60 COMMISSION OF CONSERVATION 



ing somewhat with the individual it will be found that they have become 

 more or less modified. Viewed from the broad side and preserving the 

 original orientation of the larval shell, the height of the spat shell soon 

 becomes greater than the length, the hinge and umbos instead of 

 being medio-dorsal or postero-dorsal, come to mark rather the narrow 

 anterior end of the growing spat, and the larval shell sinks into insignifi- 

 cance in comparison v/ith the spat shell. Its left valve may become ob- 

 literated by growth of the surface of attachment, but its right valve may 

 often be found until late in the life of the spat, although it is liable to be- 

 come destroyed by weathering of the umbo-region of the latter. While 

 its position marks the anterior end of the oyster, it has long since been 

 carried up by the umbo of the spat shell out of reach of the hinge or of the 

 growing parts, but its anterior end still points in a general way in the 

 direction of the anterior end of the adult. This position and relation is 

 correlated, as will be seen later, with the increase of growth of the lower 

 and posterior parts of the oyster's body, which occasions more or less of a 

 rotation round the great adductor muscle, and resulting in longer or 

 rounder forms of shells with straighter dorsal and much curved ventral 

 border. The right, upper valve remains flatter and smoother, the left, 

 lower or attached valve, more deeply hollowed, rougher, with more con- 

 spicuous concentric furrows or sharp over-lapping edges, and often fluted 

 with blue-tipped projecting processes. 



Asymmetry, or the greater convexity of the left valve over that of the right side, 

 can not be due to the operation of external conditions during the present life-time of 

 the individual (ontogenetic development). Shells that are fixed on the under sides of 

 stones are similarly a symmetrical with those on the upper sides, although in the former 

 case it is the right side which is turned downwards. It is inconceivable that the pheno- 

 menon is due to the mere contact of the present lifeless shell. Spats that become early 

 separated from their attachment continue to grow asymmetrical. The asymmetry, 

 whether of the shell or of the living soft parts, is hereditary. _ 



Neither can the asymmetry of the present oyster have been brought about dimng 

 the larval life oi' any or of all of the ancestors of the oyster (phylogenetic development of 

 the larva), since both valves in that case would be developed mider exactly the same 

 conditions'as in the life-time of the present larva, which is at first symmetrical. 



Asymmetry must have originated during the post-larval existence (phylogenetic 

 development of post-larval stages) of some of the later ancestors of the oyster._ Smce 

 most bivalves are symmetrical, and since the yomig larvae of all are symmetncal, we 

 must believe that oysters sprang from symmetrical ancestors. Moreover, since asym- 

 metry in its extremest fonns is always associated with fixation on one side (Ostrea, 

 Anomia), while in less marked forms it is to be found in deep-bodied species that habitu- 

 ally fall over on one side as soon as locomotion ceases (Pecten, Mya), we are constrained 

 to connect asymmetry with lateral fixation or rest. It is conceivable that gravitation 

 was the prime incentive to the occasion for fixation as well as asymmetry. That the 

 larvEe of all of these species possess byssus-glands shows that fixation preceded asym- 

 metry. A byssus-gland is the agent of fixation (i. e. of the conversion of a larval into 

 a post-larval state), which is the pre-requisite for asymmetry. 



We may consider that a difference in the conditions of the two sides was first oc- 

 casioned when some remote but free and symmetrical ancestors of the present oyster 

 took to a fixed mode of life, becoming attached by a sticky secretion from the foot; that 

 the deep shell tended to fall over on one side (say the left), permitting the secretion to 

 flow between the shell and the substratum; that the difference in the condition of the 

 two sides gradually brought about a difference in the size and structure of the twc» sides; 

 that this difference became so deeply impressed into the system as to become hereditary; 

 and that, further, it began to precede in the life of the individual the penod of fixation 



