68 FIRST LESSONS IN ZOOLOGY. 



freshen the blood ; and the heart, which is composed of two 

 chambers (ve and an] collects the blood by three blood- 

 vessels (br) on each side. The blood thus freshened enters 

 the auricle of the heart (u), and then passes into the 

 ventricle (ve), to be distributed through two arteries which 

 branch out all over the body. 



Now we will see how the oysters grow. Many millions 

 of eggs are laid by the oyster, and they pass directly out 



FIG. 68. Oyster, ften, gland in which the eggs are formed; nv, oviduct, from 

 which the eggs are discharged; mus, adductor muscle; H, heart; utt, mantle; 

 P, palp; G, gills. After Ryder. 



into the sea-water. A very large proportion come to 

 naught. Mr. Ryder tells us that probably only a fraction 

 of 1 per cent of all that are laid ever develop or fix them- 

 selves, even under favorable conditions.* 



* A German naturalist (Professor Moebius) estimates that each Eu- 

 ropean oyster which is born has TTrinijd f a chance to survive and 

 reach adult age, so numerous and effective are the adverse conditions 

 which surround the millions of ego's: and Mr. Ryder adds that those 

 of the American oyster, whose yield of eggs is much greater, have, 

 on account of their smaller size, probably still fewer chances of sur- 

 vival. This is the case with most of the lower animals. 



