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



composed of four equidistant simple radial canals (nr), which arise from the distal or 

 abaxial end of the primary peduncular canal, run in the subumbrella to the margin of 

 the jelly-bell, and are there united by a circular canal (nc), placed above the insertion of 

 the velum (v). Compare PI. VII. figs. 39, 48. 



Truncus. — The common stem of the corm, or the coenosarc, exhibits in the Auronectae 

 a most remarkable form and structure, very different from that of all other Siphono- 

 phorse. It is a large solid bulb of a cartilaginous consistence, sometimes subspherical 

 (PI. IV. fig. 15, a), at other times more spindle-shaped or truncately conical (PI. VII. 

 fig. 40, a). Its size is usually about equal to or double that of the pneumatophore. 

 The solid mass of the cartilaginous trunk is structureless and colourless, hyaline, 

 very similar to the hyaline fundamental substance of common cartilage. It is 

 traversed everywhere by a dense network of innumerable small canals, anastomosing 

 one with another, and with the large hypocystic cavity (PI. V. fig. 24, ac). The 

 network is very similar to that in the fleshy coenosarc of the common Alcyonium ; but 

 the numerous asteroidal connective cells, which are scattered in the fundamental 

 substance of this latter between the vessels, are wanting in the Auronectae ; excep- 

 tionally here a few mesenchymatous entoderm-cells step out from the vessels and remain 

 isolated in the fundamental substance. The network of these nutritive vessels or 

 gastro-canals forms on the surface of the trunk a superficial net, from which the canals 

 of the cormidia arise. The entodermal epithelium which lines the canals is composed 

 of high cylindrical cells, forming a single layer (PI. V. fig. 29). 



The Stephalidae (Stephalia, PI. VII. fig. 40) differ from the larger Rbodalidoe 

 (Rhodalia, PI. IV. fig. 15) in the possession of a wide, cylindrical, central canal (ca), 

 which descends vertically in the main axis of the turnip-shaped trunk and opens at its 

 distal pole by a mouth. This terminal mouth is sometimes much larger than the mouth- 

 openings of all the other siphons (PL VI. figs. 32, 33, ap). There can be no doubt in my 

 opinion that this important axial canal is the gastral cavity of the protosiphon, or 

 the primary siphon of the larva, which is the manubrium of the original medusome. 

 Its distal opening is the original Medusa-mouth. This explanation becomes evident by 

 the comparison with the youngest larva observed (Auronula, PI. VII. fig. 50). The 

 entire siphosome is here represented by the single primary siphon. By thickening of 

 its wall and development of nutritive canals in it arises the vascular bulbous trunk of 

 the Auronectae. It corresponds to the basal protosiphon at the distal end of the 

 Physalidae, and to the sterile central siphon of the Disconectas. 



Cormidia. — The entire surface of the bulbous trunk beyond the corona of 

 nectophores is in all Auronectae densely covered with numerous cormidia. Their 

 number is in the smaller Stephalidae twenty to fifty or more, in the larger Pikodalidae 

 sixty to eighty, often more than one hundred, or even several hundreds. The cormidia 

 are always monogastric and originally ordinate, arranged in regular circles or spiral coibs 



