DEVELOPMENT AND LIFE HISTORY. 401 
sensory cells. The foot is not yet developed, but from the position 
which it will afterwards occupy there hang long attaching threads of 
‘*byssus,” which moor the larva. If it manage to anchor itself on the 
tail, fins, or gills of a fish, the Glochidium shuts its valves and fixes 
itself more securely, and is soon surrounded by a pathological growth of 
- its host’s skin. 
In this parasitic stage a remarkable metamorphosis occurs. The 
sensory or tactile patches not unnaturally disappear; the ‘‘ byssus” 
Fic. 215.—Development of Azodonta.—After Goette. 
x. Section of blastosphere. s.d., Shell gland; ¢.d., ciliated disc; e., 
beginning of ectodermic invagination, Note mesoderm cells in 
the cavity. 
2. Later stage. 2., Mesoderm. 
3. Embryonic shell has appeared. 
4. Glochidium larva; note byssus threads, and teeth on shell 
valves. 
and the embryonic ‘‘ byssus glands” vanish, but a true byssus gland 
(which remains quite rudimentary in Anodonta) appears; the single 
adductor atrophies, and is replaced by two; the foot and the gills 
make their appearance ; the embryonic mantle lobes increase greatly, 
or are replaced by fresh growths ; and the permanent shell begins to be 
made. 
After -this metamorphosis, when the larva has virtually become a 
miniature adult, no longer so liable to be swept away, it drops from its 
temporary host to the bottom of the pond or river pool. : 
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