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



this seems to confirm my opinion that the centradenia is not only a hepatic and a renal, 

 but at the same time a gas-secreting gland. 



Canal System. — The system of gastral vessels or entodermal canals is in the Disco- 

 necta? very different from that of all the other Siphonophorse (or the Siphonantha?) ; it is 

 far more complicated and exhibits from the beginning quite a different type ; it is 

 originally octoradial, determined by eight primary perradial gastral canals, which arise 

 from the periphery of the base of the stomach and correspond to the eight subumbrellar 

 radial canals of a common octoradial Medusa (Trachynema, Pectyllis). This regular, 

 strictly octoradial type is retained through life in the central part of the canal system of 

 all Discalidse and Porpitidse, whilst in the amphithect Velellidae it is found only in early 

 stages, and afterwards becomes bilaterally modified, according to the different growth of 

 the two horizontal cruciate axes. No trace of this octoradial canal system is found in the 

 trunk of any other Siphonophora? ; but a similar type is apj^arently marked by the 

 octoradial structure of the pneumatosaccus in many Physonectse. 



The peculiar development of the gastro-canals in the umbrella of the Disconectse is 

 produced, firstly, by the voluminous expansion of the pneumatophore and the invagination 

 of the exumbrella connected with it ; and secondly, by the development of the centra- 

 denia between the latter and the subumbrella. The following eight parts of the canal 

 system may be distinguished in the most highly developed Disconecta?, as we ascend from 

 the central siphon to the top of the pneumatophore : — (1) The system of subumbrellar 

 radial canals (originally eight), arising from the fundus of the stomach, and running 

 horizontally and centrifugally or ascending in the subumbrella towards the margin of 

 the umbrella ; they usually form a complicated network of dichotomously branched 

 radial canals. (2) The marginal canal, which connects the distal ends of the latter ; it is 

 placed in the true margin of the umbrella, and corresponds to the circular canal of the 

 Hydromedusse. (3) The system of renal canals or white excretory vessels, containing 

 green crystals of guanin ; it is formed by a network of branches of the subumbrellar 

 canals, which is placed usually in the basal or inferior part of the centrodenia. (4) The 

 system of hepatic canals or brown pigment vessels ; it is formed by the apical or superior 

 part of the canal network in the centradenia, in the surface of which it forms a 

 regular octoradial " liver-star." (5) The system of exumbrellar or pallial radial canals 

 (originally eight) ; these run centripetally and more or less horizontally in the exumbrella 

 towards the centre of the pneumatophore, where they are united by a small ring, 

 surrounding the apical stigma (" mantle-star ") ; they may be compared to the eight 

 radial pouches of the pneumatophore in many Siphonanthae. (G) The system of gono- 

 stylic cavities, or the gastral cavities of the polyps which bear the gonophores (palpons in 

 the Discalidse, siphons in the Porpitidse and Velellidae) ; they arise as simple subumbrellar 

 diverticula from the inferior branches of the centradenial system. (7) The canal system 

 of each single gonophore, composed of four radial canals and a connecting ring-canal, 



