i6o 



SIPHONOPHORA. 



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 ten- 

 tacle (/), and the aboral of 

 a nectocalyx (nc}. 



The former of these 

 elongates itself in succeed- 

 ing stages into a process of 

 both epiblast and hypoblast. 

 The central part of the 

 nectocalyxontheotherhand 

 appears to originate from a 

 thickening of the epiblast 

 in which the cavity of the 

 bell becomes 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 

 become to a certain extent 

 established a cavity the 

 commencement of the 



FlG. 75. AN ADVANCED LARVA OF EPI- 

 BULIA AURANTIACA WITH ONE LARGE NECTO- 

 CALYX. (After Metschnikoff.) 



so. somatocyst ; nc. second imperfectly de- 

 veloped nectocalyx; hph. hydrophyllium ; po. 

 polypite; /. tentacle. 



