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



a right angle to the umbrella margin. All eight tentacles have the same form and 

 structure ; the four primary perraclial tentacles are, however, twice as long as the four 

 secondary interradial, the former are also inserted somewhat higher, and the clasps of the 

 latter are consecmently somewhat shorter. The four perraclial tentacles are somewhat 

 longer than the largest diameter of the umbrella, the four interradial only about half so 

 long. The free cylindrical tentacle filament (fig. 5, longitudinal section ; fig. 6, seen 

 from the outside) is more than a millimeter thick at the base, decreases towards the 

 point like an awl, and is shaped like a mouse's tail ("myosura"). The solid axis 

 resembles a rouleau of coin, and consists of a single row of discoid chorda! cells whose 

 nuclei be in the centre, one behind the other (fig. 5, ym ; comp. also PL XII. figs. 10, 11). 

 The elastic structureless supporting plate enclosing this column of chordal cells (2), is 

 covered by a layer of longitudinal muscular fibres (fig. 6, mt), above and outside which 

 lies the single layered epithelium of the ectoderm (d). The spheroidal nematocysts (m) in 

 the exoderm lie thickly together on the dorsal (abaxial) side of the tentacles, and form a 

 raised urticating band (fig. 6, m), whilst they are only scantily distributed and of smaller 

 size on the other sides of the tentacles. 



The peronia or " umbrella clasps," which serve to connect the base of the tentacle 

 with the urticating ring of the umbrella margin, are eight thick urticating streaks, 

 gradually increasing in breadth from the top to the bottom (figs. 1, 2, 4, en). They 

 appear egg-shaped in transverse section (figs. 7, en; 12, en), and under higher magni- 

 fying power they prove to be composed of the peculiar " peronial tissue " or " urticating 

 skeletal tissue," which is the most important element in the urticating ring, and in the 

 peronia and otoporpse of the Narcornedusse. This tissue (fig. 12, en) consists of com- 

 pacted exodermal thread cells, varying greatly in shape and size. The roundish thread 

 cells containing nematocysts enclose a long urticating thread, wound thickly and spirally ; 

 they have very thick walls, and are partly much larger (three to four times as large) than 

 the ordinary largest nematocysts of the tentacles. These nematocysts are plainly incapable 

 of throwing out their threads, but only serve with their thickened wall as firm " support- 

 ing cells." The inner axial side of the peronia is then closed on the exodermal epithe- 

 liuin of the subumbrella (figs. 7, 12, qw) ; also on the peronial canals, touching them 

 laterally (cts) by a thick supporting plate, whilst its outer abaxial side touches the 

 gelatinous substance of the umbrella (wg). 



The tentacle roots (fig. 11, tr) are, as usual, conical, being a centripetal prolongation of 

 the tentacle axis, consisting of a few large chordal cells of the endoderm, and having their 

 points directed centripetally. They are covered by a structureless supporting plate, but 

 have no exodermal epithebum. Their dorsal and their lateral surfaces are enclosed in 

 the gelatinous substance of the umbrella, whUst their ventral surface Hes immediately on 

 the cover of the stomach (or the dorsal gastral wall), which it serves at the same time to 

 support. 



