70 COMMISSION OF CONSERVATION 



The foregoing account of the apparent ontogenetic development of 

 the gills of the oyster differs markedly from the recorded phylogentic de- 

 velopment of the gills of lamellibranchs. This difference, like the asym- 

 metry of the prodissoconch, may be understood as a crenogenetic modi- 

 fication of biogenesis — a sort of short-cut to the final structure. 



Note on the Phylogeny of the Lamellibranchiate Gill. — For a long time it has been 

 customary among zoologists to speak as if the oyster or other bivalve mollusk, possesses 

 four gills, which it really appears to do, two on each side of the body. Comparative 

 anatomy and comparative embryology of the different classes have led to the view 

 that bivalves have been developed from a primitive, symmetrical, gastropod-like an- 

 cestor, having a simple head in front bearing two tentacles, two eyes, and a mouth with 

 a rasping tongue; a low conical shell above, that could be drawn down over all the soft 

 parts, and lined by a mantle that secretes its material; a flat, creeping and clinging foot 

 below; and two feather-shaped gills, disposed right and left, projecting backwards. 

 Each of these was a symmetrically constructed, bi-pectinate gill or ctenidium, having a 

 central axis with two rows of filaments, an upper and a lower. There are still living, 

 limpet-like gastropods along our coasts possessing such characters, although no one 

 species retains them all or in the most primitive form. 



The class of mollusks to which the oyster belongs has in the course of time suffered 

 modifications of the characters mentioned. Its members have taken to a more quies- 

 cent mode of life, such as burrowing in sand or fixing to rocks, and in consequence have 

 largely lost those external organs of locomotion, plunder, and special sense, so necessary 

 to free-roving animals. The absence of a head has been regarded as characteristic 

 (hence the class has been called Acephala) ; in place of a single piece they have developed 

 a shell with two valves (Bivalva) ; the foot in by far the greatest number of forms has 

 become somewhat hatchet-shaped (Pelecypoda) ; the gills are leaf-like, each separable 

 into flat plates (Lamellibranchia). If all bivalves had become equally modified it 

 would certainly have been difficult to determine the origin of the group, but owing to the 

 diversity of natural conditions and the reactions of these upon living organisms, certain 

 species have been forced to pursue special lines of action in order to better their chances 

 for life, with the result that the organs chiefly concerned have become more special- 

 ized, while other organs have retained much of their original structure, and it is just 

 these latter that are especially valuable in tracing ancestral affinities. As long ago as 

 1848 Lenckart comprehended the unity of structure in the gills of Mollusca, and his 

 views have been supported by Menegaux, Pelseneer, and many others. Peck has 

 studied the Lamellibranch T gill. Lankester, Hatschek, Thiele, Lang, and others have 

 worked out the phylogeny. 



In the adult oyster there is but one gill on each side, comparable with a ctenidium, 

 and composed of two hemibranchs (gill-leaves or branchial foliae); each hemibranch 

 i- : split lengthwise, from above nearly to the lower free edge, but not through it, into 

 outerl'and inner plates (lamellae), or subdivided transversely into numerous V-shaped 

 filaments of which one-half belongs to the outer lamella and the other half to the inner 

 lamella of the hemibranch. In the full-grown larvae of the oyster it is possible to recog- 

 nize the ctenidial axis with its lower series of filaments, but we have to await the spat 

 of 2 • 5 to 3 mm. to see the upper series, which, it may be conceived, has had to rotate 

 outwards nearly half way round the axis in order to be accommodated within the 

 branchial chamber. 



In the course of phylogenetic development the originally straight filaments have 

 become bent upon themselves to permit of greater length (breathing surface) and still 

 be protected in the branchial chamber — those of the ventral series were folded inwards 

 and those of the dorsal (but now lateral) series were folded outwards, while their tips, 

 coming in contact with the foot in the one case or the mantle in the other, clung for 

 support, were directed upwards, and finally became united along the side of the body or 

 along the inner surface of the mantle. At places their ciliated surface became knit together 

 for further support (inter-filamentar and inter-lamellar junctions), leaving intervening 

 gill-slits between contiguous filaments and ascending water-tubes between opposed 

 lamellae. At the level of union of the gill-filaments with the body and wit li the mantle, 

 they separate off a branchial chamber below and between the suspended hemibranchs 

 from a BUpra-branchi&l (or cloacal) chamber above, and by the activity of the cilia, 

 water is drawn into the former, directed through the gill-slits, up the water-tubes, and 

 out by the cloacal chamber. The position of the original ctenidial axis lies above and 

 between the lines of origin of the filaments of each pair of hemibranchs, and is marked 



