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



The family Rhizophysidse comprises all Cystonectae polygastricse witli a long tubular 

 stem, bearing numerous monogastric cormidia. The trunk of the corm is a prolonged and 

 very contractile tube, as in the succeeding Salacidse ; but the cormidia of these latter 

 are polygastric. Many different species, and mainly deep-sea forms of gigantic size, 

 seem to belong to this interesting family ; but only a few species have been described 

 hitherto with sufficient accuracy. All species have been united up to this time in a 

 single genus, Rhizophysa. 



The oldest known form of Rhizophysidae is the Mediterranean Rhizophysa fliformis, 

 described as early as 1775 by the first author on Siphonophorae, Forskal, under the name 

 Physophora Jiliformis (11, p. 120, Tab. xxxiii. fig. F). A similar species from the 

 Atlantic was figured in 1807 by Pdron and Lesueur under the name Rhizophysa 

 planostorna (14, pi. xxix. fig. 3). A third species of the same genus was observed in 

 1827 by Mertens in the Northern Pacific, and described by Brandt in 1835 asJSpibulia 

 mertensii (25, p. 33). Unfortunately Mertens' excellent figure, drawn from life and 

 exhibiting distinctly the characters of the genus Rhizophysa, has never been published. 

 Brandt established for these forms the family Rkizophysidae (loc. cit., p. 33). 



The first accurate anatomical description was that given in 1854 by Gegenbaur of 

 the Mediterranean Rhizophysa Jiliformis (7, p. 324 ; 10, p. 78). It was afterwards 

 supplemented by Huxley (9, p. 90), Fewkes (41, p. 292), and especially by Chun (47, 

 p. 404; 48, p. 529; 86, p. 1169). Two new genera of this family, Cannophysa and 

 Nectophysa (Pis. XXIII. , XXIV.), were observed by me in 1866 off the Canary Islands. 

 Two other genera, Aurophysa and Linophysa, both inhabiting the deep sea, were 

 described in 1878 by Studer as species of Rhizophysa (40, p. 4, Taf. L). 



Truncus. — The common stem of the corm is in all Rhizophysidae a very long and 

 slender cylindrical tube ; its contractility is so great that, in the expanded state, it 

 may be ten to twenty times as long as in the contracted state (compare PI. XXIII. 

 figs. 1-3 and PI. XXIV. figs. 1-3). At the same time it is so sensitive that a slight 

 touching of the stem is sufficient to effect suddenly its strongest contraction. The 

 wall of the tubular trunk is rather thick, with a thin layer of circular muscles arising 

 from the entoderm, and a thick layer of longitudinal muscles arising from the exoderm ; 

 the latter are arranged, as usual, in parallel bundles along the radial folds of the 

 fulcrum, which separates the two layers. The exoderm is often coloured yellowish, rose, 

 or brown. 



Cormidia. — The numerous cormidia, which arise from the long tubular stem of the 

 corm, exhibit in the Rhizophysidae (as in the Agalmidae) a double shape and arrange- 

 ment, according to which two subfamdies may be separated, the Cannophysidae and 

 Nectophysidae. The Cannophysidae (PI. XXIV.) possess ordinate cormidia, which are 

 separated by long, free, and naked internodes of equal length (similar to those of the 

 Stephanomidse) ; each cormidium is composed of a siphon, a tentacle, and a gonostyle ; 



