MOLLUSC A. 223 



with those of Braun. I was however unsuccessful in carrying on my 

 observations till the young mussel left its host. 



The free Glochiclia very soon attach themselves to the gills, fins, or 

 other parts of fish which are placed in the tank containing them; after 

 attachment they become covered by a growth of the epidermic cells of 

 their host, and undergo their metamorphosis. 



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 wh : ch 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.acfy. 



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

 somewhat knobbed papillae 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 coimted 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 natter side of the shell were the anterior. I 

 have followed Eabl and Flemming in the determinations of the anterior and posterior 

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

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

 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 mesoblast 

 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. 



