338 



INVEETEBKATA 



CHAP. 



embryo bursts the egg-raembrane and enters on its free-swininimg 



life as a Trocliophore 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 

 ~p.tr view would bring the develop- 

 ment of Pelecypoda into 

 harmony with that of other 

 MoUusca, and should be 

 definitely teited. 



The Trochophore larva soon 

 passes into the condition of a 

 Veliger larva. This change 

 takes place by the enlargement 

 of the prototroch into the 

 velum and by the growth of 

 the bivalve shell. Behind the 

 prototroch several rows of 



Fig. 267. — Sagittal section through a young 

 Trochophore larva of Dreissensia polymorpha. 

 (After Meisenheimer. ) 



Letters as in Figs. 265 and 266. In addition, co^, 

 group of cells from which the coelom (pericardium) 

 later develops ; int, intestine ; sk, primary shell (tlie 

 adult llinge). 



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

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

 (After Meisenheimer.) 



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



large cells are differentiated ; they are covered with numerous fine 

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



