28 THE VOYAGE OF H.M.S. CHALLENGER. 



of gonostyles, which bear the gonophores (mouthless palpons in the Discalidse, mouth- 

 bearing siphons in the Porpitidae and Velellidse) ; (III.) a simple or multiple corona of 

 submarginal tentacles. 



Umbrella. — That part of the body, which is the true homologue of the umbrella in 

 the common Medusae, is in the Disconectse the most voluminous part of the whole cormus. 

 It includes the polythalamous pneumatocyst, and bears all the other organs on its lower 

 face. The latter corresponds to the subumbrella of the typical Medusae, while the upper 

 face is homologous with their exumbrella. Both faces are separated by the peripheral 

 margin or limb of the umbrella, which constantly bears a corona of muciparous glands. 

 The exodermal epithelium of the umbrella everywhere includes scattered nematocysts. 

 Beyond it there is a nervous plate, mainly developed on the limb and the upper face. 

 The muscles of the umbrella are more developed on the upper than on the lower face ; 

 they are composed of an outer layer of longitudinal or radial fibres, and an inner layer of 

 transverse or circular fibres. 



Exumbrella. — The superior (apical or proximal) part of the umbrella, which is sepa- 

 rated from the inferior part (or subumbrella) by the glandiferous limb, is sometimes 

 a nearly even horizontal disc, at other times more or less convex. Its upper free face is 

 either smooth or papillate, always pierced by stigmata ; its lower face is in contact with 

 the centradenia. The exumbrella in all Disconectse is composed of two parallel plates, 

 which are separated by the reticulate plexus of the pallial canals ; the outer plate is the 

 pneumatocodon, the inner is the pneumatosaccus. 



Pneumatophore, — The hydrostatic apparatus, or the swimming-bladder, which we 

 call pneumatophore, has in the Disconectse another and a far more complicated structure 

 than in all the other Siphonophorse (or the Siphonanthas). Only the first beginning can 

 be common to the two subclasses, viz., a simple pneumadenia, or a gas-producing gland of 

 the exumbrella. But this is centrally placed in the Disconanthse, excentrically in the 

 Siphonanthse ; and further, the chitinous pneumatocyst, which covers the inside of the 

 pneumadenia, is a simple monothalamous cyst in the latter, a multiple polythalamous 

 cyst in the former. Another important difference is furnished by the openings of the 

 pneumatocyst ; the simple pneumatocyst of the Siphonanthse is either closed, or has 

 only a simple opening (infundibulum) at the lower pole, and another simple mouth 

 (stigma) on the upper pole of its main axis. The polythalamous pneumatocyst of the 

 Disconanthse, however, has numerous openings on both faces, internal tracheae on the 

 lower face, external stigmata on the upper face. 



Pneumatocodon. — The pneumatophore as a whole, or the hydrostatic apparatus, is 

 composed of all the above-mentioned parts, and of the two plates of the exumbrella also 

 already referred to. The outer or upper of these is the pneumatocodon, the uppermost 

 lamella of the entire umbrella. It is composed of three layers, an outer exodermal 

 epithelium, a nervous plate (composed of a loose reticulum of branched ganglion cells), 



