REPORT ON THE SIPHONOPHOR^. 29 



and a double muscle plate, with outer radial and inner circular fibres. The plexus of 

 pallial canals separates the pneumatocodon from the pneumatosaccus. 



Pneumatosaccus. — That lamella of the exoderm which surrounds immediately the 

 chitinous pneumatocyst, is the pneumatosaccus. It represents the invaginated part of 

 the exumbrella, and has taken its origin from a simple bottle-shaped gland in its apex. 

 This gas-secreting gland — or pneumadenia — originally small and occupying only the 

 apical centre of the exumbrella, has afterwards become so extraordinarily expanded that 

 it usually forms the most voluminous part of the entire umbrella. The exodermal 

 epithelium of the pneumatosaccus is a simple stratum of glandular cells, which secrete 

 the cuticular chitinous plate of the adjacent pneumatocyst. The basal part, or the 

 inferior face of the former, is probably also the matrix of the centradenia or the so-called 

 " liver." The exodermal cells, and cnidoblasts, which constitute the solid parenchyma of 

 the latter, are probably derived from the basal part of the pneumatosaccus. 



Pneumatocystis. — The chitinous polythalanious float filled with air, which we call 

 pneumatocyst (formerly called " inner shell "), exhibits in all Disconectae a rather com- 

 plicated structure. Its general form is circular, and originally octoradial in the Discalidae 

 and Porpitidae, elliptical or nearly quadrangular (parallelogram) in the Velelbxke ; but 

 also in the young larvae of the latter its first rudiment is octoradial. It always commences 

 with the formation of a simple central chamber, which is situated in the centre of the 

 exumbrella, just above the gastral base of the large central siphon. It opens outside by 

 a central stigma in its upper face. Around this primary central chamber (the chitinous 

 lining of a central pneumadenia of the exumbrella) a peripheral corona of eight radial 

 chambers is next formed, each provided with an outer stigma on its upper side, and with 

 an articulate trachea on its lower side. These eight radial chambers are equal and 

 regularly radial in the Discalidae and Porpitidae, while they are more or less amphithect 

 and somewhat bilaterally disposed in the Velellidae. Sometimes in the latter family they 

 are more or less obliterated. 



In the simplest case (Discalia, PI. XLIX.) the formation of the pneumatocyst is 

 complete with the eight radial chambers ; in all the other Disconectae a different number 

 of peripheral concentric chambers is formed around their octoradial corona. All these 

 tertiary chambers are simple rings without radial partitions ; they open outside (in the 

 exumbrella) by a different number of stigmata, inside (in the centradenia) by a number 

 of open tracheae. The rings are circular in the Discalidae and Porpitidae, elliptical or 

 quadrangular (parallelogram) in the Velellidaa. In these latter there usually arises after- 

 wards a solid vertical crest, placed diagonally on the upper side of the horizontal disc. 



The general opinion regarding the physiological function of the polythalanious pneuma- 

 tocyst of the Disconectae may be summed up in the following propositions : — (1) The 

 Disconectae are exclusively pelagic animals, always floating on the top of the ocean, and 

 never sinking below its surface ; (2) the air contained in the pneumatocyst is atmo- 



