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 for Some Weeks.
br. branchiæ; 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, (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 Lymnæus 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.