C,s THE VOYAGE OF H.M.S. CHALLENGER. 



very peculiar, but could not be diagnosed satisfactorily from the only spirit-specimen 

 examined. The ectodermal epithelium (q) contains numerous nematocysts. The muscular 

 plate (m) appears thinned away on both lateral surfaces of the tentacles, but on the other 

 hand thickened so remarkably on the inner and outer side that it projects in the form of 

 two strong band-shaped longitudinal muscles. The external or axial longitudinal muscle 

 springs from the pedalia, and usually occupies only the proximal third or fourth of the 

 length of the tentacle. The inner or axial longitudinal muscle runs through the entire 

 length of the tentacle, and is split up above into two conical root muscles (ink, figs. 22, 29). 

 These invaginate the distal margin of the corresponding coronal pouch, divide it into an 

 external velar pouch and an internal avelar pouch, diverge into the " funnel of the 

 tentacle " (it), formed in this way between the velar and avelar pouches and run as the 

 proximal margin of the coronal muscle, where they are inserted (comp. below). If the 

 internal longitudinal muscle is strongly contracted, the tentacle appears rolled up spirally 

 and laid in deep transverse folds (d). A structureless thin, but very firm, supporting 

 plate ("lamina fulcralis," fig. 21, z) lies under the muscular plate (m) and under the 

 supporting plate, the enclodermal epithelium of the tentacle canal (d). The nature of the 

 latter is very remarkable ; it consists of large vesicular cells, and rises in the shape of a 

 thick spongy cord (fig. 21, d") on the abaxial side of the canal wall. This cord consists 

 of an accumulation of very large vesicular cells, and fills like marrow nearly the half of 

 the lumen of the tube (ct). It would recpiire to be more minutely investigated in living 

 and well-preserved animals. So would another most peculiar arrangement of the ten- 

 tentacles ; a strong, double- valved aperture lying inside the base of the tentacle immedi- 

 ately at the point where the tentacle roots diverge (PL XXII. fig. 22, yk). The elastic 

 fulcral lamella is swollen there into a thick gelatinous plate containing cells, and forms two 

 horizontal vent-valves lying above one another, by means of which the cavity of the 

 tentacle can be completely closed. Even by strong pressure from within the tentacle 

 cavitv it was impossible to overcome the antagonism of the double valve. The cavity of 

 the vent-hole (fig. 22, ex) between the distal (yk") and the proximal valve (y¥) is nearly 

 as hioh as broad. 



The marginal sense clubs (" rhopalia") of Periphylla (PI. XVIII. figs. 1-5 ; PL XXII. 

 fig. 22, or; PL XXIII. 31, 32, or) have been already described by me in Periphylla hyacin- 

 th i 'nam my System der Medusen (1879, taf. xxiii. figs. 9-12). They appear to have 

 essentially the same formation in Periphylla m irabilis, and represent very composite organs 

 of sense connected among the forms hitherto known, with the rhopalia of the Cubornedusaa 

 on the one side and with those of the Nausithoidae on the other. As in all Peromedusa? 

 there arc only four interradial rhopalia, which lie in the radii of the septal nodes and 

 the tseniola. They were, unfortunately, very badly preserved in the spirit-specimen 

 examined ; a complete and correct insight into their very complicated minute structure 

 could only have been obtained by examination and special preparation of fresh rhopalia. 



