REPORT ON THE SIPHONOPHOR^E. 187 



times cylindrical ; it is more or less variable, and may be changed by the contraction of 

 its muscular walls. The apex of the float is usually coloured by pigment (red or brown) ; 

 sometimes this forms a regular octoradial star, with a colourless centre. 



Pi(c>nnatosaccus. — Since the float is developed by an invagination of the original 

 exumbrella, — comparable to a simple air-secreting gland of the exoderm (p. 12), — the 

 central air-flask or pneumatocyst, filled with gas, is always surrounded by a double 

 wall ; the inner or invaginated wall (comparable to the entoderm of a gastrula) is the 

 pneumatosaccus (or " Luftsack "), which secretes the structureless thin chitin-plate of the 

 pneumatocyst ; the outer or non-invaginated wall (comparable to the exoderm of a 

 gastrula) is the pneumatocodon (or " Luftschirm "). The cavity between the two walls 

 (cavum pneumatophoraB) is everywhere closed and filled with the nutritive fluid of the 

 axial trunk, with which it communicates at its distal or basal end. This cavity is usually 

 divided by a number of equidistant vertical septa into radial pouches, which correspond 

 to those in the umbrella of a Medusa (e.g., JEgina, Cunina). I find constantly eight 

 radial pouches, regularly disposed around the pneumatosac, in the great majority of the 

 Physonectse ; the number, however, is not cruite constant ; single individuals have seven 

 or nine, instead of eight ; some species (Halistemma, Nectalia) possess only four, other 

 species twelve or sixteen. Sometimes the radial septa divide the whole cavity of the 

 pneumatophore into chambers, at other times only its basal or inferior part ; this 

 remains simple in the Athoridse and Apoleniidas, where no septa are developed. 



Pneumatocyst. — The delicate chitinous air-flask, which is produced (as a cuticle) and 

 immediately surrounded by the exodermal invaginated pneumatosac, seems to be closed 

 in all Physonecta3 at the apex (or the upper pole of its vertical axis) ; it is open at its 

 thickened annular base (or the lower pole). This opening is the circular pneumatopyle 

 or funnel aperture (" Trichterpforte," Chun, 48, p. 512). It corresponds to an annular 

 constriction of the surrounding pneumatosac, by which this is divided into two portions ; 

 the larger superior (or apical) portion alone secretes the chitinous plate of the flask ; the 

 smaller inferior (or basal) portion secretes no chitinous cuticle, and has a stratified 

 exodermal epithebum of a peculiar shape and a yellowish or greenish colour ; this is the 

 important pneumadenia or the " air- funnel," which secretes the gas (" Lufttrichter," 

 Chun, 48, p. 512). The glandular epithelium of the pneumadenia often passes, owing to 

 a secondary growth and further expansion, through the pneumatopyle into the cavity of 

 the pneumatocyst and lines its basal portion — usually only one-fourth or one-third of its 

 inner face (" secondary exoderm," Chun, 48). The pneumadenia is then divided by the 

 thickened chitinous ring of the pneumatopyle into a superior (endocystal) and an inferior 

 (hypocystal) portion. At other times the pneumadenia gives off peripheral branches or 

 lateral solid cord-shaped apophyses which enter into the septa and were formerly described 

 as peculiar csecal canals by Glaus (74, p. 22) and Korotneff (50, p. 272). This is the ease 

 in the Discolabidse (Family XVI.) : in these, and perhaps also in other Physonectfe, the 



