The pit has its opening turned towards the sub-umbral cavity, while it base or fundus forms a bulge, more or less pronounced, on the ex-umbral side of the velum.
Vesicle (the fundus of the original pit) or on its sides; their arrangement and number vary greatly and furnish useful characters for distinguishing genera.
At the fundus are placed the concrement-cells with their conspicuous otoliths (con) and the inconspicuous auditory cells, which are connected with the subumbral nerve - ring.
Next comes the gall bladder, a pear-shaped bag, the fundus of which is in front and below, the neck behind and above.
The ureters open into the fundus of the bladder as in some Rodents.