
and shell develop rapidly, but the digestive organs are not 
developed until six months or a year afterward. When the 
embryo, or glochidium, leaves the parent it has the form shown 
in Fig. 10. It is encased in an egg shell (e) in which are 
Fia. 10. 
Anterior view of ‘‘Glochidium” of ANoponTa, enclosed in the eggshell, 
X100 (W. K. Brooks, Handbook of Inv. Zodl., p. 330, fig. 174). b, byssus; 
bg, byssus organ; e, eggshell; h, hooks, ls, left valve of shell; m, posterior 
adductor muscle; rs, right valve of shell; s. setze; v, velum. 
enveloped the two she//s (ls, rs) united by a stout hinge whichis 
so elastic that the two valves are frequently seen in the same 
plane. The ventral edge of the shell forms a toothed hook or 
hooks, which are movable and from which *the larva receives its 
name. ‘The valves of the shell are lined by large spherical cells 
from some of which large see (s) project into the mantle 
cavity. There is a stout adductor muscle (m) and the space 
between the two sides of the mantle is filled by a long, elastic, 
brown, coiled, thread-like dyssws (b) which is formed in the tub- 
*For further information the reader is referred to the most interesting articles (to which 
the author is indebted for the notes used) by Dr. V. Sterki, in ‘‘ The Nautilus,"’ Vol. IX 
No. 8, p. 91 (November, 1895), and by Mr. Chas. T. Simpson in ‘The Nautilus,” Vol, XI. 
No. 2, p. 18, June, 1397. 
