IX 



MOLLUSCA 



301 



and Haliotis, which, like Patella, are members of the division 

 Aspidobranchiata. 



Patten made out some other interesting points in the develop- 

 ment of Patella. The radula sac appears as a ventral pocket-like 

 outgrowth of the stomodaeum (r.s, Fig. 236). The anterior ends of 

 the mesodermic bands break up into loose tissue, like mesenchyme. 

 Some of this tissue develops into long spindle-shaped cells, which are 

 muscular and which are inserted at one end into the ectoderm of 

 the visceral hump and at the other into the sides of the stomach. 



Fig. 236. — Side view of a yeliger larva of Patdla coerulea before torsion has taken place. 



(After Patten. ) 



a, position of anus ; /, foot ; int, intestine ; mj, mantle Told ; mnse, retractor muscle of the shell ; 

 0, mouth ; r.s, radula sac ; sK, shell ; st, stomach ; stom, stomodaeum ; Mr, telotroch ; V, velum ; 

 v.h, visceral hump. 



Others of these cells form muscles connecting the apical plate with 

 the sides of the oesophagus (as in Polygordius). 



The otocysts are formed quite early, whilst the foot is still 

 quite inconspicuous, as two elongated pits situated just behind the 

 mouth on either side. Later, when the foot grows out and becomes 

 very conspicuous, two ectodermic thickenings appear on its anterior 

 surface, and are interpreted as the rudiments of the pedal ganglia. 



What are most probably the rudiments of the cerebral ganglia 

 are shown in Fig. 239, where, at the sides of the persistent apical 

 organ with its powerful cilia, two ectodermic thickenings are seen. 



