LIFE-HISTOR Y OF ANODONTA. 3 1 1 



convenient cradle, safe and yet continually freshened by 

 water-currents. There, moreover, the developing embryos 

 seem to be nourished by gelatinous stuff secreted from the 

 gills. The larvEB remain in this shelter until they are 

 ready for liberation, and may be retained beyond the usual 

 period if there are no fish near. For it is to fish, such as 

 sticklebacks and perches, that the liberated larvae fix them- 

 selves. The larva is called a Glochidium. 



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 

 "byssus," which serve to moor the larva. If it manage to 

 anchor itself on the tail, fins, or gills of a fish, the Glochidium 

 claps 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 small gland which made it 

 vanish, but a new byssus gland (which remains quite rudi- 

 mentary 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. 



Classification of Bivalves. 



Order i. IsoMYA. Adductor muscles approximately equal. 



Sub-order i . Integripallia. The mantle's line of attachment to 

 the shell is not broken by a sinus into which 

 inhalent and exhalent siphons may be retracted, 

 but in most these siphons are present. 

 Area (Noah's Ark shell), Unio and Anodonta (fresh- 

 water), Lucina, Cyprina, Cardium (cockle), Cyclas 

 (fresh-water), Tridacna. 



