REPORT ON THE SIPHONOPHOR^E. 309 



Pneumatosaccas. — The invaginated portion of the apex of the trunk, which forms 

 the air-sac, hangs in all CystonectaB freely in the pericystic cavity, or the wide interval 

 between the inner and outer wall of the pneumatophore. These two walls are not connected 

 by radial septa, as in most Physonectse, and are united only on the margin of the apical 

 stigma. The uppermost portion of the air-sac, which surrounds this stigma, is usually 

 intensely coloured (mostly red or brown), or covered by a hemispherical pigment-cap 

 (mitra ocellaris). A ring-like constriction (the pylorus infundibuli, " Trichterpforte ") 

 separates incompletely the larger apical and the smaller basal portion of the air-sac. 

 The former is the pericystic sac, which secretes the chitinous pneumatocyst (or the 

 cuticular air-flask) ; the latter is the hypocystic infundibulum or the pneumatochone 

 (air-funnel, " Lufttrichter"), which together with the tapetum endocystale forms the 

 air-secreting gland, or the pneumadenia. The basal or inferior opening of the 

 pneumatocyst (opposite to the superior or apical stigma) is often surrounded by an 

 annular thickening of the cuticle, the annulus pylori (PI. XXII. figs. 7,8). 



Pneumadenia. — The exodermal epithelium of the pneumatosac (or strictly speaking 

 of its basal portion), which secretes the gas filling the float, in all Cystonectae is a 

 gas-gland or pneumadenia of considerable size. It exhibits the simplest shape in the 

 monogastric Cystalidas (PI. XXII. fig. 5) ; it is here a spheroidal vesicle of thickened 

 glandular exoderm, which communicates by a proximal opening (the pylorus) with the 

 cavity of the pneumatocyst ; this simple " air-funnel " is very similar to that of the 

 Apolemidse among the Physonectse (50, Taf. xix. fig. 93). The pneumadenia of the 

 Physalidte, the largest of all, is originally of the same simple shape as in the 

 Cystalidse, but expands afterwards unilaterally, on the ventral side of the pneumato- 

 phore, and forms there a large circular or oblongish air-secreting plate (" Luftplatte," 

 Chun, 83, p. 569) ; in Caravella maxima it reaches the extraordinary size of 100 to 

 150 mm. This peculiar growth along the ventral side of the float is effected by the 

 extraordinary expansion, which the air-sac reaches in the Physalidse, growing down- 

 wards into the cavity of the trunk, along its dorsal side ; and by the consequent 

 asymmetrical development. 



The three other families (Rhizophysidae, Pis. XXIII. , XXIV.; Salacidse, PI. XXV.; 

 and Epibulidas, PI. XXII. figs. 6-8) agree in a peculiar structure of the pneumatophore 

 and differ essentially from the two preceding (Cystalidaa and Physalidse). The beginning 

 of the structure is the same as in the latter ; but afterwards the pneumadenia exhibits 

 a further very remarkable development, firstly in the endocystic tapetum, and secondly 

 in the hypocystic villi. The glandular exodermal epithelium of the primary spheroidal 

 pneumadenia (or the air-funnel) grows in apical direction, passes through the pylorus 

 into the cavity of the pneumatocyst, and expands into its basal portion, lining its inside 

 (in younger floats only one-third, in older more than two-thirds). The extraordinary 

 development of the gas-gland in these CystonectaB corresponds to its important physio- 



