PHYLOGENY OF THE PELECYPODA. 303 



of the young- oyster is effected by means of a byssus of however short a duration. Tn my 

 studies of the young of Avicula, Perna, Pecten and Anomia, Pis. xxvii-xxrx, it is shown 

 that a notch for the projection of the byssus, or foot, is marked in the dissoconch shell im- 

 mediately after the close of the prodissoconch stage. On most careful study no such indi- 

 cations of a byssal attachment were seen on the lower valve of attached 03^sters, and as 

 they Avere growing on transpai-ent media, fullest oppoi'tunity was enjoyed to make such 

 observations. In view of the evidence it is therefore safe to conclude that the oyster does 

 not have a byssus at any period of its development. The preliminary fixation is probably 

 effected by means of the reflected mantle border, as described by Ryder, and is then im- 

 mediately succeeded by a cementing conchyolin attachment of the extreme edge of the 

 lower left prodissoconch valve. 



The fiict that a velum, or swimming organ, exists up to the period of permanent fixa- 

 tion, accounts for the great reduction of the foot, because that organ is unnecessary while 

 the animal is provided with another locomotive organ, and is useless for progression af- 

 ter the animal is permanently attached. . The reduction of the foot is clearly attributable 

 to disuse and a high degree of concentration of development. This is another of the cases 

 of extreme concentration of development seen so markedly in the development of the 

 oyster. 



The gills of the young oyster, PI. xxiv, figs. 1-2, and fig. 6, consist of two rows of fila- 

 mentous appendages, one on either side. They are fixed to the body mass anteriorly, 

 but are free posteriorly, and are capable of considerable dorsal and ventral rotation. 

 Eight club-shaped filaments were counted in either row at this early stage. Each tila- 

 memt is ciliated and perforated by a tube which expands dorsally into a canal connect- 

 ing the tubes of the several filaments. The filaments are entirely free and separate from 

 one another excepting on their dorsal border and are not bent back upon themselves on 

 their ventral margins as they are in later stages. Each filament represents a hollow bag, 

 or pouch, in which the blood is aerated by water kept in constant motion by the active 

 cilia. This stage of development of the gills is exactly comparable to that figured in the 

 young of Mytilus by Lacaze-Dathiers (41). The condition is also comparable to that de- 

 scribed by Mitzukuri in the adult of Nucula excepting that in that genus a pair of filif- 

 erous branchiae exist on either side. After tracing the further development to the adult 

 condition in Ostrea it will be seen that the gills of the other Pelecypoda described are 

 only modifications of this simple early type. 



V. Ostrea : Development of the Soft Pakts. 



The Spat. 



At the close of the period just described, the yoimg oyster enters on stages of growth 

 both in the soft and hard parts which are rapidly approaching the conditions character- 

 istic of the family to which it l)elongs. They are stages distinctly post-embryonic and 

 I shall consider them as the nepionic or babyhood stages.^ 



Tile young oyster, Pi. xxiv, figs. 1-2, growing on a glass slide, was kept alive in a 



' For a UiscussiDii ol' llio ndiiu'iicluliiru oi' stau'es vide sccliDii 111. 



