338 



INVERTEBRATA 



CHAP. 



embryo bursts the egg-membrane and enters on its free-swimming 



life as a Trochophore larva (Fig 266). 



In Physa, as we have already seen, Wierzejski has traced the 



pericardium back to its origin in the derivatives of the mother 



mesoderm cells, through an 

 unbroken series of stages. 

 For these reasons we reject 

 Meisenheimer's view of the 

 origin of these cells, and 

 believe that they are derived 

 from the mother mesoderm 

 cells after the latter have 

 given off the mesenchymatous 

 tissue alluded to above. This 

 view would bring the develop- 

 ment of Pelecypoda into 

 harmony with that of other 

 Mollusca, and should be 

 definitely tested. 



The Trochophore larva soon 

 passes into the condition of a 

 Veliger larva. This change 

 takes place by the enlargement 

 of the prototroch into the 



int- 



coe 



ttr 



FIG. 267. Sagittal section through a young 

 Trochophore larva of Dreissensia polymorpha. 

 (After Meisenheimer. ) 



Letters as in Figs. 265 and 266. In addition, coe, 

 group of cells from which the coeloin (pericardium) 

 later develops ; int, intestine ; sh, primary shell (the 

 adult hinge). 



velum and by the growth of 

 the bivalve shell. Behind the 

 prototroch several rows of 



FIG. 268. Transverse section of the ventral portion of a young Veliger larva of Dreissensia 

 polymorpha to show the origin of the mantle-groove and of the pedal ganglia. 

 (After Meisenheimer.) 



m.c, mantle-groove ; p.g, thickenings of ectoderm which will give rise to the pedal ganglia ; xh, shell. 



large cells are differentiated ; they are covered with numerous fine 

 cilia and reinforce the action of the prototrochal girdle ; this enlarged 



