132 



SJPHONOPHORA. 



There is a regular segmentation, unaccompanied by the formation 

 of a segmentation cavity. At its close the ovum becomes a spherical 

 ciliated embryo. This embryo soon becomes elongated, and its cells 

 differentiate themselves into a central and a peripheral layer the 

 epiblast and the hypoblast (fig. 74 A). At this stage the larva has the 

 typical planula form. The epiblast is especially thickened at a pole, 

 which may be called the oral pole, and towards the side of this, which 

 will be spoken of as the ventral side. Adjoining this thickened layer 

 of epiblast a special thin layer of hypoblast becomes differentiated, 

 which in opposition to the main mass of large nutritive cells forms the 

 true hypoblastic epithelium (fig. 74 B, hy}. On this thickening two 

 prominences make their appearance (fig. 74 B). The oral of these is 

 the rudiment of a tentacle (i), and the aboral of a nectocalyx (??c). 



The former of these elongates itself in succeeding stages into a 

 process of both epiblast and hypoblast. The central part of the 



nectocalyx on the other hand 

 appears to originate from a 

 thickening of the epiblast in 

 which the cavity of the bell be- 

 comes subsequently hollowed 

 out. Between this part and 

 the external epiblast which 

 gives origin to the outermost 

 layer of the nectocalyx a layer 

 of hypoblast is interposed. 

 When the nectocalyx has be- 

 come to a certain extent esta- 

 blished a cavity the com- 

 mencement of the primitive 

 gastrovascular cavity of the 

 adult appears in the general 

 hypoblast between the epithe- 

 lial and nutritive layers in the 

 immediate neighbourhood of 

 its attachment. This cavity 

 becomes prolonged into the 

 nectocalyx to form the four 

 gastrovascular canals ; while 

 the hypoblast at the upper 

 end of the nectocalyx forms 

 the somatocyst (fig. 74 C, so}. 

 The primitive enteric cavity 

 once formed rapidly extends, 

 especially in an oral direction 

 (fig. 74 C), and forms a widish 

 cavity in the oral part of the 

 embryo. At the pole of this 



FIG. 75. AN ADVANCED LAEVA OF EPIBULIA 



AURANTIACA WITH ONE LAEGE NECTOCALYX. (After 



Metscknikoff.) 



so. somatocyst; nc. second imperfectly de- 

 veloped nectocalyx ; hph. hydrophyllium ; po. 

 polypite; t. tentacle. 



part (fig. 74, po) is eventually formed the opening of the mouth, and 



