32 THE VOYAGE OF H.M.S. CHALLENGES. 



is elliptical or sometimes lanceolate in the Velellidse ; often the periphery is more or less 

 lobate, corresponding to the form of the surrounding peripheral part of the umbrella. 

 The upper or proximal surface of the centradenia is attached to the lower or distal 

 surface of the pneumatosaccus, and separated from it by a thinner or thicker fulcrum, a 

 firm and elastic, structureless, supporting plate. But this supporting septum is wanting 

 in the young larvaa, where the exodermal epithelium of the pneumatosac is in immediate 

 contact and connection with the upper face of the centradenia, 



All the various and often complicated apophyses (radial ribs, circular rings, &c.) 

 which are formed by the distal or inferior face of the pneumatocyst, and its enveloping 

 pneumatosac, are surrounded by corresponding furrows or envelopes of the proximal or 

 superior face of the centradenia ; the latter being always immediately attached to the 

 supporting plate, which separates it from the former in the adult Disconectse. 



The lower or distal surface of the centradenia is in its central part in contact with 

 the base of the central siphon, in its peripheral part with that portion of the subumbrella 

 which bears the gonostyles. The fundus of the central siphon, or the uppermost part of 

 its cavity, is separated from the attached centradenia by a strong support, that sup- 

 porting plate, which has been described above as " lamina gastrobasalis." This elastic 

 and structureless plate is pierced in the periphery of the basal part of the stomach by 

 eight or sixteen (sometimes more) radial canals, which arise from the fundus of the 

 stomach itself and pass into the vascular system of the centradenia. We call these the 

 primary perradial gastro-canals. There are originally eight equal and regularly disposed 

 radial canals in the Discalidse and Porpitidse. In the Velellidse, on the other hand, they 

 are arranged in a bilateral manner ; two larger radial canals (a dorsal and a ventral) 

 arising from the opposite poles of the longer (sagittal) axis, two smaller (right and left) 

 from the poles of the shorter (transverse) axis, and four other (diagonal) canals in the 

 middle between the latter and the former. But usually the number of intercalated 

 secondary canals is here much larger, and their arrangement more or less irregular. 



Origin of the Centradenia. — For the right understanding of this peculiar organ of 

 the Disconectas, a knowledge of its origin and development is essential. This question 

 may be solved by the comparison of very young Disconectse and mainly of the Discalidae. 

 These simplest and most primitive Disconectse remain permanently in a lower stage 

 of development, which is transitory in the Porpitidse and Velellidse. In the smallest 

 Disconula-larvae which I observed the centradenia is a small circular, biconvex, lenticular 

 disc ; its upper face is in direct contact with the pneumatosaccus, whilst its lower face is 

 separated from the central siphon by the gastrobasal plate ( :< plancher" of Bedot). The 

 entire mass of this solid disc is composed solely of exodermal cells and cnidoblasts ; it is 

 not traversed by any canals. The only canals of the centradenia are the eight simple 

 radial canals which run upon its upper face ; they arise from the eight ostia of the basi- 

 gaster, embrace the surface of the lenticular centradenia like eight meridian lines, and 



