222 



L A MEL LI BRA NCHIA TA . 



With the development of the shell, the mantle, and the sense-organs, 

 the young mussel reaches its full larval development, and is now known as 

 a Glochidium (Fig. 121 A). 



If the parent, with Glochidia in its gills, is placed in a tank with fish, 

 it very soon (as I have found from numerous experiments) ejects the larvse 

 from its gills, and as soon as this occurs the larvse become free from the 

 egg-membrane, attach themselves by the byssus-cord, and when suspended 

 in this position continually close and open their shells by the contraction 

 of the adductor muscle. If the mussels are not placed in a tank with fish 

 the larva? may remain for a long time in the gills. 



FIG. 121. 



A. GLOCHIDIUM IMMEDIATELY AFTER IT is HATCHED. 

 ad. adductor; sh. shell; by. byssus cord ; s. sense organs. 



B. GLOCHIDIUM AFTER IT HAS BEEN ON THE FISH FOB SOME WEEKS. 

 br. branchiae; au.v. auditory sack; /. foot; a. ad. and p. ad. anterior and posterior 

 adductors; al. mesenteron; mt. mantle. 



Before passing on to state what is known with reference to the larval 

 metamorphosis, it may be well to call attention to certain, and to my 

 mind not inconsiderable, difficulties in the way of accepting in all particulars 

 Rabl's account of the development. 



In all Gasteropod Molluscs the lower or vegetative pole of the ovum is 

 ventral, not dorsal as Rabl would make it in Unio. The blastopore in other 

 Molluscs always coincides either with the mouth or anus, or extends between 

 the two. The surface on which the foot is formed is the ventral surface. 

 On the dorsal surface are placed, (1) the velum near the mouth, (2) the 

 shell-gland near the anus. In Anodon the velum is placed just dorsal to 

 the mouth, then according to Rabl follows the blastopore, and in the region 

 of the blastopore is formed the shell. The blastopore is therefore dorsal in 

 position. It occupies in fact the ordinary place of the shell-gland, and 

 looks very much like this organ (which is not otherwise present in Anodon 

 and Unio). Without necessarily considering Rabl's interpretations false, 

 I think that the above difficulties should have been at any rate discussed in 

 his paper. More especially is this the case when there is no doubt that 

 Rabl has made in his paper on Lymnaeus a confusion between the mouth 

 and the shell- gland. 



Investigations on the post-embryonic metamorphosis of Glochidium 

 have been made by Braun (No. 287), and several years ago I made a series 

 of observations on this subject, the results of which agree in most points 



