162 SIPHONOPHORA. 



second polypite early become visible ; while a hydrophyllium is 

 formed as a bud which covers over the first polypite and tentacle 

 (hp/i). With the development of the hydrophyllium the first 

 segment, if the term may so be used, is complete. The second 

 segment of which a rudiment is already present as a second 

 polypite is intercalated between the first segment and the necto- 

 calyces. 



Amongst the Physophoridae there is a considerable range of 

 variation in development ; though the variations concern for the 

 most part not very important points. The simplest type hitherto 

 observed is that of Stephanomia (Halistemma) pictum. The 

 segmentation and formation of a two-layered planula (fig. 76) 

 take place in the usual way. Between the solid central mass of 

 nutritive hypoblast cells and the epiblast an epithelial hypoblastic 

 layer becomes interposed which undergoes a special thickening 

 at the aboral pole. At this pole a solid involution of epiblast 

 next becomes formed, to which a layer of hypoblast becomes 

 applied. The structure so formed is the rudiment of the pneuma- 

 tocyst (ep}. In the next stage the air-cavity of the pneumatocyst 

 becomes established within the epiblast. 



The gastrovascular cavity is formed in the midst of the 

 nutritive hypoblast cells, which then become rapidly absorbed 

 leaving the gastrovascular cavity entirely enclosed by the epi- 

 thelial layer of hypoblast (fig. 76 B). 



By the above changes the more important organs of the larva 

 have become established. The one end forms the pneumatophore, 

 and the other, the oral part, the polypite. Between the two there 

 is already present the rudiment of a tentacle, and a second tenta- 

 cle soon becomes formed. The mouth arises as a perforation at 

 the oral end of the larva. 



The pneumatophore contains a prolongation of the gastro- 

 vascular cavity, the fluid in which bathes the outer hypoblastic 

 wall of the pneumatocyst. It has however no communication 

 with the enclosed cavity of the pneumatocyst. In the later 

 developmental stages the size of the pneumatophore becomes 

 immensely reduced in comparison with the remainder of the 

 larva. 



The development of Physophora agrees closely with that of Stephanomia 

 except in one somewhat important point, viz. in the development of a 



