42 THE HYDROMEDUSAE 



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

 pneumatophore 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). 



(b) 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, by pores in their 

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

 plate thus composed 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 penetrate the centradenia. 

 There is reason for supposing that the pneumatophore even in the 

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

 PN; 50). 



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

 acteristic of the Auronectae ; it is placed at the side of the pneumato- 

 phore, is ovoid in shape, and is traversed by a minute canal which 

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

 this canal lies the pistillum, a mass of muscle enveloped in a strong 

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

 gloea perforated by branching endodermal tubes, and the super- 

 ficial ectoderm (Figs. 48fi, 48c). The function and homologies of 

 the aurophore are most obscure (Fewkes, 43). 



TYPES OF SIPHONOPHORE COLONIES : 



The polymorphic individuals above described are very differ- 

 ently combined in the different sub-orders of the Siphonophora. 



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

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

 dactyldzooids. They all spring from a mass of coenosarc which 

 underlies the pneumatophore, composed of ectoderm, mesogloea, and 

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

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

 ture is termed the centradenia, and is perforated by tracheae. The 

 coenosarc entirely envelops the pneumatophore, and projects 

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

 cular canal. 



(b) In the Calyconectae (Figs. 46, 47) no pneumatophore is 

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

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

 bonectes) or a tube closed at the upper end (Diphyes) the hydrocciuw 

 or infundibulum, lined by ectoderm. From the upper end of this 

 cavity spring both the pedicle of the second nectophore when present, 

 and the long tubular coenosarc (Diphyes, Fig. 46); or the nectophores 



