DEVELOPMENT AND LIFE HISTORY. 



371 



an invagination establishes tlie archenteion and the embryo becomes a 

 gastrula (see Fig. 120). 



The shell sac forms an embryonic shell, and many of the mesoderm 

 cells combine in an adductor muscle. The mouth of the gastrula closes, 

 and a definite mouth is subsequently formed by an ectodermic invagina- 

 tion. Gradually a larva peculiar to fresh water mussels, and known as 

 a Glochidium, is built up. 



The Glochidium has two triangular, delicate, and porous shell valves, 

 each with a spiny incurved tooth on its free edge. The valves clap 

 together by the action of the adductor muscle. The mantle lobes are 

 very small, and their margins bear on each side three or four patches of 

 sensory cells. The foot is not yet developed, but from the position 

 which it will afterwards occupy there hang long attaching threads of 



Fig. 120. — Development of Anodonta. (Alter Giktte.) 



1. .Seclion of blaslosphere. sd., shell gland ; c,/., ciliated disc ; 

 i:., beginning of endodermic invaginalion. Note mesoderm cells 

 in the cavity. 



2. Later stage, in. mesoderm. 



3. Embr>'onic shell has appeared. 



4. Glochidium larva; note byssus threads, and teeth on shell 

 valves. 



" 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 and the 

 embryonic byssus glands vanish, but a new byssus gland (which remains 



