344 



INVEETEBEATA 



CHAP. 



to a horizontal position, and the loop of the intestine is straightened 

 out. 



After metamorphosis the labial palps, so characteristic of the 

 adult mussel, are formed in an amazing manner. The cerebral pit, 

 from which the rudiments of the cerebral ganglia have been separated, 

 flattens out and forms two lateral bands of ciliated epithelium above 

 and at the sides of the mouth. These develop into the upper and 

 outer labial palps, while downgrowths from their inner ends give rise 

 to the inner and lower labial palps (Fig. 276). 



The shell begins to alter its larval shape by a preponderant growth 

 of its posterior and lower angle, and by a certain amount of growth 



Fig. 274. — Horizontal .section through the anterior portion of an old Veliger of DreUsensia 

 polymorpha in order to show the differentiation of the cerebral ganglia from the 

 cerebral pit. (After Meisenheimer. ) 



eg, cerebral ganglion ; c.-p, cerebral pit ; oes, oe.sopl)agu.s ; pl.g, rudiment of pleural ganglia ; 

 sh, sbell ; V, velum. 



of its anterior angle also. The gill papillae grow into long filaments, 

 which become locked together by the longer cilia on their lateral 

 faces. The development of an additional outer row of gill filaments, 

 and the bending up of the ends of the filaments to form the reflected 

 portions of the gill lamellae, take place later in life (Fig. 275). 



The rudiments of the kidneys acquire cavities. The ring of cells 

 surrounding the intestine becomes double and the two layers separate 

 from one another, the cavity between them being the pericardium, 

 i.e. the coelom. The cavity of the heart is the space between the 

 inner of these layers and the intestine. The kidney becomes U-shaped, 

 and the inner limb of the U on either side (which will form the 

 ureter) coalesces with the corresponding part of the other kidney so 

 as to form a transverse space beneath the intestine. Into this space 

 opens, on either side, a diverticulum of the mantle-cavity, and in this 



