DISCONECTAE. 143 



centradenial canals, a set of canals the pallia! canals on the ex- 

 umbrella or upper surface of the pneumatocyst, which unite over the 

 centre of the pneumatophore (round cp). This pallial system is 

 the result of the invagination of ectoderm to form the pneumato^feore. 



The constitution and function of the centradenia, tracheae, and pneumatocyst 

 of the Disconectae is disputed. Haeckel's account has been followed in the text. 

 He regards the supposed ectodermal portion of the centradenia or liver as gas- 

 secreting in function, and corresponding to the gas-secreting portion of the 

 pneumatocyst of the Siphonanthae (see below), the tracheae being for the purpose 

 of carrying the gas, so secreted, into the pneumatocyst, which is entirely hydro- 

 static. Chun, on the other hand, holds that in the Disconanthae, which 

 habitually float on the surface, the pneumatocyst has no gas-gland, and that the 

 cell mass of the centradenia, which Haeckel calls ectoderm,, is endoderm with a 

 rich development of thread cells ; further, that the tracheae often end in places 

 where this tissue is absent. He considers that these tubes are really tracheae 

 for the conveyance of oxygen to the thick glandular endoderm, and that the 

 pneumatocyst in this group is a breathing organ. In confirmation of this view 

 he states that the living Velella does periodically contract its body as though it 

 were expelling air from the air-sac. The elastic chitinous lining receives its 

 explanation also on this view, as it would by its elasticity tend, in regaining its 

 original form, to suck air in through the stigmata. 



The gas-secreting ectoderm of the pneumatocyst is present in the young forms, 

 which apparently live below the surface, and probably in the deep sea forms ; 

 and no doubt the function of the air-sac is, in these cases, purely hydrostatic, 

 as in other Siphonophora in which there is a gas-gland and no tracheae. 



Chun C. , Bericht ub. cine nach d. Canarischen Inseln ausgef. lieise. Sitzb. Acad. 

 Wiss. Berlin, 1888. 



The gonophores which are produced on the gonostyles are small 

 4-radiate medusae, which do not produce sexual cells until after 

 detachment, when they are known as CJirysomitra. 



It is probable that the young of the Disconectae pass through a larval stage 

 resembling in structure Discalia. This would be the so-called Disconula, a form 

 actually met with in Discalia and presenting an 8-radiate medusiform structure 

 with eight radial canals, eight marginal tentacles, and a dorsal 8-radiate in- 

 vagination of ectoderm the pneumatocyst. 



Fam. 1. Discalidae. From the deep sea. Ex-umbrella without crest, 

 gonostyles without mouth, pneumatocyst divided into a central chamber sur- 

 rounded by 8 radial chambers, to which may be added a still more circumferential 

 arrangement of 5 to 10 concentric ring-chambers. These chambers communicate 

 with each other by the apertures called pneumothyrae, and some of them with 

 the exterior by stigmata. The tentacles have terminal cnidospheres.f Discalia 

 H. ; Disconalia H. 



Fam. 2. Porpitidae. Circular umbrella without crest ; pneumatocyst divided 

 into an octoradiate central part and numerous concentric rings. The gonostyles 

 have mouths. Pneumothyrae* are present. With many stalked cnidospheresf 

 on the tentacles. 



* Pneumothyrae are communications between the concentric chambers of the 

 pneumatocyst. f Cnidospheres are spherical knobs composed of cnidobjasts. 



