THE LAMELLIBRANCHIA 251 
Ce IN EEE ana 
called a “glochidium” (Fig. 228). This larva is characterised by 
the possession of hooks in the middle of the lateral borders of its 
valves and by its larval byssus—which is not homologous with that 
of other Lamellibranchs. This byssus appears to issue from, but 
in reality winds round, the adductor muscle, and originates from a 
single glandular epithelial cell, deeply embedded in the tissues on 
aad 
Fic. 228. 
Two stages in the development of Anodonta ; both figures represent the ‘‘glochidiumn ” stage. 
A, when free swimming, shows the two dentigerous valves widely open. B, a later stage, after 
fixation to the fin ofa fish. a.ad and ad, anterior adductor muscle ; al, alimentary canal ; au.v, 
otocyst ; br, branchial filaments ; by, byssus ; f, foot; mt, mantle-flap ; p.ad, posterior adductor ; 
s, teeth of the shell; sh, shell. (From Lankester, after Balfour.) 
the dorsal and anterior side of the muscle. The glochidia swim 
actively by clapping together the valves of the shell, and eventually 
attach themselves to the gills or fins of a fish, and become encysted 
in consequence of a pathological development of the epithelium of 
their host. This parasitic existence lasts for a period varying from 
two to six weeks, during which the glochidia are nourished by the 
epidermic elements of their host, absorbing them by means of the 
ectodermic cells of the embryonic mantle. During this time most of 
the definite organs of the adult, the 
foot, otocysts, gills, ete., which were 
not required in larval life, are de- 
veloped, largely as the result of the 
proliferation of the cells of two sym- 
metrical cavities situated behind the 
adductor muscle. In a general way 
the development of the organs follows 
the normal course, but some—the 
borders of the mantle, for instance— 
are formed anew. ‘The glochidium a 
5 ° Parasitic larva of Anodonta on the 
shell is not cast off but persists, though eighth day of parasitic life ; ventral view. 
it undergoes a considerable change /,'00h; 23h ame eto ee 
of shape. The posterior ciliated shield 
and the byssus disappear. During the early part of the parasitic 
life the mouth acquires an opening into the previously closed 
endodermic cavity or archenteron, but the anus is not formed till 
Fig. 229. 
