•28(5 THE VOYAGE OF H.M.S. CHALLENGER. 



be a bundle of parallel spindle-shaped muscle-cells, and is probably an inner prolongation 

 of the exodermal muscle-stratum of the outer wall. The inner insertion of the 

 pistillum forms a broad circular ring in the " foveola auropylse" of the pneumatophore. 

 This foveola (fig. 16, pi) contains the auropyle or the inner opening which leads 

 from the axial canal of the aurophore into the large cavity of the pneumatophore. 

 The longitudinal muscle-fibres of the pistillum diverge here in a radial direction hori- 

 zontally, and are inserted at the circular margin of the foveola, ending abruptly with 

 a sharp boundary line on the pneumatocyst (fig. 24, Ip 1 ). 



Judging from these peculiar structures of the pistillum, we suppose that it acts as a 

 strong muscle, by the contraction of which the aurophore is opened and the air contained 

 in the pneumatophore expelled. Its morphological explanation is very difficult ; one 

 might suppose it to be a part of a modified stomach (manubrium) of the medusoid 

 person ; more probably, however, it is a secondary apophysis of the exoderm only 

 (similar to the endocystic tapetum of the Physonectae), grown inside from the spira- 

 culum into the central cavity of the aurophore, which corresponds to the umbrella- 

 cavity of the Medusa. In this case the margin of the aurostigma (lo) may be compared 

 perhaps with the umbrella margin of the Medusa, and the pistillum with its velum 

 turned inside into the umbrella cavity (?). 



Nectophore^ («). — The nectocalyces or swimming-bells form an elegant corona round 

 the base of the pneumatophore. This corona is simple in Stephalia (PI. VII. figs. 39, 

 40, 48) and in Auralia ; it is multiple in Stephonalia and Rhodalia (PI. I. fig. 1 ; PI. II. 

 fig. 6 ; PI. III. figs. 13, 14). The circular corona is bisected in the sagittal plane of the 

 body, on the dorsal side by the aurophore (I), on the ventral side by the set of buds (?'). The 

 nectophoivs are pyriform medusoid persons of equal size ; their number is eight to six- 

 teen in Stephalia, twenty to thirty in Stephonalia, fifty to eighty or more in Rhodalia. 



Pedicles of the Nectophores. — The swimming-bells are attached on the periphery of the 

 cylindrical nectosome (or the upper half of the bulbous trunk) by means of large lamellar 

 pedicles, similar to mesenterial plates (PL III. figs. 13, 14). Each pedicle is a thin trans- 

 parent lamella of quadrangular or nearly square form, and consists of a cartilaginous 

 vertical jelly-plate placed in a meridional plane of the trunk. The thinner upper and the 

 thicker lower margins of the pedicle are free ; the inner or axial margin is thickened and 

 arises by a broad base from the ccenosome ; the outer or abaxial margin passes over 

 into the conical apical part of the nectophore (PI. IV. fig. 16, np). A wide canal, the 

 peduncular canal of the nectophore (PI. V. fig. 31, ns), arising from the network of canals 

 in the ccenosome, and placed radially to its vertical main axis, runs horizontally along 

 the thickened lower margin of each pedicle, and gives off at right angles a series of 

 twenty to thirty small, lateral, vertically ascending branches. These branches, or the 

 "secondary peduncular canals" (nl), are therefore directed parallel to one another and to 

 the vertical main axis of the trunk ; they are single, blind, slightly curved or undulating, 



