OYSTERS AND METHODS OF OYSTER-CULTURE. 271 



A circular tuft of long hairs or cilia has now made its appearance at what is thus 

 marked as the anterior end of the body, and as soon as these hairs are formed they 

 begin to swing backward and forward in such a way as to constitute a swimming 

 organ, which rows the little animal up from the bottom to the surface of the water, 

 where it swims around very actively by the aid of its cilia. This stage of develop- 

 ment, plate VII, fig. 12, which is of short duration, is of great importance in raising the 

 young oysters, for it is the time when they can l)e8t be siphoned oft" into a separate 

 vessel and freed from the danger of being killed by the decay of any eggs which 

 may fail to develop. On one surface of the body at this stage, the dorsal surface, 

 there is a well-marked groove, and when a specimen is found in a proper position for 

 examination the opening into the digestive tract is found at the bottom of this groove. 

 Plate VII, fig. 13, is a sectional view of such an emljryo. It is seen to consist of a cen- 

 tral cavity, the digestive cavity, which opens externally on the dorsal surface of the 

 body by a small orifice, the primitive mouth, and which is surrounded at all points, 

 except at the mouth, by a wall which is distinct from the outer wall of the body. 

 Around the primitive mouth these two layers are continuous with each other. 



The way in which this cavity, with its wall and external opening, has been formed 

 will be understood by a comparison of plate vii, fig. 13, with plate vii, fig. 8. The 

 layer which is below in plate vii, fig. S, has been pushed upward in such a way as to 

 convert it into a long tube, and at the same time the outer layer has grown downward 

 and inward around it, and has thus constricted the opening. The layer of cells which 

 is below in plate vii, fig. 8, thus becomes converted into the walls of the digestive 

 tract, and the space which is outside and below the embryo, in plate Vll, fig. 8, becomes 

 converted into an inclosed digestive cavity, which opens externally by the primitive 

 mouth. 



This stage of development, in which the embryo consists of two layers, an inner 

 layer surrounding a cavity which opens externally by a mouth-like opening, and an 

 outer layer which is continuous with the inner around the margins of the opening, 

 is of very frequent occurrence, and it has been found, with modifications, in the most 

 widely separated groups of animals, such as the starfish, the oyster, and the frog; 

 and some representatives of all the larger groups of animals, except the protozoa, 

 appear to pass during their development through a form which may be regarded as 

 a more or less considerable modification of that presented by our embryo oyster. 

 This stage of development is known as the f/astrida stage. 



The edges of the primitive mouth of the oyster continue to approach each other 

 and finally meet and unite, thus closing up the opening, as shown in plate vii, fig. 16 

 and leaving the digestive tract without any communication with the outside of the 

 body, and entirely surrounded by the outer layer. The embryo shown in plate vii, 

 figs. 12 and 16, are represented with the dorsal surface below, in order to facilitate 

 comparison with the adult, but in plate vii, fig. 17, and most of the following figures, 

 the dorsal surface is uppermost, for more ready comparison with the adult. 



In other lamellibrauclis, and doubtless also in the oyster, the shell 

 begins as a deposit in an invagination or pocket on the dorsal side of 

 the body. In its manner of formation this shell-gland resembles the 

 l)rimitive mouth for which it has been more than once mistaken by 

 investigators. In some forms the shell is at first single, but in the 

 oyster they are said to be separated from each other from the beginning, 

 and appear independently. Dr. Brooks says further: 



Soon after they make their appearance, the em1)ryos cease to crowd to the surface 

 of the water and sink to various dejiths, although they continue to swim actively in 

 all directions, and may still be found occasionally close to the surface. The region 

 of the body which carries the cilia now becomes sharply defined, as a circular pro- 

 jecting pad, the velum, and this is present and is the organ of locomotion at a much 

 later stage of development. It is shown at the right side of the figure in plate vii, 



