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



centradonial system of canals, on the other hand, is the consequence of the centripetal 

 immigration of exodermal parenchyma between pneumatosac and gastrobasal plate, 

 producing the centradenia. 



The innumerable ramifying and anastomosing branches of the above-mentioned vessels, 

 which form a very complicated network in the majority of Disconectae, are secondary 

 productions. Their mutual relations are best understood when we compare them with 

 the typical organisation of the simplest forms of that order, Discalia and Disconalia 

 (Pis. XLIX., L.). The canal system exhibits here three typical octoradial horizontal 

 stars : — (l) The pallial star above the pnoumatocyst ; (2) the hepatic star on the upper 

 face of the centradenia ; (3) the gastral star below the gastrobasal plate. The eight 

 peripheral main rays of these stars are united by the coronal ring vessel, which runs in 

 the coronal groove around the equator of the umbrella (PI. XLIX. fig. 4). 



Gonostyles. — The sexual persons of the cormus, or the zooids which produce the 

 medusiform gonophores of the Disconectse, are arranged in a girdle around the base of 

 the central siphon ; they occupy a smaller or broader zone of the subumbrella between it 

 and the corona of tentacles (gonostylar zone). These gonostyles or gonoblastidia are 

 mouthless palpons in the Discalidae, whilst they are either cystons or true siphons, pro- 

 vided with a terminal mouth, in the Porpitidae and Velellidse. They are originally nothing 

 else than secondary manubria of a single Medusa person, budding from the radial canals 

 of the subumbrella. They may consequently be regarded as secondary persons, compar- 

 able to the buds of the Gastroblasta (described by Keller and Lang), and of other Hydro- 

 medusas budding from the subumbrella. On the other hand they may be compared also 

 to the genital sacs which depend from the radial canals of the subumbrella in the Trachy- 

 nemidae, Aglauridae, Pectyllidae, and in other families of Hydrornedusae. If we imagine 

 that these genital sacs, instead of themselves producing ova and spermatozoa in their 

 exodermal wall, produce by budding Medusae which afterwards become sexually mature, 

 we shall understand how the Disconectae have originated from Trachomedusae. 



The mouthless medusiferous palpons of the Discalidae, and the similar secondary 

 siphons of the Porpitidae and Velellidae (differing only in the possession of a mouth at 

 the distal end), have the same structure as the large primary central siphon, but are 

 always much smaller. Their number is originally eight, and they form a regular octo- 

 radial corona in some smaller and simpler genera [Discalia, PI. XLIX. figs. 1-4 ; 

 Porpalia, PL XLVIII. ; Rataria, PI. XLIV.). Sometimes there are sixteen, e.g., in 

 Disconalia (PI. L.) and Porpitella (PI. XL VI.). But usually their number is much 

 increased, and amounts in the larger species to some hundreds. These cover the greatest 

 part of the subumbrella, the large gonostylar zone between the central siphon and the 

 corona of submarginal tentacles, often densely crowded without intervals. The form 

 and size of the gonostyles are very variable, owing to their great contractility. Some- 

 times they are more spindle-shaped, at other times more cylindrical or pyriform, with a 



