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SCIENCE 



[N. S. Vol. XXXIX. No. 1006 



internal, medial in position, and confined 

 strictly within the hinge area) ; (4) by the 

 presence of an anterior adductor muscle im- 

 pression indicating the persistence of the 

 anterior adductor in the adult stage (in Ostrea, 

 the anterior muscle has been observed by a 

 number of embryologists in larval forms 

 though never in stages later than the 

 nepionic) ; (5) by the presence of pedal im- 

 pressions indicating a well-defined, though 

 probably reduced foot (in Ostrea, the loco- 

 motor organ is the velum which persists 

 throughout the free swimming stage, and the 

 presence of a well-defined foot and byssus have 

 never been established even in the embryo). 



These observations have led Jaworski to the 

 theory that the new form, the Heierostrea 

 steinmanni Jaworski, represents the mid- 

 Jurassic ancestral type from which the true 

 oysters of the Cretaceous, Tertiary, and Recent 

 were directly derived. In order to establish 

 this new hypothesis, it is necessary to first 

 overthrow some of the critical work that has 

 already been done along this line and from 

 which quite different conclusions have been 

 drawn. The three most notable researches are 

 those made by Jackson, of Harvard, Douville, 

 of the French Geological Survey, and Stein- 

 mann, of Bonn. Jackson's work was done at 

 the Museum of Comparative Zoology, and his 

 approach to the problem was by way of the 

 embryology of the oyster and a number of 

 closely related mono- and heteromyarian 

 groups. The oysters were watched through all 

 stages of development of the egg, the spat 

 encouraged to attach themselves to trans- 

 parent media and the growth phenomena 

 studied under magnification. He was thus 

 able to confirm and elaborate many of the ob- 

 servations of Huxley, Horst and Brooks, 

 notably those on the number and position of 

 the adductor muscles during the earlier de- 

 velopment of the form. 



Ostrea, like all other pelecypods in which 

 the embryonic development has been carefully 

 worked out, passes through a monomyarian 

 stage in which, curiously enough, the single 

 adductor muscle is anterior in position, al- 

 though it is not the antecedent of the anterior 



adductor of the adult. Then there is a short 

 period at the end of the prodissoconch and the 

 beginning of the dissoconch stages in which 

 both the anterior and posterior adductors are 

 present and typically dimyarian in position. 

 About the third day after the animal has be- 

 come attached, the anterior adductor evanesces, 

 and only the posterior remains. The atrophy 

 of the anterior adductor has been explained by 

 purely mechanical action: in the dimyarian 

 stage, the orientation of the soft parts is 

 similar to that of the typical pelecypod; the 

 mouth is approximately midway between the 

 hinge line and the ventral margin and the 

 antero-posterior axis is approximately parallel 

 to the hinge; as soon as the spat become fixed, 

 there is a shifting of this axis through almost 

 45 degrees so that the mouth lies close up 

 under the hinge. This change in position 

 brings the anterior adductor so near the hinge 

 that it loses most of its efficiency; there is, 

 therefore, a compensating increase in the size 

 and effectiveness of the posterior adductor 

 which is gradually shifted to a point of van- 

 tage near the central portion of the valves. 



The character and mode of development of 

 the soft parts, as a whole, have led Jackson to 

 the belief that Ostrea is the sessile analogue 

 and the direct descendant of the free swim- 

 ming Perna. The difference in habit would 

 readily explain the unequal valves and the 

 absence of the foot in the one form and the 

 equal valves and well-developed foot in the 

 other. Perna is characterized, however, by a 

 series of vertical cartilage grooves while in 

 typical Ostrea there is a single trigonal sub- 

 umbonal pit. Jaworski's criticism of Dr. 

 Jackson's theory of the relationships seems 

 pertinent: the very fact that the young of 

 Perna possess a single sub-umbonal pit similar 

 to that of Ostrea and the adults a series of pits 

 implies a highly specialized type. The simi- 

 larity of the young of Ostrea and Perna may 

 very properly be due to deseendence along 

 collateral lines from a common aviculoid an- 

 cestor, but there is small evidence of any 

 more direct relationship. Jackson considers 

 Exogyra and Gryphwa derivative forms of 

 Ostrea which have become highly modified. 



