268 LAMELLIBRANCHIATA. 



p. ad 



B. 

 FIG. 121. 



A. GLOCHIDIUM IMMEDIATELY AFTER IT is HATCHED. 

 ad. adductor ; s/t. shell ; by. byssus cord ; s. sense organs. 



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

 br. branchiae ; au. v. auditory sack ; f. 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, (i) 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 with 

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

 till the young mussel left its host. 



The free Glochidia 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. 



