ECHINODERMATA 



401 



. mesenchyma at the thickened pole is already under way (Fig. 182). It 

 j .-,^ can be observed in the living blastula that the shortened cells elongate, 



I ad soon attain the length of the surrounding cells. The mesenchyma, 

 len, does not take its origin from primitive mesenchyma cells, but by 

 le proliferation of a large number of cells. Moreover, the migratory 

 »lls which have entered the blastocoele do not form mesenchyma bands, 

 Qt are irregularly scattered. 

 Gastrulation in the Echinoidea takes place after the 

 etachment of the mesenchyma cells, in the ordinary manner 

 (Selexka, No. 53). The archenteron grows oat in half a day 

 to a comparatively long sac-like tube. Between it and the 

 ectoderm there are frequently stretched some of the raesen- 



mesenchyma cells stretch across suspensor-like between the ectoderm and archen- 

 teron. 



chyma cells, which probably serve as suspensors for the 

 archenteron (Fig. 186). The larval mouth in the Echinoidea 

 is formed in a direct manner, for the end of the archenteron 

 (after the abstriction of the entero-hydrocoele) bends toward 

 the ventral surface and fuses with the ectoderm, whereupon 

 the mouth-opening breaks through. The gastrula mouth 

 here too becomes the anus. 



Asteroidea.— Cleavage, although unequal, differs only a 

 little from the equal type. The difPerence in the size of the 



K.H.E. , ^^ 



