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



either form a simple ring of eight or sixteen gonostyles around the central siphon, or 

 they occupy a broader gonostylar zone, often the whole suburnbrella between the central 

 siphon and the submarginal corona of tentacles. At first there is a corona of eight 

 gonostyles only, regularly placed around the central siphon, and this state is permanent in 

 some species of Porpalia (PI. XLVIIL). In Porpitella (PL XLVI.) there is a girdle of 

 sixteen gonostyles. But usually their number is soon increased, and they form several 

 concentric circles, more or less regular. Finally, in the largest Porpitidse their number 

 amounts to several hundreds, and they are densely crowded. Their form and structure 

 have been described above (p. 36). 



Tentacles. — The corona of submarginal tentacles in all Porpitidse is originally 

 regularly octoradial. In all young larvae of this family there occurs a stage in which the 

 umbrella-margin bears only eight tentacles regularly disposed at equal intervals (PI. L. 

 figs. 9, 10), as permanently in Discalia (PL XLIX. figs. 1-4). But their number is soon 

 increased either by interpolation of eight secondary interradial tentacles (in the middle 

 between the former), or by budding of several secondary tentacles at the base of each 

 primary one, so that eight regular bunches arise {Porpalia, PL XLVIIL). Sometimes 

 sixteen bunches are formed {Porpitella, PL XLVI.). Usually the number is so increased 

 that hundreds or thousands of tentacles, densely crowded, cover the whole zone beyond 

 the margin of the disc. Often three to nine or more concentric circles may be discerned, 

 and then the uppermost are the smallest, the lowermost the longest. Sometimes their 

 bases are so densely crowded in the tentacular area, that after removing the tentacles 

 there appears an elegant reticulated girdle beyond the margin ; each rhomboidal dimple 

 of the reticulum is the place of the basal insertion of a lost tentacle (PL XL VII. 

 fig. 3, tu; PL XLVIIL fig. 2, tu). 



The, general structure of the tentacles is the same in the Porpitidse as in the other Dis- 

 conectse ; but they differ from those of the Velellidse in their peculiar constant form and 

 structure. The youngest larvae of the Porpitidse bear eight simple radial tentacles with a 

 terminal cnidosphere (PL L. fig. 9) like those of Discalia. The next larval stage exhibits 

 four pediculate cnidospheres at the distal end of each tentacle, one of which is the primary 

 terminal knob, the three others basal branches of it (fig. 10). The number of the latter 

 is soon multiplied, and the older and longer tentacles, which are club-shaped and 

 thiekened at the rounded distal end, bear always three longitudinal rows of pedunculated 

 cnidospheres ; one odd inferior series in the middle line of the lower or distal face, and two 

 paired (lateral) series on the two lateral faces ; the upper or proximal face is always smooth. 1 

 Each cnidosphere is a thin lateral branch of the tentacle, having the same structure as 

 the latter, and bearing a terminal sphere composed of radially crowded cnidoblasts. The 

 peduncles of the latter are sometimes shorter, at other times longer. Their length often 

 increases towards the distal end of the tentacle. Their number is very variable in different 



1 57, pis. ix., x. 



