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" 



FIG. 215. Development of Anodonta. After Goette. 



1. Section of blastosph ere. s.d., Shell gland ; c.d., ciliated disc ; e. y 



beginning of ectodermic invagination. Note mesoderm cells in 

 the cavity. 



2. Later stage. m.> Mesoderm. 



3. Embryonic shell has appeared. 



4. Glochidium larva ; note byssu 



valves. 



. 



sus threads, and teeth on shell 



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. 



