328 FOSSIL OSTEEID^ OF NORTH AMERICA. 



is ovei, tc probably again develop to great ])roportious by the time the 

 next spawuiug season arrives. The organs also vary with the size of 

 the individual, and a large female American oyster may contain more 

 than 100,000,000 ova ; a small one, 3,000,000 to 4,000,000, or less. They 

 begin spawning at one year old. 



At a temperature of 75° to 80° Fahr. the period of incubation of the 

 American oyster is only five to six hours, when the young commence to 

 lead an independent active existence, which is in the most striking con- 

 trast with the permanent sedentary habit of the spat and adult condi- 

 tions. In the European oyster the young are retained in the folds of 

 the mantleand about the gills of the parent for apparently a much longer 

 time, the length of which does not, however, yet seem to have been 

 determined. 



The young, when first hatched, are ovoidal in form, not much, if any, 

 larger in bulk than the egg, and they have a slight depression on the 

 back or dorsal side, which marks the position of the shell gland or first 

 rudiment of the mantle organ properly so-called. Here, as development 

 advances, the shell is formed as a very thin saddle-shaped structure, 

 the right and left lobes of which grow in size with the development of 

 the embryo. Soon these two halves of the larval shell become very con- 

 vex and cover the soft parts of the young oyster on either side almost 

 enlirely ; the mantle m and velum v alone iJrojecting somewhat past their 

 margins, as shown in Fig. 1 in the accompanying Plate LXXV, which 

 reiJreseuts a young American oyster in the larval or fry stage enlarged 250 

 times. The velum v consists of a cushion-shaped anterior projection of 

 the soft parts, which bears two circles of long, very minute, thread-like 

 appendages or cilia, which aie incessantly vibrating, and which con- 

 stitute the locomotive organs of the fry. The rapid movement of these 

 filaments propels the young oyster through the water, and probably 

 also carries minute particles of food to the mouth, situated immediately 

 below the velum. The intestine and stomach are developed by this 

 time, and there are also delicate muscles formed which retract the velum 

 and draw the valves together. 



A portion of the nervous system is developed in the center of the 

 velum, which answers to the supraoesophageal ganglion «</, Fig. 1, Plate 

 LXXIII (of the adult), which consists of a pair of knots of nervous 

 matter, which lie above and at either side of the mouth, and which are 

 connected on either side of the body by a commissure or nervous thread, 

 with the larger hindmost ganglion jpg below the adductor muscle. The 

 mantle border is innervated from this hindmost nervous mass, and radi- 

 ating threads pass out from it on either side in all directions to the edge 

 of the mantle and tentacles. The nervous system of the fry or larvte is 

 much simpler than that of the adult, yet they are apparently sensitive 

 to external stimuli, such as raps or blows struck on the table on which 

 the microscope rests under which one is observing them. 



The duration of the locomotive stage of development of the larvaa 



