134 SCIENCE FROM AN EASY CHAIR 



driven along into the openings of the tubular reproductive 

 sacs, and into those sacs in the case of those oysters 

 which are at the time producing eggs. 

 There they fertilise the eggs. The 

 \. V <4 J " J~ minute eggs begin to develop whilst still 

 T^ within the parent's body, and continue 

 /- r* to do so whilst remaining within the 

 4 X shelter of the shell, adhering to the 

 gill-plates (Fig. 33). In a day or two 

 each fertilised egg has developed into a 

 very mmute crea ture, provided with a 

 tiny circlet of cilia or vibratile hairs, the 

 FIG. 32. The sperms movements of which cause it to swim 



or spermatozoa of /T -,. ,-,, ~, 



a ripe oyster, as ( Fl - 33*> The parent oyster is now 

 seen swimming in a said to be " white-sick." In the course 

 drop of sea water : o f a CO uple of days the young oyster 

 dmeTHneat 2OCO still within its parent's shell becomes dark 

 in colour, and has formed on its surface 

 a pair of symmetrical shells, not like those of an adult 

 oyster, but convex (Fig. 34) like those of a clam or 

 a cockle. The head region, with its circlet of vibrating 

 cilia, can be projected between the open shells or with- 

 drawn between them when the shells are shut. The 

 mother oyster, laden with these little dark specks, is now 

 said to be " black-sick." 



In the course of a week or so the brood of dark 

 young oysters escapes by thousands from the parent's 

 shell into the surrounding water. They swim by their 

 circlet of vibrating hairs, or " velum," as it is called, 

 towards the surface, and are carried far and wide by the 

 tides. They are active, transparent little " dots," very 

 unlike their parent (Fig. 34). The next thing that 

 happens to them after a few days, perhaps weeks is 

 that owing to the increasing weight of their shells, they 

 sink to the bottom. More than half perish by dropping 



