The first change that takes place is the disappearance of the byssus and the byssus organ. This occurs very soon; shortly afterwards all traces of the velum and sense organs also become lost.
At the time of the disappearance of these bodies, at the point of the projection from which the byssus cord arose, and very possibly from this very projection, the foot arises as a rounded process which rapidly grows and soon becomes ciliated ([fig. 121] B, f).
The single adductor muscle begins to atrophy very early, but before its entire disappearance rudiments are formed at the two ends of the body, which at a later period can be distinctly recognised as the anterior and posterior adductor muscles ([fig. 121]1 B, a.ad and p.ad).
After the formation of these parts the gills arise as solid and at first somewhat knobbed papillæ covered with a ciliated epidermis, on each side of, but somewhat in front of (!) the foot ([fig. 121] B, br). In the foot there soon appear the auditory sacks (au.v), and the foot itself becomes a long tongue-like ciliated organ projecting backwards[110].
The mantle lobes undergo great changes, and indeed by Braun the mantle lobes are stated to be formed almost entirely de novo. The permanent shell is (Braun) formed on the dorsal surface of the still parasitic larva in the form of two small independent plates. I have not followed the changes of the alimentary canal, etc., but at an early stage there is visible, dorsal to the foot, a simple enteric sack.
By the time the larva leaves its host all the organs of the adult, except the generative organs, have become established.
The post-embryonic development of the organs of Glochidium is similar in the main to that of other Lamellibranchiata. This fact is of some importance on account of the peculiarities of the earlier developmental stages.
The byssus organ, the toothed processes of the shell, and the sense organs of the Glochidium can hardly be ancestral rudiments, but must be organs which have been specially developed for the peculiar mode of life of the Glochidium. Whether the single muscle is to be counted amongst such provisional organs is perhaps a more doubtful point, but I am inclined to think that it ought to be so.
If however the single muscle is an ancestral organ, it is important to observe that it entirely disappears as development goes on and the two adductor muscles in the adult are developed independently of it.