42 THE HYDROMEDUSAE 



secretion. Between the air gland and the outer wall of the 

 pneiimatophore lie in many cases radial pouches and septa of 

 varying number, which perhaps correspond to the radial canals 

 of a medusoid. The air sac may be closed (most Physonectae) 

 or open by a pore to the exterior (most Cystonectae). 



(/') In the Disconectae the pneumatophore is at first a single 

 chitinous chamber ; round this are added concentrically and in 

 one plane chitinous tubes of varying number, which communicate 

 with each other, and with the central chamber, l)y ))ores in their 

 walls, and in some places open also to the exterior. The chitinous 

 plate thus comi)Osed may bear a crest or sail, set at right angles to 

 the plane of the plate, but obliquely to its longer axis (Velella), 

 and is covered on all sides by the ectoderm which secretes it. Air 

 tubes or tracheae from the pneumatophore ])enetrate the centradenia. 

 There is reason for supposing that the pneumatophore even in the 

 Disconectae is derived from a highly modified medusoid (Figs. 48, 

 rS ; 50). 



8. The mtrophore is perhaps also a highly modified medusoid, char- 

 acteristic of the Auronectae ; it is i)laced at the side of the pneum:it(»- 

 phore, is ovoid in shape, and is traversed 1)y a minute canal which 

 leads from the cavity of the pneumatophore to the exterior. Round 

 this caiuil lies the pint ill uiit, a mass of muscle envelojied in a strong 

 chitinous tube ; external to this lie successively ectoderm, meso- 

 gloea perforated by branching endodermal tubes, and the super 

 ficial ectoderm (Figs. 48/', 48c)- The function and homologies of 

 the aurophorc are most obscure (Fewkes, 43). 



TyI'ES ok SlPHoNorilOKK COLONIKS: — 



The polymorphic individuals above described are very differ- 

 ently combined in the ditiercnt sub-orders of the Sii)honophora. 



(«) In the Disconectae (Fig. 48) a single gastrozooid is sur- 

 rounded by numerous blastostyles, and, beyond these, by numerous 

 dactylozooids. They all spring from a mass of coenosarc whicli 

 utidcrlios the ])neumatoj)hore, composed of ectoderm, mesogloca, and 

 ramifying enclodermal tubes; the cells of the latter are apparently in 

 some places renal, in others hepatic, in function ; the whole struc- 

 ture is termed the ceutrtK/cnia, and is perforated by tracheae. The 

 coenosarc entirely envelops the pneumatophore, and project.- 

 laterally for some distance beyond it ; at its edge runs a cii- 

 cular canal. 



(It) In the Calyconectae (Figs. 46, 47) no pneumatophore i- 

 developed. There are one or two, rarely more, large nectophures, the 

 uppermost of which has on one side either an open groove (Cym 

 bonectes) ora tube closed at theui)[)erend (Diphyes) — the lii/ilrorriuin 

 or infundibuhuii, lined by ectoderm. From the u])per end of this 

 cavity si)riiig both the pedicle of the second nectophorewhen present, 

 and the long tubular coeno.sirc (Diphyes, Fig. 4li); or the necto[)hore.- 



I 



