344 



MOLLUSCA. 



laid in spring or summer, and pass into the space between the lamellae of the 

 inner gill, and thence into that between the lamellae of the outer gill at the 

 posterior end of the gill, where these spaces communicate. In this space they 

 remain as in a brood-pouch, and undergo the first stages of their development. 

 These take two months. They then cease to develop, and pass the winter in the 

 brood-pouch. In the following spring they pass out by the exhalent orifice 

 under the larval form, known as Glochid ium (Fig. 269). This possesses a bivalve 

 shell with hooks in the middle of the ventral edge of the valves, a single 

 adductor (the anterior), and a byssus thread which appears to arise just behind 

 the adductor. They swim by snapping their valves, and attach themselves to 

 the skin (gills or fin) of a fish, in which they become embedded. Here they 

 remain as parasites from two to five weeks, and undergo further- development ; 

 but they are not fully formed for some time after leaving their host. The gills 

 grow, and the external lamina is not developed till the third year. Sexual 

 maturity is not attained till the fifth year, and growth continues later. 



FIG. 269. Vontral view of the Glochidium of Anodonta (from Pelseneer after Schierholtz). 

 a bunch of sotae ; I visceral ganglia; c stomodeal imagination ; d ciliated patch; 

 e enteron ; / lateral pits ; g hooks on the edge of the valves ; h byssus filament ; 

 i the single adductor. 



The Lamellibranchs are all aquatic animals, for the most part 

 marine, but a few are fresh-water. They feed on microscopic, 

 mainly vegetable organisms (Diatoms, etc.), which float in the water, 

 and are conveyed to them by currents set up by the cilia of the 

 mantle-chamber. The Septibranchs alone are carnivorous. A great 

 many forms live partly or wholly embedded in sand or mud, procuring 

 food and water and getting rid of waste by their siphons which 

 project at the surface. Many are sedentary, being attached to 

 foreign objects by their byssus, or by one of the valves of their 

 shell (Ostrea, Spondylus). Some are borers into wood (Teredo) or 

 into stone (Lithodomus, Pholas, Clavagella). Some build nests by 

 means of their byssus (Lima) ; while some are commensals Modio- 



