SNAILS AND MUSSELS 



dividual and the sperm cells by another. Eggs pass from the 

 ovary into the mantle cavity and thence into the gills of the 

 parent mussel. Each gill is a flattened sac, partitioned off 

 into water tubes, and currents of water carry the eggs into 

 their open tops. Sperm cells are discharged into the water 

 by a male which is usually nearby. They are carried through 

 the incurrent siphon and finall>' into the gill of the female 

 where fertilization of the eggs takes place. Thus the thin 

 curtain-like gill becomes the brood pouch (Fig. 262) and each 

 water tube is filled with eggs like peas packed in a pod. 

 There the developing embryos remain until they have be- 

 come glochidia, larval mussels with their first juvenile shells. 



A brood pouch is brown or whitish according to the color 

 of the eggs or embryos contained in it and so full that the 

 slightest tear in it will bring them streaming out like sand. 

 The number of embryos in the pouches varies with the species; 

 according to counts and computations there may be from 

 seventy-five thousand to three millions of them to a parent. 

 In the little glochidia the shells gape wide open/ but are 

 snapped together intermittently by the single add'uctor 

 muscle. If a few glochidia are put in a drop of water on a 

 piece of glass their convulsive snapping can be seen easily 

 with a hand-lens. The least jar in the water will start them 

 clapping almost in unison and if a drop of fish blood or a little 

 salt is dropped into the water there will be the wildest excite- 

 ment. 



Their persistent snapping stands the glochiciia in good stead 

 when they are thrown out of the brood pouch. Then their 



^ Fig. 263. — Minnow with larval musses or glochid- 

 ia clinging to its fins and body, taken from a brook 

 in midwinter. 



3M9 



