•j;;s THE VOYAGE OF H.M.S. CHALLENGER. 



Pneumatophore. — The float at the top of the trunk is relatively small, usually ovate 

 or pyrilbrm, and pigmented in the apical part. Its outer wall (pneumatocodon) is 

 connected with the inner wall (pneumatosaccus) by a variable number of vertical radial 

 septa : four in Strobalia, six to eight or twelve in Forshalia and Forshaliopsis, sixteen 

 in Bathyphysa. The radial pouches or chambers of the pericystic cavity, which are 

 separated by these septa, are closed above, but open below in the common trunk, 

 beyond the funnel-cavity of the pneumatosac. 



Nectophores. — The numerous nectocalyces, owing to their multiserial and spiral 

 arrangement on the nectosome, differ in form considerably from those of the other 

 Physonectse. Usually they are attached to the common trunk by a long pedicle of 

 conical or pyramidal form. Their umbrella is prismatic, with polygonal faces, produced 

 by the mutual pressure of the densely associated nectophores. The subumbrella is more 

 or less compressed in the sagittal direction, dilated laterally, often provided with two 

 lateral horns. Sometimes the exumbrella of the apical part, corresponding to the latter, 

 bears two prominent apophyses. The four radial canals of the nectosac, according to 

 that dorso-ventral compression, are usually of different sizes, the two lateral longer than 

 the two sagittal vessels. The ring-canal which connects them at the constricted ostium 

 of the nectosac is sometimes circular, at other times elliptical. Often some ocelli, red or 

 brown pigment-spots, are visible above the small velum, at the inosculation of the radial 

 canals. From the proximal union of the latter in the top of the nectosac arises a long 

 peduncular canal which runs through the pedicle of the nectophore and opens into the 

 axial canal of the trunk (PI. VIII. figs. 2, 3). 



Siphosome. — Corresponding to the high development of the nectosome in the 

 Forskalidas, this interesting family surpasses all the other Physonectse also in the 

 complicated composition and extraordinary size of the svphosorae ; it attains in the 

 largest species a diameter of more than one metre in the fully expanded state ; in 

 Bathyphysa probably four to six metres or more ; in the strongly contracted state it 

 is much smaller. The numerous siphons are attached to the long trunk of the 

 siphosome by verj?' long peduncles, and these, as well as the trunk itself, are densely 

 covered with innumerable bracts. These envelop, densely crowded, the outer surface 

 of the contracted siphosome like a protecting carapace of scales. The entire form of 

 the siphosome is sometimes more cylindrical or inversely conical, at other times more 

 ovate or hemispherical ; fully expanded, with widely prominent and brilliantly coloured 

 appendages, it presents a most splendid spectacle. 



Cormidia. — The number of the cormidia which compose the siphosome is usually 

 very large, thirty to fifty or more, in the larger species several hundreds (sometimes 

 more than five hundred). They are arranged around the axial trunk in a continuous 

 spiral, the turning of which is usually right-handed (or dexiotropic), in contrast to the 

 left-handed (or Inotropic) spiral of the nectosome. The trunk itself is correspondingly 



