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



The obliquely egg-shaped subumbrella is much more vaulted in the ventral than in 

 the dorsal half; the diameter of its aperture is about equal to its height. The velum 

 (figs. 1, v, 2, v) is small. The four radial canals of the subumbrella are arranged 

 symmetrically ; the dorsal canal (figs. 1, 2, cd) is shorter, the ventral (cv) longer than 

 the two lateral canals {ex right, cl left). The apical canal (fig. 7, en), which arises from 

 their proximal junction, is short, and passes obliquely through the jelly substance of the 

 top in ventral direction to the somatocyst (cs). 



Apical Bract (figs. 1, 6, 2, b, 7 b). — The peculiar organ, which we call the apical 

 bract, distinguishes at once Mitrophyes from all other Monophyida?, and from all Caly- 

 conectee in general. It covers the exumbrella of the nectophore in the same manner as 

 a cap covers the head of a man. The bract is a circular concave-convex shield, the 

 diameter of which nearly equals that of the subjacent nectophore. 



Its morphological signification is difficult to make out ; it may be nothing other than 

 the reduced umbrella of a medusome, which has lost all its other parts ; but it may also 

 be the remnant of the primary nectophore, the place of which is taken by the permanent 

 nectophore afterwards developed. These two pieces are connected by a narrow short 

 pedicle (figs. 2, 7) which is probably the uppermost part of the original trunk. The jelly 

 substance of the bract is rather thin in the peripheral, thicker in its central part, and 

 includes here three short blind radial canals, a longer dorsal (fig. 1, cb) and two smaller 

 lateral canals ; from their junction arises a short bracteal canal (fig. 7, cb) which passes 

 through the pedicle to the somatocyst {cs). 



Somatocyst (figs. 1, 2, 7, cs). — The somatocyst, or the coryphal cavity of the stem, is 

 a slender conical canal, placed nearly horizontally in the gelatinous umbrella of the 

 nectophore, in its sagittal plane, and directed towards the dorsal side. Its direct 

 continuation towards the ventral side is the axial canal of the trunk. Its proximal apex 

 is closed. From its distal base arise two lateral branches nearly opposite ; proximally the 

 peduncular canal of the bract (fig. 7, cb), and distally the peduncular canal of the 

 nectophore (fig. 7, en). 



Hydrcecium. — Mitrophyes does not possess a distinct hydrcecium, but it has a very 

 small cavity, which may be considered as the rudimentary homologue of such a " funnel 

 cavity" (figs. 1, 7, ui). This rudiment of a hydrcecium is placed nearly in the apex of 

 the nectophore, and represents a very small funnel-shaped foveola of its exumbrella, 

 which surrounds the origin of the free trunk (a). 



Siphosome (figs. 1, 2, 7, a). — The common trunk or stem of the siphosome in the 

 expanded state (fig. 1) is a very long thin cylindrical tube, attaining a length of 20 to 

 30 mm. The internodes between the ordinate cormidia are twice as long as these. In 

 the contracted state (fig. 2) the internodes disappear, and the convoluted stem becomes 

 hidden between the nectophore and bract. The number of the cormidia in the largest 

 specimen observed was between thirty and forty. 



