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



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genitalia fixed on its subumbral side, It is very much thickened in the centre of the 

 apex, and projects into the fundus of the gastral cavity in the form of a short, conical, 

 gelatinous appendage (fig. 2, nk). On the other hand, the gelatinous substance is much 

 thinner in the lower oral half, hardly one-fourth or one-fifth as thick as in the upper 

 half, from which it is sharply divided (half-way up the height of the umbrella) by the 

 exumbral circular furrow. An enormous number of sinous elastic fibres, running from 

 the exumbral to the subumbral wall, traverse the gelatinous substance of the umbrella, 

 to which they give a considerable degree of firmness ; they are placed together in dainty 

 pyramids (fig. 8, iif), whose points touch the exumbrella (e), and their bases the sub- 

 umbrella (w) ; these pyramids form regular longitudinal series, corresponding to the 

 exumbral radial ribs. 



The umbrella margin (" margo umbralis," figs. 1, 11, 12, 20) is not so visibly lobed in 

 this genus of the Pectyllidaj as in the two others, but rather appears to be of equal thick- 

 ness all over, and closely beset with an enormous number of short sucking-tentacles 

 placed in several rows above each other. Closer consideration, however, shows that this 

 garniture of the umbrella margin is by no means equally distributed, but rather arranged 

 in eight larger and thirty-two smaller groups. These, however, hang closely together, 

 and are not separated by depressions as in Pectyllis and PectardMs. A similar lobed 

 formation of the umbrella margin also exists fundamentally in Pedis, though it is not so 

 apparent externally as in the other two genera. Each of the thirty-two small groups of 

 tentacles (fig. 20, td) consists of from 30 to 40 solid tentacular appendages. Of these 

 the 16 to 20 upper (proximal) are short-stalked, pyriform, or club-shaped sucking-cups, 

 the 12 to 16 lower (distal) on the contrary are somewhat longer tentacles, partly with a 

 terminal sucking-cup, partly apparently forming feelers. These " tactile tentacles " with- 

 out sucking-cup (fig. 17) were mostly torn away, the longest barely above 1 mm. in 

 length. They are probably much longer in the living animal (as in Pectyllis arctica, 

 PI. III. fig. 1). The numerous sucking-cups form G to 8 alternating rows placed one 

 above the other on the umbrella margin ; in each of the thirty-two small groups, which 

 have an almost rhomboidal outline, they are placed in 5 to 6 diagonal rows, each with 

 4 to 5 tentacles (fig. 20). The size of the sucking-cups, which enclose a visible conical 

 ectodermal sucking cavity, decreases gradually from above downwards (fig. 12). A 

 larger sucking-cup is placed above somewhat further on the umbrella margin (fig. 20, 

 st). All the tentacles of this genus are solid ; their endodermal axis consists of large 

 clear chordal cells which are sometimes placed in a discoid row one behind the other 

 (fig. 17, dt), sometimes more numerous and in a more complete arrangement beside 

 each other (fig. 15, dt). This vesicular axial tissue is covered by very powerful internal 

 annular muscles, which thicken at the end into a strons; annular swelling ; single bundles 

 of external longitudinal muscles, very much thickened at the exumbral side of the 

 tentacles, extend out above these annular muscles ; they run out below the sucking- 



