IX 



MOLLUSCA 



341 



in the posterior dorsal region, and all pass forwards and slant down- 

 wards. The uppermost of these is the dorsal retractor, the fibres 

 of which pass forwards and diverge to the right and left and are 

 inserted into the upper parts of the velum ; below it lies the median 

 retractor, which sends fibres to the lateral and ventral parts of the 

 velum ; whilst below this again lies the ventral retractor, which is 

 inserted into the anterior portions of the right and left mantle-lobes. 

 All these muscles are of a transitory character and disappear when 



Pig. 271. — Older Veliger larva of Dreissensia polymorpha, seen from the side. This stage 

 is the one which immediately precedes the metamorphosis. (After Meisenheimer. ) 



Letters as in two previous figures. In addition, aild.a, anterior adductor muscle"; add.p, posterior 

 adductor muscle ; &r, rudiments of gill-papillae ; iyss, byssus gland ; h, rudiment of kidney ; of, otocyst ; 

 per, rudiment of pericardium. 



the free-swimming life is given up; but the anterior adductor 

 muscle, passing from one valve of the shell to the other, is already 

 formed at this stage, and it persists into the adult. 



The three pairs of ganglia characteristic of Mollusca make their 

 appearance at this stage. Of these the cerebral ganglia owe their 

 origin to a bilobed pit, termed the cerebral pit, situated within the 

 velar area in front of and below the ciliated apical plate. This pit 

 evidently corresponds to the two lateral thickenings of the velar area 

 in Patella. From the bottom of this pit a bilobed mass of cells is 



