THE DEVELOPMENT OF LAMELLIBRANCHS. 329 



There is now a well-developed mantle chamber, into 

 which all the soft parts are retracted while the animal is at 

 rest. The velum (Fig. 173, v), is very large, and it fills 

 the ventral half of the anterior end of the cavity between 

 the shells, when retracted, but while the animal is swim- 

 ming, the velum is protruded from between the valves. 

 In most forms its outer surface is sunken, thus forming a 

 conical basin, with a fringe of locomotor cilia, (c, v), around 

 its rim. The depression in the centre allows the organ to 

 be folded together when withdrawn into the shell, but 

 when it is expanded, in swimming, it is nearly flat. In 

 most forms, a long flagellum (/*), arises from the bottom of 

 the depression, an(l projects beyond the cilia. There are 

 two large, flat muscles (v, m), on each side, to retract the 

 velum. 



The mouth, being behind the velum, is in the posterior 

 half of the shell, and a long ciliated oesophagus (oe), runs 

 upwards and forwards through the liver (/), to the stom- 

 ach (s). A small tongue-like process from the posterior 

 wall of the oesophagus, runs out into its cavity, just below 

 the liver. The long, twisted intestine (z) , is freely movable 

 in the body cavity (6) , and the anus is near the mouth. In 

 most specimens, the auditory organs (e), can be seen a 

 little posterior to the oesophagus, and there are usually 

 two small pigmented eyes (not shown in the figure) 

 carried upon short, blunt tentacles, at the base of the 

 velum, anterior to the oesophagus. 



The two renal organs, or organs of Bojanus, soon ap- 

 pear, as a pair of little tubular diverticula from the intes- 

 tine, near the anus, and at about the same time the ante- 

 rior adductor muscle (, a), and, soon after, the posterior 

 adductor, appears. The three pairs of ganglia appear 

 before the commissures between them. The velum, ten- 



