THE TRANSITION TO THE ADULT. 59 



rates completely later, so that the two adductors of the adult must 

 be regarded as new formations. In opposition to this view we have 

 the statement of SCHIERHOLZ that the larval muscle only partly 

 degenerates, some of it passing over into the anterior adductor of 

 the adult. This latter condition would agree with the fact that the 

 anterior adductor appears first in most Lamellibranchs, and for along 

 time is the only adductor present (p. 48) ; BBAUN, however, has 

 maintained his original view against that of SCHIERHOLZ. 



The formation of the intestine is also apparently greatly influenced 

 by the specialised conditions of the larva. The archenteron had 

 already lost its connection with the ectoderm before the commence- 

 ment of parisitism, and lay in contact with the ectoderm as an 

 entodermal vesicle closed on all sides. In this condition it remains 

 for a very long time ; the larva either does not require nourishment 

 or obtains it as described above through the mushroom-shaped growth 

 of the mantle. The small entoderm-vesicle is now found in the pos- 

 terior part of the larva lying rather closely applied to the ectoderm. 

 The swelling carrying an invagination known by authors as the oral 

 shield (Fig. 26 A, m) has also shifted posteriorly. The sac of the 

 oral or middle shield of authors is the rudiment of the stomodaeum, 

 and appears as a transverse slit (Fig. 26, m). By the development 

 of the foot this organ is pressed forward. The entoderm-vesicle also 

 lengthens from behind forward and fuses with the ectodermal rudi- 

 ment of the stomodaeum. At the posterior end where the entoderm 

 vesicle is in contact with the ectoderm, the anus now breaks through, 

 without the formation of an ectodermal invagination (F. SCHMIDT, 

 SCHIERHOLZ). The formation of the other organs, in so far as they 

 present peculiar features, will be described later. 



When the young Lamellibranch leaves the fish, it moves about 

 with great activity by means of its foot, which has in the meantime 

 become perfected, having lengthened very much and become geni- 

 culate. On its lower surface it carries a groove which represents the 

 rudiment of the byssal gland. The latter arises, as in Cyclcut, in the 

 form of two pits situated posteriorly on the pedal swelling. In conse- 

 quence of an invagination which forms later, these pits come to lie at 

 the base of a funnel-shaped pit which is afterwards continued into 

 the longitudinal groove just mentioned. The persistent byssal gland 

 of other Lamellibranchs exhibits similar morphological conditions 

 to those already described in connection with the Unionidae and 

 Gyda*. 



