102 HYDBOZOA. 



and, the free extremity of the pneumatophore 

 being likewise perforate, a communication exists, 

 in these genera, between the cavity of the pneuma- 

 tocyst and the surrounding medium. In Rhizo- 

 jjliysa, moreover, peculiar long branched processes 

 freely depend from the distal surface of the pneu- 

 matocyst. Each process consists of a layer of the 

 investing endoderm containing in its axis clear cel- 

 l^form bodies, -02 of an inch long, each of which 

 includes an opaque oval endoplast, about -i-th of 

 these dimensions, and this, in its turn, a more mi- 

 nute particle or nucleolus, oval or circular in form, 

 and 'OOoSth of an inch in diameter. In Agahna 

 and Forshalia radiating membranous partitions 

 connect the walls of the pneumatophore with those 

 of the pneumatocyst, below which each terminates 

 in a free arcuated edge. In Velelia and Forpita 

 the pneumatocyst is furnished with several open- 

 ings, or stigmata, communicating with the exterior, 

 while to its distal surfa^ce are attached a number 

 of long slender processes enclosing air, and hence 

 termed the ^ pneumatic filaments.' 



Excepting the presence of the pneumatophore 

 and the absence of a somatocyst, the general plan 

 of structure in these Hydrozoa differs little from 

 that of the Calycophoridw. In Apolemia, as in 

 Dipliyes, the numerous groups of appendages are 

 supported at intervals along a slender, unbranched, 

 connecting stem. Physophora, the type of the 

 order, has a filiform, but comparatively short, coe- 

 nosarc, terminated proximally by a pneumato- 

 phore of moderate size, below which the greater 

 portion of its length is occupied by a double 

 series of nectocalyces, each alternating with its 

 successor on the opposite side, and deeply grooved 



