134 SIPHONOPHORA 



of the pneumatocyst (ep\ In the next stage the air-cavity of the 

 pneumatocyst becomes established within the epiblast. 



The gastro vascular cavity is formed in the midst of the nutritive 

 hypoblast cells, which then become rapidly absorbed leaving the 

 gastrovascular cavity entirely enclosed by the epithelial 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 tentacle soon 

 becomes formed. The mouth arises as a perforation at the oral end 

 of the larva. 



The pneumatophore contains a prolongation of the gastrovascular 

 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 Physopliora agrees closely with that of Stephanomia 

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

 provisional hydrophyllium. This arises as a prominence at the aboral 

 pole, containing a prolongation of the gastrovascular cavity. Between the 

 epiblast and hypoblast of the prominence gelatinous tissue becomes deposited, 

 and the liydrophyllium is thus converted into a large umbrella-like organ 

 enclosing the polypite. The two together have a close resemblance to an 

 ordinary Medusa, the polypite forming the manubrium, and the hydro- 

 phyllium the umbrella. The hydrophyllium is eventually thrown off. 



An important type of Physophorid development is exemplified in 

 Crystalloides, a genus closely allied to Agalma. In this type the greater 

 part of the original ovum, instead of directly giving rise to the polypite, 

 becomes a kind of yolk-sack, from which the polypite is secondarily budded 

 (fig. 77, yk}. Agalma sarsii is in this respect intermediate between Crystal- 

 loides and Physopliora. Both these types are remarkable for developing a 

 series of provisional hydrophyllia (fig. 77, h.ph.). In both genera the first 

 of these develops as in Physopkora, and for a long time is the only one 

 functional. 



The conclusions to be drawn from the above description may be 

 summed up as follows. In all the Siphonophora, so far observed, the 

 starting-point for further development is a typical ciliated two- 

 layered planula. The inner layer or hypoblast is mainly formed of 

 large nutritive cells. From these cells an epithelial hypoblastic 

 layer becomes secondarily differentiated, the exact relations of which 

 differ somewhat in the various types. The nutritive cells themselves do 

 not appear to become directly converted into the permanent hypo- 

 blastic tissues. The development of the adult from the planula com- 

 mences by the thickening of the epiblastic layer, usually at one pole 

 (the future proximal or aboral pole), and the formation at this pole of a 

 series of bud-like structures (in the growth of which both embryonic 



