MOLLUSCA. 269 



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,/). 



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 B, a.ad and p.ad). 



After the formation of these parts the gills arise as solid and at first 

 somewhat knobbed papilla? 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 1 . 



The mantle lobes undergo great changes, and indeed by Braun the 

 mantle lobes are stated to be formed almost entirely de novo. The perma- 

 nent 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. 



1 The position of the foot and gills in the larva represented in Fig. 119 B would be 

 more normal if the convex and not the flatter side of the shell were the anterior. I 

 have followed Rabl and Hemming in the determinations of the anterior and posterior 

 end of the embryo, but failed to rear my larvae up to a stage at which the presence of 

 the heart or some other organ would definitely confirm their interpretation. I ori- 

 ginally adopted myself the other view, and in case they are mistaken, the so-called 

 velum would be a circum-anal patch of cilia, while the position of the primitive meso- 

 blast cells as well as of the byssus would better suit my view than that adopted in the 

 text on the authority of the above observers, 



