302 INVERTEBRATA CHAP. 



The cerebral ganglia of Patella, on this supposition, would arise in the 

 same way as those of Polygordius, at the sides of the apical plate 

 (see Chap. VII). 



The rudiments of the eye cups probably arise on the velar area at 

 the sides of the apical plate, where two groups of three or four clear 

 cells are seen, around which pigment is developed (o.c, Fig. 237). 

 Curious prominences bearing stiff setae appear on the velar area 

 at the sides of the apical plate. One is tempted to regard these as 

 the rudiments of the tentacles, but according to Patten they 

 disappear. 



As the foot grows large the otocysts, which at first lie in front 



m.c. 



FIG. 237. Side view of a veliger larva of Patella coerulea after torsion has taken place. 



(After Patten. ) 



Letters as in previous figure. In ^idition, m.c, mantle cavity ; o.c, rudiment of eye ; 

 op, operculum ;$>. g, rudiment of pedal ganglion. 



of it and which have become detached from the ectoderm, move into 

 it. On the posterior aspect of the foot a thin operculum is developed, 

 which is large enough to close the aperture of the shell. The shell 

 itself, which was at first thin and chitinous, becomes much thicker 

 and calcareous, with a corrugated surface. At the latest period at 

 which Patten observed the prototrochal girdle or velum, it consisted 

 of three concentric rows of cells, a middle row of very large tall cells 

 carrying very powerful cilia, and an anterior and posterior circle of 

 cells carrying much smaller cilia. 



It will be seen that a thorough investigation of the manner in 

 which the organs of Patella are built up has yet to be made ; 

 Patten's description must be regarded as a first sketch. He did not 



