338 



INVEKTEBKATA 



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 



unbroken 



For these 



Meisenheimer's 



origin 



int- 



an 



series of stages. 

 reasons we reject 

 view of the 

 of these cells, and 

 believe that they are derived 

 from the mother mesoderm 

 cells after the latter have 

 given off the mesenchymatoiis 

 tissue alluded to above. This 

 p.tr view would bring the develop- 

 ment of Pelecypoda into 

 harmony with that of other 

 Mollusca, and should be 

 definitely tested. 



The Trochophore larva soon 



FIG. 267.-Sagittal section through a young P ass . es int the Condition of a 



,/>/!. Vellger larva. This change 



takes place by the enlargement 



Letters as in Figs. 265 and 260. In addition, cue, of the prototl'Och into the 



ve l um an( l | )y t { ie o TO \vth of 

 Ve U J 



the bivalve shell. Behind the 

 prototroch several rows of 



stom 



coe 



ttr 



Trochophore larva of Dreissensia 



(After Meisenheimer.) 



group of cells from which the coelom (pericardium) 

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



hinge). 



FIG. 268. Transverse section of the ventral portion of a young Veliger larva of l)n-i.wnxi 

 polymorpha to show the origin of the mantle-groove and of the j><'</,/l <nni<jliti. 

 (After Mei.senheimer. ) 



m.c, mantle-groove ; p.<j, thickenings of ectoderm which will sive rise to the pedal i;an.u'lia ; xli, shell. 



large cells are differentiated ; they are covered with numerous fine 

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



